View source for Paleocene–Eocene Thermal Maximum

{{short description|Global warming about 55 million years ago}}
{{redirect-distinguish|PETM|PETN}}
{{cs1 config|name-list-style=vanc}}
[[File:65 Myr Climate Change.png|thumb|upright=1.8|Climate change during the last 65 million years as expressed by the oxygen isotope composition of benthic foraminifera. The Paleocene-Eocene thermal maximum (PETM) is characterized by a brief but prominent excursion, attributed to rapid warming. Note that the excursion is understated in this graph due to the smoothing of data.]]

The '''Paleocene–Eocene thermal maximum''' ('''PETM'''), alternatively "'''Eocene thermal maximum 1 (ETM1)'''" and formerly known as the "'''Initial&nbsp;Eocene'''" or "'''Late Paleocene thermal maximum'''", was a geologically brief time interval characterized by a {{convert|5–8|C-change|0}} global average temperature rise and massive input of carbon into the ocean and atmosphere.<ref name="HaynesHönisch2020">{{cite journal |last1=Haynes |first1=Laura L. |last2=Hönisch |first2=Bärbel |date=14 September 2020 |title=The seawater carbon inventory at the Paleocene–Eocene Thermal Maximum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=117 |issue=39 |pages=24088–24095 |doi=10.1073/pnas.2003197117 |pmid=32929018 |pmc=7533689 |bibcode=2020PNAS..11724088H |doi-access=free}}</ref><ref name=McInerney2011>{{cite journal |last1=McInerney |first1=Francesca A. |last2=Wing |first2=Scott L. |title=The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future |journal=Annual Review of Earth and Planetary Sciences |date=30 May 2011 |volume=39 |issue=1 |pages=489–516 |doi=10.1146/annurev-earth-040610-133431 |bibcode=2011AREPS..39..489M }}</ref> The event began, now formally codified, at the precise time boundary between the [[Paleocene]] and [[Eocene]] geological [[Epoch (geology)|epochs]].<ref name=Westerhold2008>{{cite journal |last1=Westerhold |first1=Thomas |last2=Röhl |first2=Ursula |last3=Raffi |first3=Isabella |last4=Fornaciari |first4=Eliana |last5=Monechi |first5=Simonetta |last6=Reale |first6=Viviana |last7=Bowles |first7=Julie |last8=Evans |first8=Helen F. |title=Astronomical calibration of the Paleocene time |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=February 2008 |volume=257 |issue=4 |pages=377–403 |doi=10.1016/j.palaeo.2007.09.016 |bibcode=2008PPP...257..377W }}</ref> The exact age and duration of the PETM remain uncertain, but it occurred around 55.8 million years ago (Ma) and lasted about 200 thousand years (Ka).<ref name=Bowen2015>{{cite journal |journal=[[Nature (journal)|Nature]] |volume=8 |issue=1 |pages=44–47 |doi=10.1038/ngeo2316 |last1=Bowen |first1=Gabriel J. |last2=Maibauer |first2=Bianca J. |last3=Kraus |first3=Mary J. |last4=Röhl |first4=Ursula |last5=Westerhold |first5=Thomas |last6=Steimke |first6=Amy |last7=Gingerich |first7=Philip D. |last8=Wing |first8=Scott L. |last9=Clyde |first9=William C. |year=2015 |title=Two massive, rapid releases of carbon during the onset of the Palaeocene–Eocene thermal maximum |bibcode=2015NatGe...8...44B}}</ref><ref name=":2">{{Cite journal |last1=Li |first1=Mingsong |last2=Bralower |first2=Timothy J. |last3=Kump |first3=Lee R. |last4=Self-Trail |first4=Jean M. |last5=Zachos |first5=James C. |last6=Rush |first6=William D. |last7=Robinson |first7=Marci M. |date=2022-09-24 |title=Astrochronology of the Paleocene-Eocene Thermal Maximum on the Atlantic Coastal Plain |journal=Nature Communications |volume=13 |issue=1 |page=5618 |doi=10.1038/s41467-022-33390-x |pmid=36153313 |pmc=9509358 |bibcode=2022NatCo..13.5618L }}</ref> 

The PETM arguably represents our best past analogue for which to understand how global warming and the [[carbon cycle]] operate in a greenhouse world.<ref name=McInerney2011 /><ref name=Gutjahr2017>{{cite journal |last1=Gutjahr |first1=Marcus |last2=Ridgwell |first2=Andy |last3=Sexton |first3=Philip F. |last4=Anagnostou |first4=Eleni |last5=Pearson |first5=Paul N. |last6=Pälike |first6=Heiko |last7=Norris |first7=Richard D. |last8=Thomas |first8=Ellen |author8-link=Ellen Thomas (scientist) |last9=Foster |first9=Gavin L. |title=Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum |journal=[[Nature (journal)|Nature]] |date=August 2017 |volume=548 |issue=7669 |pages=573–577 |doi=10.1038/nature23646 |pmid=28858305 |pmc=5582631 |bibcode=2017Natur.548..573G}}</ref><ref name = "JonesSM2019">{{cite journal |last1= Jones|first1= S.M.|last2= Hoggett|first2= M.|last3= Greene|first3= S.E.|last4= Jones|first4= T.D.|title= Large Igneous Province thermogenic greenhouse gas flux could have initiated Paleocene-Eocene Thermal Maximum climate change|journal=[[Nature Communications]]|volume= 10|issue= 1|page= 5547|date= 2019|doi= 10.1038/s41467-019-12957-1|pmid= 31804460|pmc= 6895149|bibcode= 2019NatCo..10.5547J|doi-access= free}}</ref> The time interval is marked by a prominent negative excursion in carbon [[stable isotope]] ({{delta|13|C|link}}) records from around the globe; more specifically, a large decrease in the <sup>13</sup>C/[[Carbon-12|<sup>12</sup>C]] ratio of marine and terrestrial [[carbonate]]s and organic carbon has been found and correlated across hundreds of locations.<ref name=McInerney2011/><ref name=Kennett1991>{{cite journal |last1=Kennett |first1=J. P. |last2=Stott |first2=L. D. |title=Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene |journal=Nature |date=September 1991 |volume=353 |issue=6341 |pages=225–229 |doi=10.1038/353225a0 |bibcode=1991Natur.353..225K }}</ref><ref name=Koch1992>{{cite journal |last1=Koch |first1=Paul L. |last2=Zachos |first2=James C. |last3=Gingerich |first3=Philip D. |title=Correlation between isotope records in marine and continental carbon reservoirs near the Palaeocene/Eocene boundary |year=1992 |journal=[[Nature (journal)|Nature]] |volume=358 |pages=319–322 |doi=10.1038/358319a0 |issue=6384 |bibcode=1992Natur.358..319K |hdl=2027.42/62634 |hdl-access=free}}</ref> The magnitude and timing of the PETM ({{delta|13|C|link}}) excursion, which attest to the massive past carbon release to our ocean and atmosphere, and the source of this carbon remain topics of considerable current geoscience research.

What has become clear over the last few decades is that [[Stratigraphic]] sections across the PETM reveal numerous changes beyond warming and carbon emission.<ref name=McInerney2011/> Consistent with an Epoch boundary, [[fossil]] records of many organisms show major turnovers. In the marine realm, a [[extinction event|mass extinction]] of [[benthos|benthic]] [[foraminifera]], a global expansion of subtropical [[dinoflagellate]]s, and an appearance of excursion taxa, including within [[planktic]] [[foraminifera]] and [[calcareous nannofossils]], all occurred during the beginning stages of the PETM. On land, many modern [[mammal]] orders (including [[primates]]) suddenly appear in Europe and in North America.<ref name="VanDerMeulen2020">{{cite journal |last1=Van der Meulen |first1=Bas |last2=Gingerich |first2=Philip D. |last3=Lourens |first3=Lucas J. |last4=Meijer |first4=Niels |last5=Van Broekhuizen |first5=Sjors |last6=Van Ginneken |first6=Sverre |last7=Abels |first7=Hemmo A. |date=15 March 2020 |title=Carbon isotope and mammal recovery from extreme greenhouse warming at the Paleocene–Eocene boundary in astronomically-calibrated fluvial strata, Bighorn Basin, Wyoming, USA |journal=[[Earth and Planetary Science Letters]] |volume=534 |article-number=116044 |doi=10.1016/j.epsl.2019.116044 |doi-access=free |bibcode=2020E&PSL.53416044V }}</ref>

{{Paleogene graphical timeline}}

==Setting==
The configuration of oceans and continents was somewhat different during the early [[Paleogene]] relative to the present day. The [[Panama Isthmus]] did not yet connect [[North America]] and [[South America]], and this allowed direct low-latitude circulation between the [[Pacific Ocean|Pacific]] and [[Atlantic Ocean]]s. The [[Drake Passage]], which now separates [[South America]] and [[Antarctica]], was closed, and this perhaps prevented thermal isolation of Antarctica. The [[Arctic]] was also more restricted. Although various proxies for past atmospheric {{CO2}} concentrations across the Cenozoic do not agree in absolute terms, all suggest that levels in the early Paleogene before and after the PETM were much higher than at present-day. In any case, significant terrestrial ice sheets and sea-ice did not exist during the late Paleocene through early Eocene.<ref name=Zachos2008>{{cite journal |last1=Zachos |first1=James C. |last2=Dickens |first2=Gerald R. |last3=Zeebe |first3=Richard E. |year=2008 |title=An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics |journal=[[Nature (journal)|Nature]] |volume=451 |issue=7176 |pages=279–283 |doi=10.1038/nature06588 |pmid=18202643 |bibcode=2008Natur.451..279Z |doi-access=free }}</ref>

Earth surface temperatures gradually increased by about {{convert|6|C-change|0}} from the late Paleocene through the early Eocene.<ref name=Zachos2008/> Superimposed on this long-term, gradual warming were at least three (and probably more) "hyperthermals". These can be defined as geologically brief (<200,000 year) events characterized by rapid global warming, major changes in the environment, and massive carbon addition. Though not the first within the [[Cenozoic]],<ref>{{cite book |last1=Gilmour |first1=Iain |last2=Jolley |first2=David |last3=Kemp |first3=David |last4=Kelley |first4=Simon |last5=Gilmour |first5=Mabs |last6=Daly |first6=Rob |last7=Widdowson |first7=Mike |title=Volcanism, Impacts, and Mass Extinctions: Causes and Effects |chapter=The early Danian hyperthermal event at Boltysh (Ukraine): Relation to Cretaceous-Paleogene boundary events |date=2014 |doi=10.1130/2014.2505(06) |isbn=978-0-8137-2505-5 }}</ref> the PETM was the most extreme hyperthermal, and stands out as a major change in the lithologic, biotic and geochemical composition of sediment in hundreds of records across Earth. Other hyperthermals clearly occurred at approximately 53.7 Ma (now called [[Eocene Thermal Maximum 2|ETM-2]] and also referred to as H-1, or the Elmo event) and at about 53.6 Ma (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 Ma (informally called K, X or ETM-3).<ref>{{cite journal |last1=Thomas |first1=Ellen |last2=Boscolo-Galazzo |first2=Flavia |last3=Balestra |first3=Barbara |last4=Monechi |first4=Simonetta |last5=Donner |first5=Barbara |last6=Röhl |first6=Ursula |date=July 2018 |title=Early Eocene Thermal Maximum 3: Biotic Response at Walvis Ridge (SE Atlantic Ocean) |journal=[[Paleoceanography and Paleoclimatology]] |volume=33 |issue=8 |pages=862–883 |doi=10.1029/2018PA003375 |bibcode=2018PaPa...33..862T }}</ref> The number, nomenclature, absolute ages, and relative global impact of the Eocene hyperthermals remain a source of current research. Whether they only occurred during the long-term warming, and whether they are causally related to apparently similar events in older intervals of the geological record (e.g. the [[Toarcian turnover]] of the [[Jurassic]]) are open issues.

==Global warming==
[[File:Global Cenozoic Temperature-en.svg|thumb|upright=1.35|A stacked record of temperatures and ice volume in the deep ocean through the Mesozoic and Cenozoic periods.<br />LPTM— Paleocene-Eocene thermal maximum<br />OAEs— oceanic anoxic events<br />MME— mid-Maastrichtian event]]
A study in 2020 estimated the [[Global surface temperature|global mean surface temperature]] (GMST) with 66% confidence during the latest Paleocene (c. 57 Ma) as {{cvt|22.3-28.3|C|F}}, PETM (56 Ma) as {{cvt|27.2-34.5|C|F}} and [[Early Eocene Climatic Optimum]] (EECO) (53.3 to 49.1 Ma) as {{cvt|23.2-29.7|C|F}}.<ref>{{cite journal |last1=Inglis |first1=Gordon N. |last2=Bragg |first2=Fran |last3=Burls |first3=Natalie J. |last4=Cramwinckel |first4=Marlow Julius |last5=Evans |first5=David |last6=Foster |first6=Gavin L. |last7=Huber |first7=Matthew |last8=Lunt |first8=Daniel J. |last9=Siler |first9=Nicholas |last10=Steinig |first10=Sebastian |last11=Tierney |first11=Jessica E. |last12=Wilkinson |first12=Richard |last13=Anagnostou |first13=Eleni |last14=de Boer |first14=Agatha M. |last15=Dunkley Jones |first15=Tom |last16=Edgar |first16=Kirsty M. |last17=Hollis |first17=Christopher J. |last18=Hutchinson |first18=David K. |last19=Pancost |first19=Richard D. |journal=[[Climate of the Past]] |title=Global mean surface temperature and climate sensitivity of the early Eocene Climatic Optimum (EECO), Paleocene–Eocene Thermal Maximum (PETM), and latest Paleocene |volume=16 |number=5 |year=2020 |pages=1953–1968 |doi=10.5194/cp-16-1953-2020 |bibcode=2020CliPa..16.1953I |doi-access=free |hdl=1983/24a30f12-51a6-4544-9db8-b2009e33c02a |hdl-access=free}}</ref> Estimates of the amount of average global temperature rise at the start of the PETM range from approximately 3 to 6&nbsp;°C<ref name="EvansEtAl2016">{{cite journal |last1=Evans |first1=David |last2=Wade |first2=Bridget S. |last3=Henehan |first3=Michael |last4=Erez |first4=Jonathan |last5=Müller |first5=Wolfgang |date=6 April 2016 |title=Revisiting carbonate chemistry controls on planktic foraminifera Mg / Ca: implications for sea surface temperature and hydrology shifts over the Paleocene–Eocene Thermal Maximum and Eocene–Oligocene transition |journal=[[Climate of the Past]] |volume=12 |issue=4 |pages=819–835 |doi=10.5194/cp-12-819-2016 |bibcode=2016CliPa..12..819E |doi-access=free}}</ref> to between 5 and 8&nbsp;°C.<ref>{{Cite journal |last1=Stuijs |first1=Appy |last2=Brinkhuis |first2=Henk |date=September 2008 |title=Rapid carbon injection and transient global warming during the Paleocene-Eocene thermal maximum |journal=Netherlands Journal of Geosciences - Geologie en Mijnbouw |volume=87 |issue=3 |pages=201–206 |doi=10.1017/S0016774600023271 |bibcode=2008NJGeo..87..201S }}</ref><ref name=McInerney2011 /> This warming was superimposed on "long-term" [[Climate across Cretaceous–Paleogene boundary|early Paleogene warming]], and is based on several lines of evidence. There is a prominent (>1[[Per mil|‰]]) negative excursion in the {{delta|18|O|link}} of foraminifera shells, both those made in surface and deep ocean water. Because there was little or no polar ice in the early Paleogene, the shift in {{delta|18|O}} very probably signifies a rise in ocean temperature.<ref name=Thomas1996>{{Cite journal |last1=Thomas |first1=Ellen |last2=Shackleton |first2=Nicholas J. |title=The Paleocene-Eocene benthic foraminiferal extinction and stable isotope anomalies |year=1996 |journal=Geological Society of London, Special Publications |volume=101 |issue=1 |pages=401–441 |doi=10.1144/GSL.SP.1996.101.01.20 |bibcode=1996GSLSP.101..401T }}</ref> The temperature rise is also supported by the spread of warmth-loving taxa to higher latitudes,<ref>{{cite journal |last1=Speijer |first1=Robert |last2=Scheibner |first2=Christian |last3=Stassen |first3=Peter |last4=Morsi |first4=Abdel-Mohsen M. |date=May 2012 |title=Response of marine ecosystems to deep-time global warming: a synthesis of biotic patterns across the Paleocene-Eocene thermal maximum (PETM) |journal=[[Austrian Journal of Earth Sciences]] |volume=105 |issue=1 |pages=6–16 |id={{CORE output|35118345}} }}</ref> changes in plant leaf shape and size,<ref>{{cite journal |last1=Wing |first1=Scott L. |last2=Harrington |first2=Guy J. |last3=Smith |first3=Francesca A. |last4=Bloch |first4=Jonathan I. |last5=Boyer |first5=Douglas M. |last6=Freeman |first6=Katherine H. |date=11 November 2005 |title=Transient Floral Change and Rapid Global Warming at the Paleocene-Eocene Boundary |journal=[[Science (journal)|Science]] |volume=310 |issue=5750 |pages=993–996 |doi=10.1126/science.1116913 |pmid=16284173 |bibcode=2005Sci...310..993W }}</ref> the Mg/Ca ratios of foraminifera,<ref name="EvansEtAl2016" /> and the ratios of certain [[organic compound]]s, such as [[TEX86|TEX<sup>H</sup><sub>86</sub>]].<ref name="HeatStressedPlankton" /> Additionally, modelling suggests the climate became more equable, with a 5 °C decrease in mean annual temperature range over most continental interiors.<ref>{{cite journal |last1=Huber |first1=Matthew |last2=Sloan |first2=L. Cirbus |title=Warm climate transitions: A general circulation modeling study of the Late Paleocene Thermal Maximum (∼56 Ma) |journal=Journal of Geophysical Research: Atmospheres |date=27 July 1999 |volume=104 |issue=D14 |pages=16633–16655 |doi=10.1029/1999JD900272 |bibcode=1999JGR...10416633H }}</ref>

Proxy data from Esplugafereda in northeastern Spain shows a rapid {{cvt|8|C-change|0}} temperature rise, in accordance with existing regional records of marine and terrestrial environments.<ref name="Adatte2014">{{cite journal |last1=Adatte |first1=Thierry |title=Response of terrestrial environment to the Paleocene-Eocene Thermal Maximum (PETM), new insights from India and NE Spain |journal=Rendiconti online della Società Geologica Italiana |date=July 2014 |volume=31 |pages=5–6 |doi=10.3301/ROL.2014.17 |bibcode=2014ROSGI..31....5A }}</ref> In the Fushun Basin, temperatures rose from {{cvt|15.6 to 19.7|C|F}}.<ref>{{cite journal |last1=Xie |first1=Yulong |last2=Wu |first2=Fuli |last3=Fang |first3=Xiaomin |title=Abrupt collapse of a swamp ecosystem in northeast China during the Paleocene–Eocene Thermal Maximum |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=June 2022 |volume=595 |article-number=110975 |doi=10.1016/j.palaeo.2022.110975 |bibcode=2022PPP...59510975X }}</ref> Southern California had a mean annual temperature of about 17&nbsp;°C ± 4.4&nbsp;°C.<ref>{{Cite journal |last1=Broz |first1=Adrian P. |last2=Pritchard-Peterson |first2=Devin |last3=Spinola |first3=Diogo |last4=Schneider |first4=Sarah |last5=Retallack |first5=Gregory |last6=Silva |first6=Lucas C. R. |date=31 January 2024 |title=Eocene (50–55 Ma) greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA |journal=[[Scientific Reports]] |volume=14 |issue=1 |page=2613 |doi=10.1038/s41598-024-53210-0 |pmid=38297060 |pmc=10830502 |bibcode=2024NatSR..14.2613B }}</ref> In Antarctica, at least part of the year saw minimum temperatures of 15&nbsp;°C.<ref>{{Cite journal |last1=Robert |first1=Christian |last2=Kennett |first2=James P. |date=March 1994 |title=Antarctic subtropical humid episode at the Paleocene-Eocene boundary: Clay-mineral evidence |journal=[[Geology (journal)|Geology]] |volume=22 |issue=3 |page=211 |doi=10.1130/0091-7613(1994)022<0211:ASHEAT>2.3.CO;2 |bibcode=1994Geo....22..211R }}</ref>

TEX<sup>H</sup><sub>86</sub> values indicate that the average [[sea surface temperature]] (SST) reached over {{cvt|36|C|F}} in the tropics during the PETM, enough to cause heat stress even in organisms resistant to extreme thermal stress, such as dinoflagellates, of which a significant number of species went extinct.<ref name="HeatStressedPlankton">{{cite journal |last1=Frieling |first1=Joost |last2=Gebhardt |first2=Holger |last3=Huber |first3=Matthew |last4=Adekeye |first4=Olabisi A. |last5=Akande |first5=Samuel O. |last6=Reichart |first6=Gert-Jan |last7=Middelburg |first7=Jack J. |last8=Schouten |first8=Stefan |last9=Sluijs |first9=Appy |date=3 March 2017 |title=Extreme warmth and heat-stressed plankton in the tropics during the Paleocene-Eocene Thermal Maximum |journal=[[Science Advances]] |volume=3 |issue=3 |article-number=e1600891 |doi=10.1126/sciadv.1600891 |pmid=28275727 |pmc=5336354 |bibcode=2017SciA....3E0891F }}</ref> Oxygen isotope ratios from Tanzania suggest that tropical SSTs may have been even higher, exceeding {{cvt|40|C|F}}.<ref>{{cite journal |last1=Aze |first1=T. |last2=Pearson |first2=P. N. |last3=Dickson |first3=A. J. |last4=Badger |first4=M. P. S. |last5=Bown |first5=P. R. |last6=Pancost |first6=Richard D. |last7=Gibbs |first7=S. J. |last8=Huber |first8=Brian T. |last9=Leng |first9=M. J. |last10=Coe |first10=A. L. |last11=Cohen |first11=A. S. |last12=Foster |first12=G. L. |date=September 2014 |title=Extreme warming of tropical waters during the Paleocene–Eocene Thermal Maximum |journal=[[Geology (journal)|Geology]] |volume=42 |issue=9 |pages=739–742 |doi=10.1130/G35637.1 |bibcode=2014Geo....42..739A |doi-access=free |hdl=1983/eb48805c-800e-4941-953c-dcbe129c5f59 |hdl-access=free }}</ref> Ocean Drilling Program Site 1209 from the tropical western Pacific shows an increase in SST from {{cvt|34|C|F}} before the PETM to ~40&nbsp;°C.<ref>{{Cite journal |last1=Harper |first1=D. T. |last2=Hönisch |first2=B. |last3=Zeebe |first3=R. E. |last4=Shaffer |first4=G. |last5=Haynes |first5=L. L. |last6=Thomas |first6=E. |last7=Zachos |first7=J. C. |date=18 December 2019 |title=The Magnitude of Surface Ocean Acidification and Carbon Release During Eocene Thermal Maximum 2 (ETM-2) and the Paleocene-Eocene Thermal Maximum (PETM) |journal=[[Paleoceanography and Paleoclimatology]] |volume=35 |issue=2 |article-number=e2019PA003699 |doi=10.1029/2019PA003699 |doi-access=free }}</ref> In the eastern Tethys, SSTs rose by {{convert|3 to 5|C-change|0}}.<ref>{{Cite journal |last1=Jin |first1=Simin |last2=Li |first2=Guobiao |last3=Li |first3=Juan |last4=Hu |first4=Xiumian |last5=Yang |first5=Huan |last6=Huang |first6=Chunju |last7=Baoke |first7=Zhantu |last8=Algeo |first8=Thomas J. |last9=Kemp |first9=David B. |date=November 2023 |title=Tropical ocean temperatures and changes in terrigenous flux during the Paleocene-Eocene Thermal Maximum in southern Tibet |journal=[[Global and Planetary Change]] |volume=230 |article-number=104289 |doi=10.1016/j.gloplacha.2023.104289 |bibcode=2023GPC...23004289J }}</ref> Low latitude Indian Ocean Mg/Ca records show seawater at all depths warmed by about {{cvt|4-5|C-change|0}}.<ref>{{cite journal |last1=Barnet |first1=James S. K. |last2=Harper |first2=Dustin T. |last3=LeVay |first3=Leah J. |last4=Edgar |first4=Kirsty M. |last5=Henehan |first5=Michael J. |last6=Babila |first6=Tali L. |last7=Ullmann |first7=Clemens V. |last8=Leng |first8=Melanie J. |last9=Kroon |first9=Dick |last10=Zachos |first10=James C. |last11=Littler |first11=Kate |date=September 2020 |title=Coupled evolution of temperature and carbonate chemistry during the Paleocene–Eocene; new trace element records from the low latitude Indian Ocean |journal=[[Earth and Planetary Science Letters]] |volume=545 |article-number=116414 |doi=10.1016/j.epsl.2020.116414 |doi-access=free |bibcode=2020E&PSL.54516414B |hdl=10023/20365 |hdl-access=free }}</ref> In the Pacific Ocean, tropical SSTs increased by about 4-5&nbsp;°C.<ref>{{cite journal |last1=Zachos |first1=James C. |last2=Wara |first2=Michael W. |last3=Bohaty |first3=Steven |last4=Delaney |first4=Margaret L. |last5=Petrizzo |first5=Maria Rose |last6=Brill |first6=Amanda |last7=Bralower |first7=Timothy J. |last8=Premoli-Silva |first8=Isabella |date=28 November 2003 |title=A Transient Rise in Tropical Sea Surface Temperature During the Paleocene-Eocene Thermal Maximum |journal=[[Science (journal)|Science]] |volume=302 |issue=5650 |pages=1551–1554 |doi=10.1126/science.1090110 |pmid=14576441 |bibcode=2003Sci...302.1551Z |doi-access=free }}</ref> TEX<sup>L</sup><sub>86</sub> values from deposits in New Zealand, then located between [[50th parallel south|50°S]] and [[60th parallel south|60°S]] in the southwestern Pacific,<ref>{{cite journal |last1=Hollis |first1=Christopher J. |last2=Taylor |first2=Kyle W. R. |last3=Handley |first3=Luke |last4=Pancost |first4=Richard D. |last5=Huber |first5=Matthew |last6=Creech |first6=John B. |last7=Hines |first7=Benjamin R. |last8=Crouch |first8=Erica M. |last9=Morgans |first9=Hugh E. G. |last10=Crampton |first10=James S. |last11=Gibbs |first11=Samantha |last12=Pearson |first12=Paul N. |last13=Zachos |first13=James C. |date=15 July 2013 |title=Erratum to "Early Paleogene temperature history of the Southwest Pacific Ocean: Reconciling proxies and models" [Earth Planet. Sci. Lett. 349 (2012) 53–66] |journal=[[Earth and Planetary Science Letters]] |volume=374 |pages=258–259 |doi=10.1016/j.epsl.2013.06.012 |bibcode=2013E&PSL.374..258H |doi-access=free}}</ref> indicate SSTs of {{cvt|26|C}} to {{cvt|28|C}}, an increase of over {{cvt|10|C-change}} from an average of {{cvt|13|C}} to {{cvt|16|C}} at the boundary between the [[Selandian]] and [[Thanetian]].<ref>{{cite journal |last1=Hollis |first1=Christopher J. |last2=Taylor |first2=Kyle W. R. |last3=Handley |first3=Luke |last4=Pancost |first4=Richard D. |last5=Huber |first5=Matthew |last6=Creech |first6=John B. |last7=Hines |first7=Benjamin R. |last8=Crouch |first8=Erica M. |last9=Morgans |first9=Hugh E. G. |last10=Crampton |first10=James S. |last11=Gibbs |first11=Samantha |last12=Pearson |first12=Paul N. |last13=Zachos |first13=James C. |date=October 2012 |title=Early Paleogene temperature history of the Southwest Pacific Ocean: Reconciling proxies and models |journal=[[Earth and Planetary Science Letters]] |volume=349-350 |pages=53–66 |doi=10.1016/j.epsl.2012.06.024 |bibcode=2012E&PSL.349...53H}}</ref> The extreme warmth of the southwestern Pacific extended into the Australo-Antarctic Gulf.<ref>{{Cite journal |last1=Frieling |first1=J. |last2=Bohaty |first2=S. M. |last3=Cramwinckel |first3=M. J. |last4=Gallagher |first4=S. J. |last5=Holdgate |first5=G. R. |last6=Reichgelt |first6=T. |last7=Peterse |first7=F. |last8=Pross |first8=J. |last9=Sluijs |first9=A. |last10=Bijl |first10=P. K. |date=16 February 2023 |title=Revisiting the Geographical Extent of Exceptional Warmth in the Early Paleogene Southern Ocean |journal=[[Paleoceanography and Paleoclimatology]] |volume=38 |issue=3 |article-number=e2022PA004529 |doi=10.1029/2022PA004529 |doi-access=free |bibcode=2023PaPa...38.4529F}}</ref> Sediment core samples from the [[East Tasman Plateau]], then located at a palaeolatitude of ~65 °S, show an increase in SSTs from about {{cvt|26 to 33|C|F}} during the PETM.<ref>{{cite journal |last1=Sluijs |first1=Appy |last2=Bijl |first2=P. K. |last3=Schouten |first3=Stefan |last4=Röhl |first4=Ursula |last5=Reichart |first5=G.-J. |last6=Brinkhuis |first6=H. |date=26 January 2011 |title=Southern ocean warming, sea level and hydrological change during the Paleocene-Eocene thermal maximum |journal=[[Climate of the Past]] |volume=7 |issue=1 |pages=47–61 |doi=10.5194/cp-7-47-2011 |doi-access=free |bibcode=2011CliPa...7...47S}}</ref> In the North Sea, SSTs jumped by {{convert|10|C-change|0}}, reaching highs of ~33&nbsp;°C,<ref name=Stokke2020>{{cite journal |last1=Stokke |first1=Ella W. |last2=Jones |first2=Morgan T. |last3=Tierney |first3=Jessica E. |last4=Svensen |first4=Henrik H. |last5=Whiteside |first5=Jessica H. |date=15 August 2020 |title=Temperature changes across the Paleocene-Eocene Thermal Maximum – a new high-resolution TEX86 temperature record from the Eastern North Sea Basin |journal=[[Earth and Planetary Science Letters]] |volume=544 |article-number=116388 |doi=10.1016/j.epsl.2020.116388 |bibcode=2020E&PSL.54416388S |hdl=10852/81373 |hdl-access=free |doi-access=free}}</ref> while in the West Siberian Sea, SSTs climbed to ~27 °C.<ref>{{Cite journal |last1=Frieling |first1=Joost |last2=Iakovleva |first2=Alina I. |last3=Reichart |first3=Gert-Jan |last4=Aleksandrova |first4=Galina N. |last5=Gnibidenko |first5=Zinaida N. |last6=Schouten |first6=Stefan |last7=Sluijs |first7=Appy |date=September 2014 |title=Paleocene–Eocene warming and biotic response in the epicontinental West Siberian Sea |journal=[[Geology (journal)|Geology]] |volume=42 |issue=9 |pages=767–770 |doi=10.1130/G35724.1 |bibcode=2014Geo....42..767F }}</ref> 

Certainly, the central Arctic Ocean was ice-free before, during, and after the PETM. This can be ascertained from the composition of sediment cores recovered during the Arctic Coring Expedition (ACEX) at 87°N on [[Lomonosov Ridge]].<ref name=Moran2006>{{Cite journal | last1 = Moran | first1 = K. | last2 = Backman | first2 = J. | last3 = Pagani | first3 = others | title = The Cenozoic palaeoenvironment of the Arctic Ocean | year = 2006 | journal = [[Nature (journal)|Nature]] | volume = 441 | issue = 7093 |pages = 601–605 | doi=10.1038/nature04800| pmid = 16738653 | bibcode = 2006Natur.441..601M| hdl = 11250/174276 | hdl-access = free}}</ref> Moreover, temperatures increased during the PETM, as indicated by the brief presence of subtropical [[dinoflagellate]]s (''Apectodinium spp.''}, and a marked increase in TEX<sub>86</sub>.<ref name=Sluijs2006>{{cite journal |last1=Sluijs |first1=Appy |last2=Schouten |first2=Stefan |last3=Pagani |first3=Mark |last4=Woltering |first4=Martijn |last5=Brinkhuis |first5=Henk |last6=Damsté |first6=Jaap S. Sinninghe |last7=Dickens |first7=Gerald R. |last8=Huber |first8=Matthew |last9=Reichart |first9=Gert-Jan |last10=Stein |first10=Ruediger |last11=Matthiessen |first11=Jens |last12=Lourens |first12=Lucas J. |last13=Pedentchouk |first13=Nikolai |last14=Backman |first14=Jan |last15=Moran |first15=Kathryn |title=Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum |journal=[[Nature (journal)|Nature]] |date=2006 |volume=441 |issue=7093 |pages=610–613 |pmid=16752441 |doi=10.1038/nature04668 |bibcode=2006Natur.441..610S |hdl=11250/174280 |hdl-access=free }}</ref> The latter record is unusual, though, because it suggests a {{convert|6|C-change|0}} rise from ~{{Convert|17|C|F|0}} before the PETM to ~{{Convert|23|C|F|0}} during the PETM. Assuming the TEX<sub>86</sub> record reflects summer temperatures, it still implies much warmer temperatures on the North Pole compared to the present day, but no significant latitudinal amplification relative to surrounding time.

The above considerations are important because, in many global warming simulations, high latitude temperatures increase much more at the poles through an [[ice–albedo feedback]].<ref name=Shellito2003>{{Cite journal | last1 = Shellito | first1 = Cindy J. | last2 = Sloan | first2 = Lisa C. | last3 = Huber | first3 = Matthew | title = Climate model sensitivity to atmospheric CO<sub>2</sub> levels in the Early-Middle Paleogene | year = 2003 | journal = [[Palaeogeography, Palaeoclimatology, Palaeoecology]] | volume = 193 | issue = 1 | pages = 113–123 | doi = 10.1016/S0031-0182(02)00718-6| bibcode = 2003PPP...193..113S}}</ref> It may be the case, however, that during the PETM, this feedback was largely absent because of limited polar ice, so temperatures on the Equator and at the poles increased similarly. Notable is the absence of documented greater warming in polar regions compared to other regions. This implies a non-existing ice-albedo feedback, suggesting no sea or land ice was present in the late Paleocene.<ref name=Bowen2015 />

Precise limits on the global temperature rise during the PETM and whether this varied significantly with latitude remain open issues. Oxygen isotope and Mg/Ca of carbonate shells precipitated in surface waters of the ocean are commonly used measurements for reconstructing past temperature; however, both paleotemperature proxies can be compromised at low latitude locations, because re-crystallization of carbonate on the seafloor renders lower values than when formed. On the other hand, these and other temperature proxies (e.g., TEX<sub>86</sub>) are impacted at high latitudes because of seasonality; that is, the "temperature recorder" is biased toward summer, and therefore higher values, when the production of carbonate and organic carbon occurred.

==Carbon cycle disturbance==
Clear evidence for massive addition of <sup>13</sup>C-depleted carbon at the onset of the PETM comes from two observations. First, a prominent negative excursion in the carbon isotope composition ({{delta|13|C}}) of carbon-bearing phases characterizes the PETM in numerous (>130) widespread locations from a range of environments.<ref name="Koch1992" /><ref>{{cite journal |last1=Li |first1=Juan |last2=Hu |first2=Xiumian |last3=Garzanti |first3=Eduardo |last4=BouDagher-Fadel |first4=Marcelle |title=Shallow-water carbonate responses to the Paleocene–Eocene thermal maximum in the Tethyan Himalaya (southern Tibet): Tectonic and climatic implications |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=January 2017 |volume=466 |pages=153–165 |doi=10.1016/j.palaeo.2016.11.026 |bibcode=2017PPP...466..153L |url=https://discovery.ucl.ac.uk/id/eprint/1529409/ }}</ref> Second, carbonate dissolution marks the PETM in sections from the deep sea.<ref name=McInerney2011/>

The total mass of carbon injected to the ocean and atmosphere during the PETM remains the source of debate. In theory, it can be estimated from the magnitude of the negative carbon isotope excursion (CIE), the amount of carbonate dissolution on the seafloor, or ideally both.<ref name=Dickens1/><ref name=Zeebe2009 /> However, the shift in the {{delta|13|C}} across the PETM depends on the location and the carbon-bearing phase analyzed. In some records of bulk carbonate, it is about 2‰ (per mil); in some records of terrestrial carbonate or organic matter it exceeds 6‰.<ref>{{cite journal |last1=Zhang |first1=Qinghai |last2=Ding |first2=Lin |last3=Kitajima |first3=Kouki |last4=Valley |first4=John W. |last5=Zhang |first5=Bo |last6=Xu |first6=Xiaoxia |last7=Willems |first7=Helmut |last8=Klügel |first8=Andreas |title=Constraining the magnitude of the carbon isotope excursion during the Paleocene-Eocene thermal maximum using larger benthic foraminifera |journal=Global and Planetary Change |date=January 2020 |volume=184 |article-number=103049 |doi=10.1016/j.gloplacha.2019.103049 |bibcode=2020GPC...18403049Z }}</ref><ref>{{cite journal |last1=Zhang |first1=Qinghai |last2=Wendler |first2=Ines |last3=Xu |first3=Xiaoxia |last4=Willems |first4=Helmut |last5=Ding |first5=Lin |title=Structure and magnitude of the carbon isotope excursion during the Paleocene-Eocene thermal maximum |journal=Gondwana Research |date=June 2017 |volume=46 |pages=114–123 |doi=10.1016/j.gr.2017.02.016 |bibcode=2017GondR..46..114Z }}</ref><ref name=Norris1999>{{cite journal |last1=Norris |first1=Richard D |last2=Röhl |first2=Ursula |title=Carbon cycling and chronology of climate warming during the Palaeocene/Eocene transition |journal=[[Nature (journal)|Nature]] |date=1999 |volume=401 |issue=6755 |pages=775–778 |doi=10.1038/44545 |bibcode=1999Natur.401..775N }}</ref> Carbonate dissolution also varies throughout different ocean basins. It was extreme in parts of the north and central Atlantic Ocean, but far less pronounced in the Pacific Ocean. With available information, estimates of the carbon addition range from about 2,000 to 7,000 gigatons.<ref name=Zeebe2009/><ref name="Panchuk2008">{{cite journal |last1=Panchuk |first1=K. |last2=Ridgwell |first2=A. |last3=Kump |first3=L.R. |title=Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison |journal=[[Geology (journal)|Geology]] |date=2008 |volume=36 |issue=4 |pages=315–318 |bibcode=2008Geo....36..315P |doi=10.1130/G24474A.1 }}</ref><ref name="Cui2011">{{cite journal |last1=Cui |first1=Ying |last2=Kump |first2=Lee R. |last3=Ridgwell |first3=Andy J. |last4=Charles |first4=Adam J. |last5=Junium |first5=Christopher K. |last6=Diefendorf |first6=Aaron F. |last7=Freeman |first7=Katherine H. |last8=Urban |first8=Nathan M. |last9=Harding |first9=Ian C. |title=Slow release of fossil carbon during the Palaeocene-Eocene thermal maximum |journal=[[Nature Geoscience]] |date=2011 |volume=4 |issue=7 |pages=481–485 |bibcode=2011NatGe...4..481C |doi=10.1038/ngeo1179 }}</ref>

==Timing of carbon addition and warming==
The timing of the PETM {{delta|13|C}} excursion is of considerable interest. This is because the total duration of the CIE, from the rapid drop in {{delta|13|C}} through the near recovery to initial conditions, relates to key parameters of our global carbon cycle, and because the onset provides insight to the source of [[Carbon-13|<sup>13</sup>C]]-depleted {{CO2}}.

The total duration of the CIE can be estimated in several ways. The iconic sediment interval for examining and dating the PETM is a core recovered in 1987 by the [[Ocean Drilling Program]] at Hole 690B at [[Maud Rise]] in the South Atlantic Ocean. At this location, the PETM CIE, from start to end, spans about {{cvt|2|m|ft}}. Long-term age constraints, through [[biostratigraphy]] and [[magnetostratigraphy]], suggest an average Paleogene sedimentation rate of about {{cvt|1.23|cm|in}} per 1,000 years. Assuming a constant sedimentation rate, the entire event, from onset though termination, was therefore estimated at 200,000 years.<ref name=Kennett1991/> Subsequently, it was noted that the CIE spanned 10 or 11 subtle cycles in various sediment properties, such as Fe content. Assuming these cycles represent [[Axial precession|precession]], a similar but slightly longer age was calculated by Rohl et al. 2000. If a massive amount of <sup>13</sup>C-depleted {{CO2}} is rapidly injected into the modern ocean or atmosphere and projected into the future, a ~200,000 year CIE results because of slow flushing through quasi steady-state inputs (weathering and volcanism) and outputs (carbonate and organic) of carbon.<ref name=Rohl2000>{{cite journal |last1=Röhl |first1=U. |last2=Bralower |first2=T.J. |last3=Norris |first3=R.D. |last4=Wefer |first4=G. |title=New chronology for the late Paleocene thermal maximum and its environmental implications |year=2000 |journal=[[Geology (journal)|Geology]] |volume=28 |issue=10 |pages=927–930 |doi=10.1130/0091-7613(2000)28<927:NCFTLP>2.0.CO;2 |bibcode=2000Geo....28..927R }}</ref> A different study, based on a revised orbital chronology and data from sediment cores in the South Atlantic and the Southern Ocean, calculated a slightly shorter duration of about 170,000 years.<ref>{{cite journal |last1=Röhl |first1=Ursula |last2=Westerhold |first2=Thomas |last3=Bralower |first3=Timothy J. |last4=Zachos |first4=James C. |date=11 December 2007 |title=On the duration of the Paleocene-Eocene thermal maximum (PETM) |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=8 |issue=12 |pages=1–13 |article-number=2007GC001784 |doi=10.1029/2007GC001784 |bibcode=2007GGG.....812002R }}</ref>

A ~200,000 year duration for the CIE is estimated from models of global carbon cycling.<ref name=Dickens2000>{{cite journal |last1=Dickens |first1=Gerald R. |title=Methane oxidation during the late Palaeocene thermal maximum |journal=Bulletin de la Société Géologique de France |date=2000 |volume=171 |issue=1 |pages=37–49 |url=https://pubs.geoscienceworld.org/sgf/bsgf/article-abstract/171/1/37/88209/Methane-oxidation-during-the-late-Palaeocene }}</ref>

Age constraints at several deep-sea sites have been independently examined using <sup>3</sup>He contents, assuming the flux of this cosmogenic nuclide is roughly constant over short time periods. This approach also suggests a rapid onset for the PETM CIE (<20,000 years). However, the <sup>3</sup>He records support a faster recovery to near initial conditions (<100,000 years) than predicted by flushing via weathering inputs and carbonate and organic outputs.<ref name=Farley2003>{{cite journal |last1=Farley |first1=K.A. |last2=Eltgroth |first2=S.F. |title=An alternative age model for the Paleocene—Eocene thermal maximum using extraterrestrial <sup>3</sup>He |year=2003 |journal=[[Earth and Planetary Science Letters]] |volume=208 |issue=3–4 |pages=135–148 |doi=10.1016/S0012-821X(03)00017-7 |bibcode=2003E&PSL.208..135F }}</ref>

There is other evidence to suggest that warming predated the {{delta|13|C}} excursion by some 3,000&nbsp;years.<ref name=Sluijs2007>{{cite journal |last1=Sluijs |first1=Appy |last2=Brinkhuis |first2=Henk |last3=Schouten |first3=Stefan |last4=Bohaty |first4=Steven M. |last5=John |first5=Cédric M. |last6=Zachos |first6=James C. |last7=Reichart |first7=Gert-Jan |last8=Sinninghe Damsté |first8=Jaap S. |last9=Crouch |first9=Erica M. |last10=Dickens |first10=Gerald R. |title=Environmental precursors to rapid light carbon injection at the Palaeocene/Eocene boundary |year=2007 |journal=[[Nature (journal)|Nature]] |volume=450 |issue=7173 |pages=1218–1221 |doi=10.1038/nature06400 |pmid=18097406 |bibcode=2007Natur.450.1218S |hdl=1874/31621 |hdl-access=free }}</ref>

Some authors have suggested that the magnitude of the CIE may be underestimated due to local processes in many sites causing a large proportion of allochthonous sediments to accumulate in their sedimentary rocks, contaminating and offsetting isotopic values derived from them.<ref>{{cite journal |last1=Baczynski |first1=Allison A. |last2=McInerney |first2=Francesca A. |last3=Wing |first3=Scott L. |last4=Kraus |first4=Mary J. |last5=Bloch |first5=Jonathan I. |last6=Boyer |first6=Doug M. |last7=Secord |first7=Ross |last8=Morse |first8=Paul E. |last9=Fricke |first9=Henry J. |date=6 September 2013 |title=Chemostratigraphic implications of spatial variation in the Paleocene-Eocene Thermal Maximum carbon isotope excursion, SE Bighorn Basin, Wyoming |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=14 |issue=10 |pages=4133–4152 |doi=10.1002/ggge.20265 |bibcode=2013GGG....14.4133B }}</ref> Organic matter degradation by microbes has also been implicated as a source of skewing of carbon isotopic ratios in bulk organic matter.<ref>{{cite journal |last1=Baczynski |first1=Allison A. |last2=McInerney |first2=Francesca A. |last3=Wing |first3=Scott L. |last4=Kraus |first4=Mary J. |last5=Morse |first5=Paul E. |last6=Bloch |first6=Jonathan I. |last7=Chung |first7=Angela H. |last8=Freeman |first8=Katherine H. |date=September 2016 |title=Distortion of carbon isotope excursion in bulk soil organic matter during the Paleocene-Eocene thermal maximum |journal=[[Geological Society of America Bulletin]] |volume=128 |issue=9–10 |pages=1352–1366 |doi=10.1130/B31389.1 |bibcode=2016GSAB..128.1352B }}</ref>

==Effects==
===Precipitation===
[[File:Azolla caroliniana0.jpg|thumb|right|''[[Azolla]]'' floating ferns, fossils of this genus indicate [[subtropic]]al weather at the North Pole]]
The climate became much wetter, with the increase in evaporation rates peaking in the tropics. [[Deuterium]] isotopes reveal a considerable increase in the amount of moisture transported to the Arctic.<ref name=Pagani2006>{{cite journal |last1=Pagani |first1=Mark |last2=Pedentchouk |first2=Nikolai |last3=Huber |first3=Matthew |last4=Sluijs |first4=Appy |last5=Schouten |first5=Stefan |last6=Brinkhuis |first6=Henk |last7=Sinninghe Damsté |first7=Jaap S. |last8=Dickens |first8=Gerald R. |year=2006 |title=Arctic hydrology during global warming at the Palaeocene/Eocene thermal maximum |journal=[[Nature (journal)|Nature]] |volume=442 |issue=7103 |pages=671–675 |doi=10.1038/nature05043 |pmid=16906647 |bibcode=2006Natur.442..671P |hdl=1874/22388 |hdl-access=free}}</ref> Warm weather would have predominated as far north as the Polar basin. Finds of fossils of ''[[Azolla]]'' floating ferns in polar regions indicate [[subtropic]] temperatures at the poles.<ref name=Speelman2009>{{cite journal |last1=Speelman |first1=E. N. |last2=van Kempen |first2=M. M. L. |last3=Barke |first3=J. |last4=Brinkhuis |first4=H. |last5=Reichart |first5=G. J. |last6=Smolders |first6=A. J. P. |last7=Roelofs |first7=J. G. M. |last8=Sangeorgi |first8=F. |last9=de Leeuw |first9=J. W. |last10=Lotter |first10=A. F. |last11=Sinninghe Damest |first11=J. S. |title=The Eocene Arctic ''Azolla'' bloom: environmental conditions, productivity and carbon drawdown |journal=[[Geobiology (journal)|Geobiology]] |date=March 2009 |volume=7 |issue=2 |pages=155–170 |doi=10.1111/j.1472-4669.2009.00195.x |pmid=19323694 |bibcode=2009Gbio....7..155S }}</ref> East Asia became wetter during the PETM.<ref>{{cite journal |last1=Chen |first1=Zuoling |last2=Dong |first2=Xinxin |last3=Wang |first3=Xu |last4=Tang |first4=Zihua |last5=Yang |first5=Shiling |last6=Zhu |first6=Min |last7=Ding |first7=Zhongli |title=Spatial change of precipitation in response to the Paleocene-Eocene thermal Maximum warming in China |journal=Global and Planetary Change |date=November 2020 |volume=194 |article-number=103313 |doi=10.1016/j.gloplacha.2020.103313 |bibcode=2020GPC...19403313C }}</ref> Central China during the PETM hosted dense subtropical forests as a result of the significant increase in rates of precipitation in the region, with average temperatures between {{cvt|21 and 24|C|F}} and mean annual precipitation ranging from {{cvt|1396 to 1997|mm|ft}}.<ref>{{cite journal |last1=Xie |first1=Yulong |last2=Wu |first2=Fuli |last3=Fang |first3=Xiaomin |date=January 2022 |title=A transient south subtropical forest ecosystem in central China driven by rapid global warming during the Paleocene-Eocene Thermal Maximum |journal=[[Gondwana Research]] |volume=101 |pages=192–202 |doi=10.1016/j.gr.2021.08.005 |bibcode=2022GondR.101..192X}}</ref> In the Jiangshan Basin, hot and humid conditions prevailed, contributing to enhanced sedimentation.<ref>{{cite journal |last1=Fan |first1=Xiaojie |last2=Wang |first2=Chunlian |last3=Teng |first3=Xiaohua |last4=Kemp |first4=David B. |last5=Lu |first5=Yangbo |last6=Yan |first6=Kai |last7=Wei |first7=Wei |last8=Zhang |first8=Jingyu |title=Mechanisms of terrestrial organic matter accumulation across the Paleocene-Eocene thermal maximum in the Jianghan Basin, Central China |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=August 2024 |volume=648 |article-number=112283 |doi=10.1016/j.palaeo.2024.112283 |bibcode=2024PPP...64812283F |hdl=1874/452885 |hdl-access=free }}</ref> Similarly, Central Asia became wetter as proto-monsoonal rainfall penetrated farther inland.<ref>{{Cite journal |last1=Meijer |first1=Niels |last2=Licht |first2=Alexis |last3=Woutersen |first3=Amber |last4=Hoorn |first4=Carina |last5=Robin-Champigneul |first5=Faez |last6=Rohrmann |first6=Alexander |last7=Tagliavento |first7=Mattia |last8=Brugger |first8=Julia |last9=Kelemen |first9=Fanni D. |last10=Schauer |first10=Andrew J. |last11=Hren |first11=Micheal T. |last12=Sun |first12=Aijun |last13=Fiebig |first13=Jens |last14=Mulch |first14=Andreas |last15=Dupont-Nivet |first15=Guillaume |date=29 January 2024 |title=Proto-monsoon rainfall and greening in Central Asia due to extreme early Eocene warmth |journal=[[Nature Geoscience]] |volume=17 |issue=2 |pages=158–164 |doi=10.1038/s41561-023-01371-4 |bibcode=2024NatGe..17..158M |url=https://hal.science/hal-04426859 }}</ref> Very high precipitation is also evidenced in the Cambay Shale Formation of India by the deposition of thick lignitic seams as a consequence of increased soil erosion and organic matter burial.<ref>{{cite journal |last1=Samanta |first1=A. |last2=Bera |first2=M. K. |last3=Ghosh |first3=Ruby |last4=Bera |first4=Subir |last5=Filley |first5=Timothy |last6=Prade |first6=Kanchan |last7=Rathore |first7=S. S. |last8=Rai |first8=Jyotsana |last9=Sarkar |first9=A. |date=October 2013 |title=Do the large carbon isotopic excursions in terrestrial organic matter across Paleocene–Eocene boundary in India indicate intensification of tropical precipitation? |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=387 |pages=91–103 |doi=10.1016/j.palaeo.2013.07.008 |bibcode=2013PPP...387...91S}}</ref> In the Arctic, precipitation rates increased, causing an increase in sedimentation.<ref>{{Cite journal |last1=Reinhardt |first1=Lutz |last2=von Gosen |first2=Werner |last3=Lückge |first3=Andreas |last4=Blumenberg |first4=Martin |last5=Galloway |first5=Jennifer M. |last6=West |first6=Christopher K. |last7=Sudermann |first7=Markus |last8=Dolezych |first8=Martina |date=February 2022 |title=Geochemical indications for the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM-2) hyperthermals in terrestrial sediments of the Canadian Arctic |journal=[[Geosphere (journal)|Geosphere]] |volume=18 |issue=1 |pages=327–349 |doi=10.1130/GES02398.1 |bibcode=2022Geosp..18..327R |doi-access=free }}</ref> Precipitation rates in the North Sea likewise soared during the PETM.<ref>{{cite journal |last1=Walters |first1=Gregory L. |last2=Kemp |first2=Simon J. |last3=Hemingway |first3=Jordon D. |last4=Johnston |first4=David T. |last5=Hodell |first5=David A. |date=22 December 2022 |title=Clay hydroxyl isotopes show an enhanced hydrologic cycle during the Paleocene-Eocene Thermal Maximum |journal=[[Nature Communications]] |volume=13 |issue=1 |page=7885 |doi=10.1038/s41467-022-35545-2 |pmid=36550174 |pmc=9780225 |bibcode=2022NatCo..13.7885W |doi-access=free}}</ref> In Cap d'Ailly, in present-day [[Normandy]], a transient dry spell occurred just before the negative CIE, after which much moister conditions predominated, with the local environment transitioning from a closed marsh to an open, eutrophic swamp with frequent algal blooms.<ref>{{cite journal |last1=Garel |first1=Sylvain |last2=Schnyder |first2=Johann |last3=Jacob |first3=Jérémy |last4=Dupuis |first4=Christian |last5=Boussafir |first5=Mohammed |last6=Le Milbeau |first6=Claude |last7=Storme |first7=Jean-Yves |last8=Iakovleva |first8=Alina I. |last9=Yans |first9=Johan |last10=Baudin |first10=François |last11=Fléhoc |first11=Christine |last12=Quesnel |first12=Florence |date=15 April 2013 |title=Paleohydrological and paleoenvironmental changes recorded in terrestrial sediments of the Paleocene–Eocene boundary (Normandy, France) |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=376 |pages=184–199 |doi=10.1016/j.palaeo.2013.02.035 |bibcode=2013PPP...376..184G |url=https://insu.hal.science/insu-00801796 }}</ref> Precipitation patterns became highly unstable along the [[New Jersey Shelf]],<ref>{{Cite journal |last1=Sluijs |first1=A. |last2=Brinkhuis |first2=H. |date=25 August 2009 |title=A dynamic climate and ecosystem state during the Paleocene-Eocene Thermal Maximum: inferences from dinoflagellate cyst assemblages on the New Jersey Shelf |journal=[[Biogeosciences]] |volume=6 |issue=8 |pages=1755–1781 |doi=10.5194/bg-6-1755-2009 |doi-access=free |bibcode=2009BGeo....6.1755S}}</ref> while the Gulf Coast in eastern Texas shows evidence of enhanced precipitation.<ref>{{cite journal |last1=Sharman |first1=Glenn R. |last2=Covault |first2=Jacob A. |last3=Flaig |first3=Peter P. |last4=Dunn |first4=Regan |last5=Fussee-Durham |first5=Preston |last6=Larson |first6=Toti E. |last7=Shanahan |first7=Timothy M. |last8=Dubois |first8=Kalli |last9=Shaw |first9=John B. |last10=Crowley |first10=James L. |last11=Shaulis |first11=Barry |title=Coastal response to global warming during the Paleocene-Eocene Thermal Maximum |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=September 2023 |volume=625 |article-number=111664 |doi=10.1016/j.palaeo.2023.111664 |bibcode=2023PPP...62511664S }}</ref> In the Rocky Mountain Interior, precipitation locally declined, however,<ref>{{cite journal |last1=Beard |first1=K. Christopher |date=11 March 2008 |title=The oldest North American primate and mammalian biogeography during the Paleocene–Eocene Thermal Maximum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=105 |issue=10 |pages=3815–3818 |doi=10.1073/pnas.0710180105 |pmid=18316721 |pmc=2268774 |bibcode=2008PNAS..105.3815B |doi-access=free}}</ref> as the interior of North America became more seasonally arid.<ref>{{cite journal |last1=Baczynski |first1=Allison A. |last2=McInerney |first2=Francesca A. |last3=Wing |first3=Scott L. |last4=Kraus |first4=Mary J. |last5=Bloch |first5=Jonathan I. |last6=Secord |first6=Ross |date=January 2017 |title=Constraining paleohydrologic change during the Paleocene-Eocene Thermal Maximum in the continental interior of North America |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=465 |pages=237–246 |doi=10.1016/j.palaeo.2016.10.030 |bibcode=2017PPP...465..237B}}</ref> Along the central California coast, conditions also became drier overall, although precipitation did increase in the summer months.<ref>{{Cite journal |last1=Zhang |first1=Xiaodong |last2=Tipple |first2=Brett J. |last3=Zhu |first3=Jiang |last4=Rush |first4=William D. |last5=Shields |first5=Christian A. |last6=Novak |first6=Joseph B. |last7=Zachos |first7=James C. |date=25 July 2024 |title=Response of coastal California hydroclimate to the Paleocene–Eocene Thermal Maximum |journal=[[Climate of the Past]] |volume=20 |issue=7 |pages=1615–1626 |doi=10.5194/cp-20-1615-2024 |bibcode=2024CliPa..20.1615Z |doi-access=free }}</ref> The drying of western North America is explained by the northward shift of low-level jets and atmospheric rivers.<ref>{{Cite journal |last1=Shields |first1=Christine A. |last2=Kiehl |first2=Jeffrey T. |last3=Rush |first3=William |last4=Rothstein |first4=Mathew |last5=Snyder |first5=Mark A. |date=April 2021 |title=Atmospheric rivers in high-resolution simulations of the Paleocene Eocene Thermal Maximum (PETM) |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=567 |article-number=110293 |doi=10.1016/j.palaeo.2021.110293 |bibcode=2021PPP...56710293S }}</ref> East African sites display evidence of aridity punctuated by seasonal episodes of potent precipitation, revealing the global climate during the PETM not to be universally humid.<ref>{{cite journal |last1=Handley |first1=Luke |last2=O'Halloran |first2=Aiofe |last3=Pearson |first3=Paul N. |last4=Hawkins |first4=Elizabeth |last5=Nicholas |first5=Christopher J. |last6=Schouten |first6=Stefan |last7=McMillan |first7=Ian K. |last8=Pancost |first8=Richard D. |date=15 April 2012 |title=Changes in the hydrological cycle in tropical East Africa during the Paleocene–Eocene Thermal Maximum |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=329-330 |pages=10–21 |doi=10.1016/j.palaeo.2012.02.002 |bibcode=2012PPP...329...10H}}</ref> The proto-Mediterranean coastlines of the western Tethys became drier.<ref>{{Cite journal |last1=Piedrahita |first1=Victor A. |last2=Roberts |first2=Andrew P. |last3=Rohling |first3=Eelco J. |last4=Heslop |first4=David |last5=Zhao |first5=Xiang |last6=Galeotti |first6=Simone |last7=Florindo |first7=Fabio |last8=Grant |first8=Katharine M. |last9=Hu |first9=Pengxiang |last10=Li |first10=Jinhua |date=15 August 2024 |title=Dry hydroclimates in the late Palaeocene-early Eocene hothouse world |journal=[[Nature Communications]] |volume=15 |issue=1 |page=7042 |doi=10.1038/s41467-024-51430-6 |pmid=39147773 |pmc=11327323 |bibcode=2024NatCo..15.7042P |doi-access=free}}</ref> Evidence from Forada in northeastern [[Italy]] suggests that arid and humid climatic intervals alternated over the course of the PETM concomitantly with precessional cycles in mid-latitudes, and that overall, net precipitation over the central-western [[Tethys Ocean]] decreased.<ref>{{cite journal |last1=Giusberti |first1=L. |last2=Boscolo Galazzo |first2=F. |last3=Thomas |first3=E. |date=9 February 2016 |title=Variability in climate and productivity during the Paleocene–Eocene Thermal Maximum in the western Tethys (Forada section) |journal=[[Climate of the Past]] |volume=12 |issue=2 |pages=213–240 |doi=10.5194/cp-12-213-2016 |doi-access=free |bibcode=2016CliPa..12..213G |hdl=11577/3182470 |hdl-access=free}}</ref>

===Ocean===
The amount of [[freshwater]] in the Arctic Ocean increased, in part due to [[Northern Hemisphere]] rainfall patterns, fueled by poleward storm track migrations under global warming conditions.<ref name=Pagani2006/> The flux of freshwater entering the oceans increased drastically during the PETM, and continued for a time after the PETM's termination.<ref>{{cite journal |last1=Bornemann |first1=André  |last2=Norris |first2=Richard D. |last3=Lyman |first3=Johnnie A. |last4=D'haenens |first4=Simon |last5=Groeneveld |first5=Jeroen |last6=Röhl |first6=Ursula |last7=Farley |first7=Kenneth A. |last8=Speijer |first8=Robert P. |date=15 May 2014 |title=Persistent environmental change after the Paleocene–Eocene Thermal Maximum in the eastern North Atlantic |journal=[[Earth and Planetary Science Letters]] |volume=394 |pages=70–81 |doi=10.1016/j.epsl.2014.03.017 |bibcode=2014E&PSL.394...70B}}</ref>

====Anoxia====
{{Main|Anoxic event}}
The PETM generated the only [[oceanic anoxic event]] (OAE) of the Cenozoic.<ref>{{cite journal |last1=Singh |first1=Bhart |last2=Singh |first2=Seema |last3=Bhan |first3=Uday |date=3 February 2022 |title=Oceanic anoxic events in the Earth's geological history and signature of such event in the Paleocene-Eocene Himalayan foreland basin sediment records of NW Himalaya, India |journal=Arabian Journal of Geosciences |volume=15 |issue=3 |at=317 |doi=10.1007/s12517-021-09180-y |bibcode=2022ArJG...15..317S }}</ref> Oxygen depletion was achieved through a combination of elevated seawater temperatures, water column stratification, and oxidation of methane released from undersea clathrates.<ref>{{Cite journal |last1=Nicolo |first1=Micah J. |last2=Dickens |first2=Gerald R. |last3=Hollis |first3=Christopher J. |date=4 November 2010 |title=South Pacific intermediate water oxygen depletion at the onset of the Paleocene-Eocene thermal maximum as depicted in New Zealand margin sections |journal=[[Paleoceanography and Paleoclimatology]] |volume=25 |issue=4 |doi=10.1029/2009PA001904 |bibcode=2010PalOc..25.4210N |doi-access=free }}</ref> Denitrification was enhanced.<ref>{{cite journal |last1=Dong |first1=Yixin |last2=Gachetti |first2=Anthony |last3=Wu |first3=Qingting |last4=De Palma |first4=Maurizia |last5=Hu |first5=Xiumian |last6=Brachfeld |first6=Stefanie |last7=Yang |first7=Zhilin |last8=Wang |first8=Jiuyuan |last9=Wang |first9=Yasu |last10=Jiang |first10=Shijun |last11=Cui |first11=Ying |title=Paleoenvironment reconstruction of the eastern Tethys during the pre-onset excursion preceding the PETM |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=August 2024 |volume=647 |article-number=112234 |doi=10.1016/j.palaeo.2024.112234 |bibcode=2024PPP...64712234D  }}</ref> In parts of the oceans, especially the North Atlantic Ocean, [[bioturbation]] was absent. This may be due to bottom-water anoxia or due to changing ocean circulation patterns changing the temperatures of the bottom water.<ref name=Panchuk2008/> However, many ocean basins remained bioturbated through the PETM.<ref name=Zachos2005/> Iodine to calcium ratios suggest oxygen minimum zones in the oceans expanded vertically and possibly also laterally.<ref>{{cite journal |last1=Zhou |first1=X. |last2=Thomas |first2=E. |last3=Rickaby |first3=R. E. M. |last4=Winguth |first4=A. M. E. |last5=Lu |first5=Z. |author3-link=Rosalind Rickaby |date=2014 |title=I/Ca evidence for global upper ocean deoxygenation during the Paleocene-Eocene Thermal Maximum (PETM) |journal=[[Paleoceanography and Paleoclimatology]] |volume=29 |issue=10 |pages=964–975 |doi=10.1002/2014PA002702 |bibcode=2014PalOc..29..964Z |doi-access=free}}</ref> Water column anoxia and euxinia was most prevalent in restricted oceanic basins, such as the Arctic and Tethys Oceans.<ref>{{cite journal |last1=Carmichael |first1=Matthew J. |last2=Inglis |first2=Gordon N. |last3=Badger |first3=Marcus P. S. |last4=Naafs |first4=B. David A. |last5=Behrooz |first5=Leila |last6=Remmelzwaal |first6=Serginio |last7=Monteiro |first7=Fanny M. |last8=Rohrssen |first8=Megan |last9=Farnsworth |first9=Alexander |last10=Buss |first10=Heather L. |last11=Dickson |first11=Alexander J. |last12=Valdes |first12=Paul J. |last13=Lunt |first13=Daniel J. |last14=Pancost |first14=Richard D. |date=October 2017 |title=Hydrological and associated biogeochemical consequences of rapid global warming during the Paleocene-Eocene Thermal Maximum |journal=[[Global and Planetary Change]] |volume=157 |pages=114–138 |doi=10.1016/j.gloplacha.2017.07.014 |bibcode=2017GPC...157..114C |doi-access=free |hdl=1983/e0d75bfc-35b5-4fbe-886b-10e04049f9e3 |hdl-access=free}}</ref> Euxinia struck the epicontinental [[North Sea Basin]] as well,<ref name=":0">{{cite journal |last1=Schoon |first1=Petra L. |last2=Heilmann-Clausen |first2=Claus |last3=Schultz |first3=Bo Pagh |last4=Sinninghe Damsté |first4=Jaap S. |last5=Schouten |first5=Stefan |date=January 2015 |title=Warming and environmental changes in the eastern North Sea Basin during the Palaeocene–Eocene Thermal Maximum as revealed by biomarker lipids |journal=[[Organic Geochemistry]] |volume=78 |pages=79–88 |doi=10.1016/j.orggeochem.2014.11.003 |bibcode=2015OrGeo..78...79S |url=http://www.vliz.be/imisdocs/publications/82/266782.pdf }}</ref> as shown by increases in sedimentary [[uranium]], [[molybdenum]], [[Sulfur|sulphur]], and [[pyrite]] concentrations,<ref>{{Cite journal |last1=Stokke |first1=Ella W. |last2=Jones |first2=Morgan T. |last3=Riber |first3=Lars |last4=Haflidason |first4=Haflidi |last5=Midtkandal |first5=Ivar |last6=Schultz |first6=Bo Pagh |last7=Svensen |first7=Henrik H. |date=2021-10-01 |title=Rapid and sustained environmental responses to global warming: the Paleocene–Eocene Thermal Maximum in the eastern North Sea |journal=Climate of the Past |volume=17 |issue=5 |pages=1989–2013 |doi=10.5194/cp-17-1989-2021 |doi-access=free |bibcode=2021CliPa..17.1989S |hdl=10852/92695 |hdl-access=free}}</ref> along with the presence of sulphur-bound isorenieratane.<ref name=":0" /> The [[Gulf Coastal Plain]] was also affected by euxinia.<ref>{{cite journal |last1=Sluijs |first1=Appy |last2=Van Roij |first2=L. |last3=Harrington |first3=G. J. |last4=Schouten |first4=Stefan |last5=Sessa |first5=J. A. |last6=LeVay |first6=L. J. |last7=Reichart |first7=G.-J. |last8=Slomp |first8=C. P. |date=25 July 2014 |title=Warming, euxinia and sea level rise during the Paleocene–Eocene Thermal Maximum on the Gulf Coastal Plain: implications for ocean oxygenation and nutrient cycling |journal=[[Climate of the Past]] |volume=10 |issue=4 |pages=1421–1439 |doi=10.5194/cp-10-1421-2014 |doi-access=free |bibcode=2014CliPa..10.1421S |hdl=1969.1/181764 |hdl-access=free}}</ref> The [[Atlantic Coastal Plain]], well oxygenated during the Late Palaeocene, became highly dysoxic during the PETM.<ref>{{Cite journal |last1=Doubrawa |first1=Monika |last2=Stassen |first2=Peter |last3=Robinson |first3=Marci M. |last4=Babila |first4=Tali L. |last5=Zachos |first5=James C. |last6=Speijer |first6=Robert P. |date=21 September 2022 |title=Shelf Ecosystems Along the U.S. Atlantic Coastal Plain Prior to and During the Paleocene-Eocene Thermal Maximum: Insights Into the Stratigraphic Architecture |journal=[[Paleoceanography and Paleoclimatology]] |volume=37 |issue=10 |article-number=e2022PA004475 |doi=10.1029/2022PA004475 |bibcode=2022PaPa...37.4475D |doi-access=free}}</ref> The [[Tasman Sea]], as evidenced by pristane/phytane ratios, likewise saw a drop in oxygen content.<ref>{{cite journal |last1=Li |first1=Zhongxuan |last2=Huang |first2=Haiping |last3=George |first3=Simon C. |title=Molecular compositional variation of organic matter deposited on the East Tasman Plateau during the Paleocene–Eocene Thermal Maximum |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=January 2025 |volume=658 |article-number=112614 |doi=10.1016/j.palaeo.2024.112614 |bibcode=2025PPP...65812614L }}</ref> The tropical surface oceans, in contrast, remained oxygenated over the course of the hyperthermal event.<ref>{{Cite journal |last1=Moretti |first1=Simone |last2=Auderset |first2=Alexandra |last3=Deutsch |first3=Curtis |last4=Schmitz |first4=Ronja |last5=Gerber |first5=Lukas |last6=Thomas |first6=Ellen |last7=Luciani |first7=Valeria |last8=Petrizzo |first8=Maria Rose |last9=Schiebel |first9=Ralf |last10=Tripati |first10=Aradhna |last11=Sexton |first11=Philip |last12=Norris |first12=Richard |last13=D'Onofrio |first13=Roberta |last14=Zachos |first14=James |last15=Sigman |first15=Daniel M. |last16=Haug |first16=Gerald H. |last17=Martínez-García |first17=Alfredo |date=16 February 2024 |title=Oxygen rise in the tropical upper ocean during the Paleocene-Eocene Thermal Maximum |journal=[[Science (journal)|Science]] |volume=383 |issue=6684 |pages=727–731 |doi=10.1126/science.adh4893 |pmid=38359106 |bibcode=2024Sci...383..727M |hdl=11392/2537634 |hdl-access=free }}</ref>

It is possible that during the PETM's early stages, anoxia helped to slow down warming through carbon drawdown via organic matter burial.<ref>{{cite journal |last1=Dickson |first1=Alexander J. |last2=Rees-Owen |first2=Rhian L. |last3=März |first3=Christian |last4=Coe |first4=Angela L. |last5=Cohen |first5=Anthony S. |last6=Pancost |first6=Richard D. |last7=Taylor |first7=Kyle |last8=Shcherbinina |first8=Ekaterina |date=30 April 2014 |title=The spread of marine anoxia on the northern Tethys margin during the Paleocene-Eocene Thermal Maximum |journal=[[Paleoceanography and Paleoclimatology]] |volume=29 |issue=6 |pages=471–488 |doi=10.1002/2014PA002629 |doi-access=free |bibcode=2014PalOc..29..471D}}</ref><ref>{{Cite journal |last1=Sluijs |first1=Appy |last2=Röhl |first2=Ursula |last3=Schouten |first3=Stefan |last4=Brumsack |first4=Hans-J. |last5=Sangiorgi |first5=Francesca |last6=Sinninghe Damsté |first6=Jaap S. |last7=Brinkhuis |first7=Henk |date=7 February 2008 |title=Arctic late Paleocene-early Eocene paleoenvironments with special emphasis on the Paleocene-Eocene thermal maximum (Lomonosov Ridge, Integrated Ocean Drilling Program Expedition 302): PALEOCENE-EOCENE ARCTIC ENVIRONMENTS |journal=[[Paleoceanography and Paleoclimatology]] |volume=23 |issue=1 |at=PA1S11 |doi=10.1029/2007PA001495 |doi-access=free}}</ref> A pronounced negative lithium isotope excursion in both marine carbonates and local weathering inputs suggests that weathering and erosion rates increased during the PETM, generating an increase in organic carbon burial, which acted as a negative feedback on the PETM's severe global warming.<ref>{{cite journal |last1=Von Strandmann |first1=Philip A. E. Pogge |last2=Jones |first2=Morgan T. |last3=West |first3=A. Joshua |last4=Murphy |first4=Melissa J. |last5=Stokke |first5=Ella W. |last6=Tarbuck |first6=Gary |last7=Wilson |first7=David J. |last8=Pearce |first8=Christopher R. |last9=Schmidt |first9=Daniela N. |date=15 October 2021 |title=Lithium isotope evidence for enhanced weathering and erosion during the Paleocene-Eocene Thermal Maximum |journal=[[Science Advances]] |volume=7 |issue=42 |at=eabh4224 |doi=10.1126/sciadv.abh4224 |doi-access=free |pmid=34652934 |pmc=8519576 |bibcode=2021SciA....7.4224P}}</ref>

====Sea level====
{{Main|Past sea level#Changes through geologic time|Sea level rise}}
Along with the global lack of ice, the sea level would have risen due to thermal expansion. Evidence for this can be found in the shifting [[palynomorph]] assemblages of the Arctic Ocean, which reflect a relative decrease in terrestrial organic material compared to marine organic matter.<ref name=Sluijs2006/> A significant marine transgression took place in the Indian Subcontinent.<ref>{{Cite journal |last1=Singh |first1=B. P. |last2=Singh |first2=Y. Raghumani |last3=Andotra |first3=D. S. |last4=Patra |first4=A. |last5=Srivastava |first5=V. K. |last6=Guruaribam |first6=Venus |last7=Sijagurumayum |first7=Umarani |last8=Singh |first8=G. P. |date=January 2016 |title=Tectonically driven late Paleocene (57.9–54.7 Ma) transgression and climatically forced latest middle Eocene (41.3–38.0 Ma) regression on the Indian subcontinent |journal=[[Journal of Asian Earth Sciences]] |volume=115 |pages=124–132 |doi=10.1016/j.jseaes.2015.09.030 |bibcode=2016JAESc.115..124S }}</ref> In the Tarim Sea, sea levels rose by 20-50 metres.<ref>{{Cite journal |last1=Jiang |first1=Jingxin |last2=Hu |first2=Xiumian |last3=Li |first3=Juan |last4=Garzanti |first4=Eduardo |last5=Jiang |first5=Shijun |last6=Cui |first6=Ying |last7=Wang |first7=Yasu |date=October 2023 |title=Eustatic change across the Paleocene-Eocene Thermal Maximum in the epicontinental Tarim seaway |journal=[[Global and Planetary Change]] |volume=229 |article-number=104241 |doi=10.1016/j.gloplacha.2023.104241 |bibcode=2023GPC...22904241J }}</ref>

====Currents====
At the start of the PETM, the ocean circulation patterns changed radically in the course of under 5,000&nbsp;years. Global-scale current directions reversed due to a shift in overturning from the [[Southern Hemisphere]] to Northern Hemisphere. This "backwards" flow persisted for 40,000&nbsp;years. Such a change would transport warm water to the deep oceans, enhancing further warming.<ref name=Nunes2006>{{cite journal |author=Nunes, F. |author2=Norris, R.D. |year=2006 |title=Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period |journal=[[Nature (journal)|Nature]] |volume=439 |issue=7072 |pages=60–63 |doi=10.1038/nature04386 |bibcode=2006Natur.439...60N |pmid=16397495 }}</ref> The major biotic turnover among benthic foraminifera has been cited as evidence of a significant change in deep water circulation.<ref>{{cite journal |last1=Pak |first1=Dorothy K. |last2=Miller |first2=Kenneth J. |date=August 1992 |title=Paleocene to Eocene benthic foraminiferal isotopes and assemblages: Implications for deepwater circulation |journal=[[Paleoceanography and Paleoclimatology]] |volume=7 |issue=4 |pages=405–422 |doi=10.1029/92PA01234 |bibcode=1992PalOc...7..405P}}</ref>

====Acidification====
[[Ocean acidification]] occurred during the PETM,<ref name="CalciumIsotopesOceanAcid">{{cite journal |last1=Fantle |first1=Matthew S. |last2=Ridgwell |first2=Andy |date=5 August 2020 |title=Towards an understanding of the Ca isotopic signal related to ocean acidification and alkalinity overshoots in the rock record |journal=[[Chemical Geology]] |volume=547 |article-number=119672 |doi=10.1016/j.chemgeo.2020.119672 |doi-access=free |bibcode=2020ChGeo.54719672F}}</ref><ref>{{Cite journal |last1=Elbra |first1=Tiiu |last2=Soták |first2=Ján |last3=Kdýr |first3=Šimon |last4=Kohout |first4=Tomáš |last5=Schnabl |first5=Petr |last6=Skála |first6=Roman |last7=Pruner |first7=Petr |date=September 2023 |title=Cretaceous to Palaeogene boundary events and palaeoenvironmental responses across pelagic sequences of the Žilina core section, Slovakia: Rock magnetic, biotic, and geochemical characterization |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=625 |article-number=111682 |doi=10.1016/j.palaeo.2023.111682 |bibcode=2023PPP...62511682E }}</ref> with seawater pH dropping by about 0.46 units.<ref>{{Cite journal |last1=Li |first1=Mingsong |last2=Kump |first2=Lee R. |last3=Ridgwell |first3=Andy |last4=Tierney |first4=Jessica E. |last5=Hakim |first5=Gregory J. |last6=Malevich |first6=Steven B. |last7=Poulsen |first7=Christopher J. |last8=Tardif |first8=Robert |last9=Zhang |first9=Haoxun |last10=Zhu |first10=Jiang |date=12 November 2024 |title=Coupled decline in ocean pH and carbonate saturation during the Palaeocene–Eocene Thermal Maximum |journal=[[Nature Geoscience]] |volume=17 |issue=12 |pages=1299–1305 |doi=10.1038/s41561-024-01579-y |bibcode=2024NatGe..17.1299L }}</ref> This caused the [[Carbonate compensation depth|calcite compensation depth]] to shoal.<ref>{{cite thesis |last=Kitch |first=Gabriella Dawn |date=December 2021 |title=Identifying and Constraining Biocalcification Stress from Geologic Ocean Acidification Events |type=PhD |chapter=Calcium isotope composition of Morozovella over the late Paleocene–early Eocene |publisher=[[Northwestern University]] |id={{ProQuest|2617262217}} }}</ref> The [[lysocline]] marks the depth at which carbonate starts to dissolve (above the lysocline, carbonate is oversaturated): today, this is at about {{cvt|4|km|mi}}, comparable to the median depth of the oceans. This depth depends on (among other things) temperature and the amount of {{CO2}} dissolved in the ocean. Adding {{CO2}} initially raises the lysocline, resulting in the dissolution of deep water carbonates. This deep-water acidification can be observed in ocean cores, which show (where [[bioturbation]] has not destroyed the signal) an abrupt change from grey carbonate ooze to red clays (followed by a gradual grading back to grey). It is far more pronounced in North Atlantic cores than elsewhere, suggesting that acidification was more concentrated here, related to a greater rise in the level of the lysocline. Corrosive waters may have then spilled over into other regions of the world ocean from the North Atlantic. Model simulations show acidic water accumulation in the deep North Atlantic at the onset of the event. 

Acidification of deep waters and the later spreading from the North Atlantic can explain spatial variations in carbonate dissolution.<ref>{{cite journal |doi=10.1038/ngeo2430 |author=Kaitlin Alexander |author2=Katrin J. Meissner |author3=Timothy J. Bralower |date=11 May 2015 |title=Sudden spreading of corrosive bottom water during the Palaeocene–Eocene Thermal Maximum |journal=[[Nature Geoscience]] |volume=8 |issue=6 |pages=458–461 |bibcode=2015NatGe...8..458A|hdl=1959.4/unsworks_59119 |hdl-access=free }}</ref> In parts of the southeast Atlantic, the lysocline rose by 2&nbsp;km in just a few thousand years.<ref name= "Zachos2005">{{cite journal |last1=Zachos |first1=James C. |last2=Röhl |first2=Ursula |last3=Schellenberg |first3=Stephen A. |last4=Sluijs |first4=Appy |last5=Hodell |first5=David A. |last6=Kelly |first6=Daniel C. |last7=Thomas |first7=Ellen |last8=Nicolo |first8=Micah |last9=Raffi |first9=Isabella |last10=Lourens |first10=Lucas J. |last11=McCarren |first11=Heather |last12=Kroon |first12=Dick |title=Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum |journal=Science |date=10 June 2005 |volume=308 |issue=5728 |pages=1611–1615 |doi=10.1126/science.1109004 |bibcode=2005Sci...308.1611Z |pmid=15947184 |hdl=1874/385806 |hdl-access=free }}</ref> Evidence from the tropical Pacific Ocean suggests a minimum lysocline shoaling of around 500 m at the time of this hyperthermal.<ref>{{cite book |last1=Colosimo |first1=A.B. |last2=Bralower |first2=T.J. |last3=Zachos |first3=J.C. |title=Proceedings of the Ocean Drilling Program, 198 Scientific Results |chapter=Evidence for Lysocline Shoaling at the Paleocene/Eocene Thermal Maximum on Shatsky Rise, Northwest Pacific |date=2006 |volume=198 |doi=10.2973/odp.proc.sr.198.112.2006 |doi-access=free }}</ref> Acidification may have increased the efficiency of transport of photic zone water into the ocean depths, thus partially acting as a negative feedback that retarded the rate of atmospheric carbon dioxide buildup.<ref>{{cite journal |last1=Ma |first1=Zhongwu |last2=Gray |first2=Ellen |last3=Thomas |first3=Ellen |last4=Murphy |first4=Brandon |last5=Zachos |first5=James C. |last6=Paytan |first6=Adina |date=13 April 2014 |title=Carbon sequestration during the Palaeocene–Eocene Thermal Maximum by an efficient biological pump |journal=[[Nature Geoscience]] |volume=7 |issue=1 |pages=382–388 |doi=10.1038/ngeo2139 |bibcode=2014NatGe...7..382M}}</ref> Also, diminished biocalcification inhibited the removal of alkalinity from the deep ocean, causing an overshoot of calcium carbonate deposition once net calcium carbonate production resumed, helping restore the ocean to its state before the PETM.<ref>{{Cite journal |last1=Luo |first1=Yiming |last2=Boudreau |first2=Bernard P. |last3=Dickens |first3=Gerald R. |last4=Sluijs |first4=Appy |last5=Middelburg |first5=Jack J. |date=November 2016 |title=An alternative model for CaCO3 over-shooting during the PETM: Biological carbonate compensation |journal=[[Earth and Planetary Science Letters]] |volume=453 |pages=223–233 |doi=10.1016/j.epsl.2016.08.012 |doi-access=free}}</ref> As a consequence of coccolithophorid blooms enabled by enhanced runoff, carbonate was removed from seawater as the Earth recovered from the negative carbon isotope excursion, thus acting to ameliorate ocean acidification.<ref>{{cite journal |last1=Kelly |first1=D. Clay |last2=Zachos |first2=James C. |last3=Bralower |first3=Timothy J. |last4=Schellenberg |first4=Stephen A. |date=17 December 2005 |title=Enhanced terrestrial weathering/runoff and surface ocean carbonate production during the recovery stages of the Paleocene-Eocene thermal maximum |journal=[[Paleoceanography and Paleoclimatology]] |volume=20 |issue=4 |pages=1–11 |article-number=2005PA001163 |doi=10.1029/2005PA001163 |bibcode=2005PalOc..20.4023K |doi-access=free}}</ref>

===Life===
Stoichiometric [[magnetite]] ({{chem|Fe|3|O|4}}) particles were obtained from PETM-age marine sediments. The study from 2008 found elongate prism and spearhead crystal morphologies, considered unlike any magnetite crystals previously reported, and are potentially of [[biogenic]] origin.<ref>{{cite journal |year=2008 |author=Peter C. Lippert |doi=10.1073/pnas.0809839105 |pmid=19008352 |pmc=2584755 |title=Big discovery for biogenic magnetite |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=105 |issue=46 |pages=17595–17596 |bibcode=2008PNAS..10517595L |doi-access=free}}</ref> These biogenic magnetite crystals show unique gigantism, and probably are of aquatic origin. The study suggests that development of thick suboxic zones with high iron bioavailability, the result of dramatic changes in weathering and sedimentation rates, drove diversification of magnetite-forming organisms, likely including eukaryotes.<ref>{{cite journal |last1=Schumann |first1=Dirk |last2=Raub |first2=Timothy D. |last3=Kopp |first3=Robert E. |last4=Guerquin-Kern |first4=Jean-Luc |last5=Wu |first5=Ting-Di |last6=Rouiller |first6=Isabelle |last7=Smirnov |first7=Aleksey V. |last8=Sears |first8=S. Kelly |last9=Lücken |first9=Uwe |last10=Tikoo |first10=Sonia M. |last11=Hesse |first11=Reinhard |last12=Kirschvink |first12=Joseph L. |last13=Vali |first13=Hojatollah |year=2008 |doi=10.1073/pnas.0803634105 |pmid=18936486 |pmc=2584680 |title=Gigantism in unique biogenic magnetite at the Paleocene–Eocene Thermal Maximum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=105 |issue=46 |pages=17648–17653 |bibcode=2008PNAS..10517648S |doi-access=free}}</ref> Biogenic magnetites in animals have a crucial role in geomagnetic field navigation.<ref>{{cite journal |last1=Strbak |first1=O. |last2=Kopcansky |first2=P. |last3=Frollo |first3=I. |title=Biogenic Magnetite in Humans and New Magnetic Resonance Hazard Questions |journal=Measurement Science Review |date=2011 |volume=11 |issue=3 |page=85 |doi=10.2478/v10048-011-0014-1 |bibcode=2011MeScR..11...85S |doi-access=free }}</ref>

====Ocean====
Shallow marine ecosystems suffered substantial ecosystem collapse.<ref>{{cite journal |last1=Tian |first1=Skye Yunshu |last2=Yasuhara |first2=Moriaki |last3=Huang |first3=Huai-Hsuan M. |last4=Condamine |first4=Fabien L. |last5=Robinson |first5=Marci M. |title=Shallow marine ecosystem collapse and recovery during the Paleocene-Eocene Thermal Maximum |journal=Global and Planetary Change |date=December 2021 |volume=207 |article-number=103649 |doi=10.1016/j.gloplacha.2021.103649 |bibcode=2021GPC...20703649T }}</ref> The PETM was accompanied by significant changes in the diversity of calcareous nannofossils and benthic and planktonic foraminifera.<ref>{{Cite journal |last1=Al-Ameer |first1=Abdullah O. |last2=Mahfouz |first2=Kamel H. |last3=El-Sheikh |first3=Islam |last4=Metwally |first4=Amr A. |date=August 2022 |title=Nature of the Paleocene/Eocene boundary (the Dababiya Quarry Member) at El-Ballas area, Qena region, Egypt |journal=[[Journal of African Earth Sciences]] |volume=192 |article-number=104569 |doi=10.1016/j.jafrearsci.2022.104569 |bibcode=2022JAfES.19204569A}}</ref> A [[mass extinction]] of 35–50% of [[wikt:benthic|benthic]] [[foraminifera]] (especially in deeper waters) occurred over the course of ~1,000 years, with the group suffering more during the PETM than during the dinosaur-slaying [[K-T extinction]].<ref>{{cite journal |author=Thomas E. |year=1989 |title=Development of Cenozoic deep-sea benthic foraminiferal faunas in Antarctic waters |journal=Geological Society of London, Special Publications |volume=47 |issue=1| pages=283–296 |doi=10.1144/GSL.SP.1989.047.01.21 |bibcode=1989GSLSP..47..283T }}</ref><ref>{{cite book |author=Thomas E. |title=Global Catastrophes in Earth History; an Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality |chapter=Late Cretaceous–early Eocene mass extinctions in the deep sea |year=1990 |series=GSA Special Papers |volume=247 |pages=481–495 |doi=10.1130/SPE247-p481 |isbn=0-8137-2247-0}}</ref><ref>{{cite book |last=Thomas |first=E. |date=1998 |chapter=The biogeography of the late Paleocene benthic foraminiferal extinction|editor-first1=M.-P.|editor-last1=Aubry |editor-first2=S. |editor-last2=Lucas|editor-first3=W. A.|editor-last3=Berggren|title=Late Paleocene-early Eocene Biotic and Climatic Events in the Marine and Terrestrial Records |publisher=Columbia University Press|pages=214–243}}</ref> At the onset of the PETM, benthic foraminiferal diversity dropped by 30% in the Pacific Ocean,<ref>{{cite journal |last1=Takeda |first1=Kotaro |last2=Kaiho |first2=Kunio |date=3 August 2007 |title=Faunal turnovers in central Pacific benthic foraminifera during the Paleocene–Eocene thermal maximum |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=251 |issue=2 |pages=175–197 |doi=10.1016/j.palaeo.2007.02.026 |bibcode=2007PPP...251..175T}}</ref> while at Zumaia in what is now Spain, 55% of benthic foraminifera went extinct over the course of the PETM,<ref>{{cite journal |last1=Alegret |first1=Laia |last2=Ortiz |first2=Silvia |last3=Orue-Extebarria |first3=Xabier |last4=Bernaola |first4=Gilen |last5=Baceta |first5=Juan I. |last6=Monechi |first6=Simonetta |last7=Apellaniz |first7=Estibaliz |last8=Pujalte |first8=Victoriano |date=May 2009 |title=THE PALEOCENE–EOCENE THERMAL MAXIMUM: NEW DATA ON MICROFOSSIL TURNOVER AT THE ZUMAIA SECTION, SPAIN |journal=[[PALAIOS]] |volume=24 |issue=5 |pages=318–328 |doi=10.2110/palo.2008.p08-057r |bibcode=2009Palai..24..318A |hdl=2158/372896 |hdl-access=free}}</ref> though this decline was not ubiquitous to all sites; Himalayan platform carbonates show no major change in assemblages of large benthic foraminifera at the onset of the PETM; their decline came about towards the end of the event.<ref>{{cite journal |last1=Li |first1=Juan |last2=Hu |first2=Xiumian |last3=Zachos |first3=James C. |last4=Garzanti |first4=Eduardo |last5=BouDagher-Fadel |first5=Marcelle |date=November 2020 |title=Sea level, biotic and carbon-isotope response to the Paleocene–Eocene thermal maximum in Tibetan Himalayan platform carbonates |journal=[[Global and Planetary Change]] |volume=194 |article-number=103316 |doi=10.1016/j.gloplacha.2020.103316 |bibcode=2020GPC...19403316L |url=https://discovery.ucl.ac.uk/id/eprint/10114012/ }}</ref> A decrease in diversity and migration away from the oppressively hot tropics indicates planktonic foraminifera were adversely affected as well.<ref>{{cite journal |last1=Hupp |first1=Brittany N. |last2=Kelly |first2=D. Clay |last3=Williams |first3=John W. |date=22 February 2022 |title=Isotopic filtering reveals high sensitivity of planktic calcifiers to Paleocene–Eocene thermal maximum warming and acidification |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=119 |issue=9 |pages=1–7 |article-number=e2115561119 |doi=10.1073/pnas.2115561119 |doi-access=free |pmid=35193977 |pmc=8892336 |bibcode=2022PNAS..11915561H}}</ref> The [[Lilliput effect]] is observed in shallow water foraminifera,<ref>{{Cite journal |last1=Khanolkar |first1=Sonal |last2=Saraswati |first2=Pratul Kumar |title=Ecological Response of Shallow-Marine Foraminifera to Early Eocene Warming in Equatorial India |date=July 2015 |journal=The Journal of Foraminiferal Research |volume=45 |issue=3 |pages=293–304 |doi=10.2113/gsjfr.45.3.293 |bibcode=2015JForR..45..293K }}</ref> possibly as a response to decreased surficial water density or diminished nutrient availability.<ref>{{Cite journal |last1=Alegret |first1=L. |last2=Ortiz |first2=S. |last3=Arenillas |first3=I. |last4=Molina |first4=E. |date=September 2010 |title=What happens when the ocean is overheated? The foraminiferal response across the Paleocene-Eocene Thermal Maximum at the Alamedilla section (Spain) |journal=[[Geological Society of America Bulletin]] |volume=122 |issue=9–10 |pages=1616–1624 |doi=10.1130/B30055.1 |bibcode=2010GSAB..122.1616A }}</ref> Populations of planktonic foraminifera bearing photosymbionts increased.<ref>{{Cite journal |last1=Shaw |first1=Jack O. |last2=D'haenens |first2=Simon |last3=Thomas |first3=Ellen |last4=Norris |first4=Richard D. |last5=Lyman |first5=Johnnie A. |last6=Bornemann |first6=André |last7=Hull |first7=Pincelli M. |date=November 2021 |title=Photosymbiosis in planktonic foraminifera across the Paleocene–Eocene thermal maximum |journal=[[Paleobiology (journal)|Paleobiology]] |volume=47 |issue=4 |pages=632–647 |doi=10.1017/pab.2021.7 |bibcode=2021Pbio...47..632S }}</ref> Extinction rates among calcareous nannoplankton increased, but so did origination rates.<ref>{{cite journal |last1=Gibbs |first1=Samantha J. |last2=Bown |first2=Paul R. |last3=Sessa |first3=Jocelyn A. |last4=Bralower |first4=Timothy J. |last5=Wilson |first5=Paul A. |date=15 December 2006 |title=Nannoplankton Extinction and Origination Across the Paleocene-Eocene Thermal Maximum |journal=[[Science (journal)|Science]] |volume=314 |issue=5806 |pages=1770–1173 |bibcode=2006Sci...314.1770G |doi=10.1126/science.1133902 |pmid=17170303 }}</ref> In the Peri-Tethyan region, Palaeocene calcareous nannoplankton declined during the recovery from the PETM negative carbon isotope excursion.<ref>{{Cite journal |last1=Shcherbinina |first1=Ekaterina |last2=Gavrilov |first2=Yuri |last3=Iakovleva |first3=Alina |last4=Pokrovsky |first4=Boris |last5=Golovanova |first5=Olga |last6=Aleksandrova |first6=Galina |date=15 August 2016 |title=Environmental dynamics during the Paleocene–Eocene thermal maximum (PETM) in the northeastern Peri-Tethys revealed by high-resolution micropalaeontological and geochemical studies of a Caucasian key section |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=456 |pages=60–81 |doi=10.1016/j.palaeo.2016.05.006 |bibcode=2016PPP...456...60S }}</ref> In the Kerguelen Plateau, nannoplankton productivity sharply declined at the onset of the negative {{delta|13|C}} excursion but was elevated in its aftermath.<ref>{{Cite journal |last1=Jiang |first1=Shijun |last2=Wise |first2=Sherwood W. |date=June 2009 |title=Distinguishing the influence of diagenesis on the paleoecological reconstruction of nannoplankton across the Paleocene/Eocene Thermal Maximum: An example from the Kerguelen Plateau, southern Indian Ocean |journal=Marine Micropaleontology |volume=72 |issue=1–2 |pages=49–59 |doi=10.1016/j.marmicro.2009.03.003 |bibcode=2009MarMP..72...49J }}</ref> The nannoplankton genus ''Fasciculithus'' went extinct,<ref>{{Cite journal |last1=Agnini |first1=Claudia |last2=Spofforth |first2=David J. A. |last3=Dickens |first3=Gerald R. |last4=Rio |first4=Domenico |last5=Pälike |first5=Heiko |last6=Backman |first6=Jan |last7=Muttoni |first7=Giovanni |last8=Dallanave |first8=Edoardo |date=11 April 2016 |title=Stable isotope and calcareous nannofossil assemblage record of the late Paleocene and early Eocene (Cicogna section) |journal=[[Climate of the Past]] |volume=12 |issue=4 |pages=883–909 |doi=10.5194/cp-12-883-2016 |doi-access=free |bibcode=2016CliPa..12..883A |hdl=11577/3183656 |hdl-access=free}}</ref> most likely as a result of increased surface water oligotrophy;<ref name="TimothyBralower2002">{{cite journal |last1=Bralower |first1=Timothy J. |date=31 May 2002 |title=Evidence of surface water oligotrophy during the Paleocene-Eocene thermal maximum: Nanofossil assemblage data from Ocean Drilling Program Site 690, Maud Rise, Weddell Sea |journal=[[Paleoceanography and Paleoclimatology]] |volume=17 |issue=2 |pages=13-1-13-12 |bibcode=2002PalOc..17.1023B |doi=10.1029/2001PA000662 |doi-access=free}}</ref> the genera ''Sphenolithus'', ''Zygrhablithus'', ''Octolithus'' suffered badly too.<ref>{{Cite journal |last1=Agnini |first1=Claudia |last2=Fornaciari |first2=Eliana |last3=Rio |first3=Domenico |last4=Tateo |first4=Fabio |last5=Backman |first5=Jan |last6=Giusberti |first6=Luca |date=April 2007 |title=Responses of calcareous nannofossil assemblages, mineralogy and geochemistry to the environmental perturbations across the Paleocene/Eocene boundary in the Venetian Pre-Alps |journal=Marine Micropaleontology |volume=63 |issue=1–2 |pages=19–38 |doi=10.1016/j.marmicro.2006.10.002 |bibcode=2007MarMP..63...19A}}</ref>

Samples from the tropical Atlantic show that overall, dinocyst abundance diminished sharply.<ref>{{cite thesis |last=Frieling |first=Joost |date=11 May 2016 |title=Climate, carbon cycling and marine ecology during the Paleocene-Eocene Thermal Maximum |degree=PhD |chapter=Tropical Atlantic Climate and Ecosystem Regime Shifts during the Paleocene-Eocene Thermal Maximum |publisher=[[Utrecht University]] |hdl=1874/334859 |hdl-access=free }}{{pn|date=August 2025}}</ref> Contrarily, thermophilic [[dinoflagellates]] bloomed,<ref>{{Cite journal |last1=Bilal |first1=Ahmer |last2=Yang |first2=Renchao |last3=Li |first3=Yang |last4=Zhang |first4=Jingzhou |last5=Janjuhah |first5=Hammad Tariq |date=March 2024 |title=Microfacies shift in the Late Paleocene–Early Eocene Patala Formation in the Upper Indus Basin (Pakistan): Implications for development of the Ceno-Tethys Ocean |journal=[[Marine and Petroleum Geology]] |volume=161 |article-number=106693 |doi=10.1016/j.marpetgeo.2024.106693 |bibcode=2024MarPG.16106693B }}</ref> particularly ''Apectodinium''.<ref>{{Cite journal |last1=Gupta |first1=Smita |last2=Kumar |first2=Kishor |date=January 2019 |title=Precursors of the Paleocene–Eocene Thermal Maximum (PETM) in the Subathu Group, NW sub-Himalaya, India |journal=[[Journal of Asian Earth Sciences]] |volume=169 |pages=21–46 |doi=10.1016/j.jseaes.2018.05.027 |bibcode=2019JAESc.169...21G }}</ref><ref>{{Cite journal |last1=Crouch |first1=Erica M. |last2=Dickens |first2=Gerald R. |last3=Brinkhuis |first3=Henk |last4=Aubry |first4=Marie-Pierre |last5=Hollis |first5=Christopher J. |last6=Rogers |first6=Karyne M. |last7=Visscher |first7=Henk |date=25 May 2003 |title=The Apectodinium acme and terrestrial discharge during the Paleocene–Eocene thermal maximum: new palynological, geochemical and calcareous nannoplankton observations at Tawanui, New Zealand |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=194 |issue=4 |pages=387–403 |doi=10.1016/S0031-0182(03)00334-1 |bibcode=2003PPP...194..387C}}</ref><ref>{{Cite journal |last1=Prasad |first1=Vandana |last2=Garg |first2=Rahul |last3=Ateequzzaman |first3=Khowaja |last4=Singh |first4=I. B. |last5=Joachimski |first5=Michael M. |date=June 2006 |title=Apectodinium acme and the palynofacies characteristics in the latest Palaeocene-earliest Eocene of northeastern India: Biotic response to Palaeocene-Eocene Thermal maxima (PETM) in low latitude |journal=Journal of the Palaeontological Society of India |volume=51 |issue=1 |pages=75–91 |doi=10.1177/0971102320060105 |doi-access=free}}</ref> This acme in ''Apectodinium'' abundance is used as a biostratigraphic marker defining the PETM.<ref>{{cite book |last1=Röhl |first1=Ursula |title=The Cenozoic Southern Ocean: Tectonics, Sedimentation, and Climate Change Between Australia and Antarctica |chapter=On the search for the Paleocene/Eocene boundary in the Southern Ocean: Exploring ODP Leg 189 holes 1171D and 1172D, Tasman Sea |date=2004 |series=Geophysical Monograph Series |volume=151 |pages=113–125 |editor-last=Exon |editor-first=Neville F. |place=Washington, D. C. |publisher=American Geophysical Union |isbn=978-0-87590-416-0 |last2=Brinkhuis |first2=Henk |last3=Sluijs |first3=Appy |last4=Fuller |first4=Mike |doi=10.1029/151gm08 |bibcode=2004GMS...151..113R |editor2-last=Kennett |editor2-first=James P. |editor3-last=Malone |editor3-first=Mitchell J.}}</ref><ref>{{Cite journal |last1=Sharman |first1=Glenn R. |last2=Szymanski |first2=Eugene |last3=Hackworth |first3=Rebecca A. |last4=Kahn |first4=Alicia C. M. |last5=Febo |first5=Lawrence A. |last6=Oefinger |first6=Jordan |last7=Gregory |first7=Gunnar M. |date=5 September 2023 |title=Carbon isotope chemostratigraphy, geochemistry, and biostratigraphy of the Paleocene–Eocene Thermal Maximum, deepwater Wilcox Group, Gulf of Mexico (USA) |journal=[[Climate of the Past]] |volume=19 |issue=9 |pages=1743–1775 |doi=10.5194/cp-19-1743-2023 |bibcode=2023CliPa..19.1743S |doi-access=free }}</ref><ref>{{Cite journal |last1=Aboutofail |first1=Salma |last2=Slimani |first2=Hamid |date=December 2024 |title=The Paleocene–Eocene Thermal Maximum (PETM) interval in the southwestern Mediterranean Tethys at Morocco: New data from a high-resolution study of dinoflagellate cysts and palynofacies in the Rif Chain |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=655 |article-number=112522 |doi=10.1016/j.palaeo.2024.112522 |bibcode=2024PPP...65512522A }}</ref> The fitness of ''Apectodinium homomorphum'' stayed constant over the PETM while that of others declined.<ref>{{Cite journal |last1=Sluijs |first1=Appy |last2=van Roij |first2=Linda |last3=Frieling |first3=Joost |last4=Laks |first4=Jelmer |last5=Reichart |first5=Gert-Jan |date=29 November 2017 |title=Single-species dinoflagellate cyst carbon isotope ecology across the Paleocene-Eocene Thermal Maximum |journal=[[Geology (journal)|Geology]] |volume=46 |issue=1 |pages=79–82 |doi=10.1130/G39598.1 |doi-access=free }}</ref>

Radiolarians grew in size over the PETM.<ref>{{Cite journal |last1=Westacott |first1=Sophie |last2=Hollis |first2=Christopher J. |last3=Pascher |first3=Kristina M. |last4=Dickens |first4=Gerald R. |last5=Hull |first5=Pincelli M. |date=January 2023 |title=Radiolarian size and silicification across the Paleocene-Eocene boundary and into the early Eocene |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=609 |article-number=111287 |doi=10.1016/j.palaeo.2022.111287 |bibcode=2023PPP...60911287W }}</ref>

Colonial corals, sensitive to rising temperatures, declined during the PETM, being replaced by larger benthic foraminifera.<ref>{{Cite journal |last1=Ali |first1=Mubashir |last2=Coletti |first2=Giovanni |last3=Mariani |first3=Luca |last4=Benedetti |first4=Andrea |last5=Munawar |first5=Muhammad-Jawad |last6=Rehman |first6=Saif Ur |last7=Sternai |first7=Pietro |last8=Basso |first8=Daniela |last9=Malinverno |first9=Elisa |last10=Shahzad |first10=Khurram |last11=Khan |first11=Suleman |last12=Awais |first12=Muhammad |last13=Usman |first13=Muhammad |last14=Castelltort |first14=Sébastien |last15=Adatte |first15=Thierry |last16=Garzanti |first16=Eduardo |date=June 2024 |title=Shallow-water carbonate facies herald the onset of the Palaeocene-Eocene Thermal Maximum (Hazara basin, Northern Pakistan) |journal=Journal of Asian Earth Sciences: X |volume=11 |article-number=100169 |doi=10.1016/j.jaesx.2023.100169 |bibcode=2024JAESX..1100169A |doi-access=free |hdl=10281/453744 |hdl-access=free}}</ref> Aragonitic corals were greatly hampered in their ability to grow by the acidification of the ocean and eutrophication in surficial waters.<ref>{{Cite journal |last1=Scheibner |first1=C. |last2=Speijer |first2=R. P. |date=November 2008 |title=Late Paleocene–early Eocene Tethyan carbonate platform evolution — A response to long- and short-term paleoclimatic change |journal=[[Earth-Science Reviews]] |volume=90 |issue=3 |pages=71–102 |bibcode=2008ESRv...90...71S |doi=10.1016/j.earscirev.2008.07.002 |url=https://www.vliz.be/imisdocs/publications/241112.pdf }}</ref> Overall, coral framework-building capacity was greatly diminished.<ref>{{Cite journal |last1=Zamagni |first1=Jessica |last2=Mutti |first2=Maria |last3=Košir |first3=Adrijan |date=February 2012 |title=The evolution of mid Paleocene-early Eocene coral communities: How to survive during rapid global warming |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=317-318 |pages=48–65 |doi=10.1016/j.palaeo.2011.12.010 |bibcode=2012PPP...317...48Z }}</ref>

The deep-sea extinctions are difficult to explain, because many species of benthic foraminifera in the deep-sea are cosmopolitan, and can find refugia against local extinction.<ref>{{cite book |last=Thomas |first=E. |title=Large Ecosystem Perturbations: Causes and Consequences |chapter=Cenozoic mass extinctions in the deep sea: What perturbs the largest habitat on Earth? |date=2007 |editor-first1=S. |editor-last1=Monechi |editor-first2=R. |editor-last2=Coccioni |editor-first3=M. |editor-last3=Rampino |series=GSA Special Papers |volume=424 |pages=1–24 |doi=10.1130/2007.2424(01) |isbn=978-0-8137-2424-9}}</ref> General hypotheses such as a temperature-related reduction in oxygen availability, or increased corrosion due to carbonate undersaturated deep waters, are insufficient as explanations. Acidification may also have played a role in the extinction of the calcifying foraminifera, and the higher temperatures would have increased metabolic rates, thus demanding a higher food supply. Such a higher food supply might not have materialized because warming and increased ocean stratification might have led to declining productivity,<ref>{{cite journal |vauthors=Winguth A, Thomas E, Winguth C |year=2012 |title=Global decline in ocean ventilation, oxygenation and productivity during the Paleocene-Eocene Thermal Maximum – Implications for the benthic extinction |journal=[[Geology (journal)|Geology]] |volume=40 |issue=3| pages=263–266 |doi=10.1130/G32529.1 |bibcode=2012Geo....40..263W}}</ref> along with increased remineralization of organic matter in the water column before it reached the benthic foraminifera on the sea floor.<ref>{{cite journal |vauthors=Ma Z, Gray E, Thomas E, Murphy B, Zachos JC, Paytan A |year=2014 |title=Carbon sequestration during the Paleocene-Eocene Thermal maximum by an efficient biological pump |journal=[[Nature Geoscience]] |volume=7 |issue=5| pages=382–388 |doi=10.1038/NGEO2139 |bibcode=2014NatGe...7..382M}}</ref> The only factor global in extent was an increase in temperature. Regional extinctions in the North Atlantic can be attributed to increased deep-sea anoxia, which could be due to the slowdown of overturning ocean currents, or the release and rapid oxidation of large amounts of methane.

In shallower waters, it's undeniable that increased {{CO2}} levels result in a decreased oceanic [[pH]], which has a profound negative effect on corals.<ref name="Langdon2000">{{cite journal |author=Langdon, C. |author2=Takahashi, T. |author3=Sweeney, C. |author4=Chipman, D. |author5=Goddard, J. |author6=Marubini, F. |author7=Aceves, H. |author8=Barnett, H. |author9= Atkinson, M.J. |year=2000 |title=Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef |journal=Global Biogeochemical Cycles |volume=14 |issue=2 |pages=639–654 |doi=10.1029/1999GB001195 |doi-access=free |bibcode=2000GBioC..14..639L }}</ref> Experiments suggest it is also very harmful to calcifying plankton.<ref name="Riebesell2000">{{cite journal |last1=Riebesell |first1=Ulf |last2=Zondervan |first2=Ingrid |last3=Rost |first3=Björn |last4=Tortell |first4=Philippe D. |last5=Zeebe |first5=Richard E. |last6=Morel |first6=François M. M. |title=Reduced calcification of marine plankton in response to increased atmospheric CO2 |journal=Nature |date=September 2000 |volume=407 |issue=6802 |pages=364–367 |doi=10.1038/35030078 |pmid=11014189 |bibcode=2000Natur.407..364R }}</ref> However, the strong acids used to simulate the natural increase in acidity which would result from elevated {{CO2}} concentrations may have given misleading results, and the most recent evidence is that [[coccolithophore]]s (''[[E. huxleyi]]'' at least) become ''more'', not less, calcified and abundant in acidic waters.<ref name="Iglesias2008">{{cite journal |last1=Iglesias-Rodriguez |first1=M. Debora |last2=Halloran |first2=Paul R. |last3=Rickaby |first3=Rosalind E. M. |last4=Hall |first4=Ian R. |author5=Colmenero-Hidalgo, Elena |author6=Gittins, John R. |author7=Green, Darryl R. H. |author8=Tyrrell, Toby |author9=Gibbs, Samantha J. |author10=von Dassow, Peter |author11=Rehm, Eric |author12=Armbrust, E. Virginia |author13=Boessenkool, Karin P. |title=Phytoplankton Calcification in a High-CO<sub>2</sub> World |journal=[[Science (journal)|Science]] |volume=320 |issue=5874 |date=April 2008 |pages=336–40 |doi=10.1126/science.1154122 |pmid=18420926 |bibcode=2008Sci...320..336I }}</ref> No change in the distribution of calcareous nannoplankton such as the coccolithophores can be attributed to acidification during the PETM.<ref name="Iglesias2008"/> Nor was the abundance of calcareous nannoplankton controlled by changes in acidity, with local variations in nutrient availability and temperature playing much greater roles;<ref>{{Cite journal |last1=Gibbs |first1=Samantha J. |last2=Stoll |first2=Heather M. |last3=Bown |first3=Paul R. |last4=Bralower |first4=Timothy J. |date=July 2010 |title=Ocean acidification and surface water carbonate production across the Paleocene–Eocene thermal maximum |journal=[[Earth and Planetary Science Letters]] |volume=295 |issue=3 |pages=583–592 |doi=10.1016/j.epsl.2010.04.044 |bibcode=2010E&PSL.295..583G |hdl=10651/10551 |hdl-access=free }}</ref> diversity changes in calcareous nannoplankton in the Southern Ocean and at the Equator were most affected by temperature changes, whereas in much of the rest of the open ocean, changes in nutrient availability were their dominant drivers.<ref>{{Cite journal |last1=Schneider |first1=Leah J. |last2=Bralower |first2=Timothy J. |last3=Kump |first3=Lee R. |last4=Patzkowsky |first4=Mark E. |date=Fall 2013 |title=Calcareous nannoplankton ecology and community change across the Paleocene-Eocene Thermal Maximum |journal=[[Paleobiology (journal)|Paleobiology]] |volume=39 |issue=4 |pages=628–647 |doi=10.1666/12050 |bibcode=2013Pbio...39..628S }}</ref> Acidification did lead to an abundance of heavily calcified algae<ref name="TimothyBralower2002" /> and weakly calcified forams.<ref name="Kelly1998" /> The calcareous nannofossil [[species]] ''Neochiastozygus junctus'' thrived; its success is attributable to enhanced surficial productivity caused by enhanced nutrient runoff.<ref>{{cite journal |last1=He |first1=Tianchen |last2=Kemp |first2=David B. |last3=Li |first3=Juan |last4=Ruhl |first4=Micha |date=March 2023 |title=Paleoenvironmental changes across the Mesozoic–Paleogene hyperthermal events |journal=[[Global and Planetary Change]] |volume=222 |article-number=104058 |doi=10.1016/j.gloplacha.2023.104058 |doi-access=free |bibcode=2023GPC...22204058H }}</ref><ref>{{Cite journal |last1=Wang |first1=Yasu |last2=Cui |first2=Ying |last3=Su |first3=Hong |last4=Jiang |first4=Jingxin |last5=Wang |first5=Yang |last6=Yang |first6=Zhilin |last7=Hu |first7=Xiumian |last8=Jiang |first8=Shijun |date=October 2022 |title=Response of calcareous nannoplankton to the Paleocene–Eocene Thermal Maximum in the Paratethys Seaway (Tarim Basin, West China) |journal=[[Global and Planetary Change]] |volume=217 |article-number=103918 |doi=10.1016/j.gloplacha.2022.103918 |bibcode=2022GPC...21703918W }}</ref> Eutrophication at the onset of the PETM precipitated a decline among K-strategist large foraminifera, though they rebounded during the post-PETM oligotrophy coevally with the demise of low-latitude corals.<ref>{{cite journal |last1=Scheibner |first1=C. |last2=Speijer |first2=R. P. |last3=Marzou |first3=A. M. |date=June 2005 |title=Turnover of larger foraminifera during the Paleocene-Eocene Thermal Maximum and paleoclimatic control on the evolution of platform ecosystems |journal=[[Geology (journal)|Geology]] |volume=33 |issue=6 |pages=493–496 |doi=10.1130/G21237.1 |bibcode=2005Geo....33..493S}}</ref>

A study published in May 2021 concluded that fish thrived in at least some tropical areas during the PETM, based on discovered fish fossils including ''[[Mene maculata]]'' at [[Ras Gharib]], Egypt.<ref>{{cite journal |author=Sanaa El-Sayed |author2=Matt Friedman |author3=Tarek Anan |author4=Mahmoud A. Faris |author5=Hesham Sallam |title=Diverse marine fish assemblages inhabited the paleotropics during the Paleocene-Eocene thermal maximum |journal=[[Geology (journal)|Geology]] |year=2021 |volume=49 |issue=8 |pages=993–998 |doi=10.1130/G48549.1 |bibcode=2021Geo....49..993E }}</ref> [[Tetraodontiformes|Tetraodontiform]] fish suffered a mass extinction.<ref>{{Cite journal |last1=Arcila |first1=Dahiana |last2=Tyler |first2=James C. |date=15 November 2017 |title=Mass extinction in tetraodontiform fishes linked to the Palaeocene–Eocene thermal maximum |journal=[[Proceedings of the Royal Society B: Biological Sciences]] |volume=284 |issue=1866 |article-number=20171771 |doi=10.1098/rspb.2017.1771 |pmid=29118135 |pmc=5698648 }}</ref>

====Land====
The increase in mammalian abundance is notable, as many mammal clades appear around this time.<ref>{{Cite journal |last1=Carlisle |first1=Emily |last2=Janis |first2=Christine M. |last3=Pisani |first3=Davide |last4=Donoghue |first4=Philip C.J. |last5=Silvestro |first5=Daniele |date=7 August 2023 |title=A timescale for placental mammal diversification based on Bayesian modeling of the fossil record |journal=[[Current Biology]] |volume=33 |issue=15 |pages=3073–3082.e3 |doi=10.1016/j.cub.2023.06.016 |pmid=37379845 |pmc=7617171 |bibcode=2023CBio...33E3073C }}</ref> Increased global temperatures may have promoted dwarfing<ref name="Gingerich2003">{{cite book |last=Gingerich |first=P.D. |title=Causes and Consequences of Globally Warm Climates in the Early Paleogene |publisher=Geological Society of America |year=2003 |isbn=978-0-8137-2369-3 |editor-last=Wing |editor-first=Scott L. |series=GSA Special Papers |volume=369 |pages=463–78 |chapter=Mammalian responses to climate change at the Paleocene-Eocene boundary: Polecat Bench record in the northern Bighorn Basin, Wyoming |doi=10.1130/0-8137-2369-8.463}}</ref><ref>{{cite journal |last1=D'Ambrosia |first1=Abigail R. |last2=Clyde |first2=William C. |last3=Fricke |first3=Henry C. |last4=Gingerich |first4=Philip D. |last5=Abels |first5=Hemmo A. |date=15 March 2017 |title=Repetitive mammalian dwarfing during ancient greenhouse warming events |journal=[[Science Advances]] |volume=3 |issue=3 |at=e1601430 |doi=10.1126/sciadv.1601430 |doi-access=free |pmid=28345031 |pmc=5351980 |bibcode=2017SciA....3E1430D }}</ref><ref name="Secord2012">{{Cite journal |last1=Secord |first1=R. |last2=Bloch |first2=J. I. |last3=Chester |first3=S. G. B. |last4=Boyer |first4=D. M. |last5=Wood |first5=A. R. |last6=Wing |first6=S. L. |last7=Kraus |first7=M. J. |last8=McInerney |first8=F. A. |last9=Krigbaum |first9=J. |title=Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum |journal=[[Science (journal)|Science]] |volume=335 |issue=6071 |pages=959–962 |year=2012 |doi=10.1126/science.1213859 |pmid=22363006 |bibcode=2012Sci...335..959S |url=https://digitalcommons.unl.edu/geosciencefacpub/310 }}</ref> – which may have encouraged speciation. Major mammalian dwarfing occurred early in the PETM, with further dwarfing taking place during the middle of the hyperthermal.<ref name="VanDerMeulen2020" /> The dwarfing of various mammal lineages led to further dwarfing in other mammals whose reduction in body size was not directly induced by the PETM.<ref>{{cite journal |last1=Solé |first1=Floréal |last2=Morse |first2=Paul E. |last3=Bloch |first3=Jonathan I. |last4=Gingerich |first4=Philip D. |last5=Smith |first5=Thierry |date=July 2021 |title=New specimens of the mesonychid Dissacus praenuntius from the early Eocene of Wyoming and evaluation of body size through the PETM in North America |journal=[[Geobios]] |volume=66-67 |pages=103–118 |doi=10.1016/j.geobios.2021.02.005 |bibcode=2021Geobi..66..103S }}</ref> Small mammals such as [[stem group]] [[Erinaceidae|erinaceids]] exhibited no measurable dwarfing, however,<ref>{{Cite journal |last1=Vitek |first1=Natasha S. |last2=Morse |first2=Paul E. |last3=Boyer |first3=Doug M. |last4=Strait |first4=Suzanne G. |last5=Bloch |first5=Jonathan I. |date=19 March 2021 |title=Evaluating the responses of three closely related small mammal lineages to climate change across the Paleocene–Eocene thermal maximum |journal=[[Paleobiology (journal)|Paleobiology]] |volume=47 |issue=3 |pages=464–486 |doi=10.1017/pab.2021.12 |bibcode=2021Pbio...47..464V |doi-access=free }}</ref> with dwarfing instead being concentrated in larger animals such as [[Equidae|equids]],<ref>{{Cite journal |last=Lee |first=Scott |date=December 2024 |title=Newton's Law of Cooling and the Size of ''Sifrhippus sandrae'', the Earliest Horse, During the Paleocene–Eocene Thermal Maximum |journal=[[The Physics Teacher]] |volume=62 |issue=9 |pages=766–767 |doi=10.1119/5.0164827 |bibcode=2024PhTea..62i.766L }}</ref> [[Mesonychidae|mesonychids]],<ref>{{Cite journal |last1=Solé |first1=Floréal |last2=Godinot |first2=Marc |last3=Laurent |first3=Yves |last4=Galoyer |first4=Alain |last5=Smith |first5=Thierry |date=9 March 2017 |title=The European Mesonychid Mammals: Phylogeny, Ecology, Biogeography, and Biochronology |journal=[[Journal of Mammalian Evolution]] |volume=25 |issue=3 |pages=339–379 |doi=10.1007/s10914-016-9371-8 }}</ref> and [[Oxyaenidae|oxyaenids]].<ref>{{Cite journal |last1=Chester |first1=Stephen G. B. |last2=Bloch |first2=Jonathan I. |last3=Secord |first3=Ross |last4=Boyer |first4=Doug M. |date=11 July 2010 |title=A New Small-Bodied Species of Palaeonictis (Creodonta, Oxyaenidae) from the Paleocene-Eocene Thermal Maximum  |journal=[[Journal of Mammalian Evolution]] |volume=17 |issue=4 |pages=227–243 |doi=10.1007/s10914-010-9141-y |url=https://digitalcommons.unl.edu/geosciencefacpub/298 }}</ref> The dwarfing is believed to have been caused by a combination of [[anagenesis]] and immigration of smaller taxa.<ref>{{Cite journal |last1=Rankin |first1=Brian D. |last2=Fox |first2=Jeremy W. |last3=Barrón-Ortiz |first3=Christian R. |last4=Chew |first4=Amy E. |last5=Holroyd |first5=Patricia A. |last6=Ludtke |first6=Joshua A. |last7=Yang |first7=Xingkai |last8=Theodor |first8=Jessica M. |date=7 August 2015 |title=The extended Price equation quantifies species selection on mammalian body size across the Palaeocene/Eocene Thermal Maximum |journal=[[Proceedings of the Royal Society B: Biological Sciences]] |volume=282 |issue=1812 |article-number=20151097 |doi=10.1098/rspb.2015.1097 |pmid=26224712 |pmc=4528525 }}</ref> Many major mammalian clades – including [[Hyaenodontidae|hyaenodontids]], [[Artiodactyla|artiodactyls]], [[Perissodactyla|perissodactyls]], and [[primates]] – appeared and spread around the globe 13,000 to 22,000 years after the initiation of the PETM,<ref>{{cite journal |last1=Bowen |first1=Gabriel J. |last2=Clyde |first2=William C. |last3=Koch |first3=Paul L. |last4=Ting |first4=Suyin |last5=Alroy |first5=John |last6=Tsubamoto |first6=Takehisa |last7=Wang |first7=Yuanqing |last8=Wang |first8=Yuan |date=15 March 2002 |title=Mammalian Dispersal at the Paleocene/Eocene Boundary |journal=[[Science (journal)|Science]] |volume=295 |issue=5562 |pages=2062–2065 |doi=10.1126/science.1068700 |pmid=11896275 |bibcode=2002Sci...295.2062B }}</ref><ref>{{Cite journal |last1=Yuanqing |first1=Wang |last2=Jin |first2=Meng |last3=Xijun |first3=Ni |last4=Chuankui |first4=Li |date=26 April 2007 |title=Major events of Paleogene mammal radiation in China |journal=[[Geological Journal]] |volume=42 |issue=3–4 |pages=415–430 |doi=10.1002/gj.1083 |bibcode=2007GeolJ..42..415Y }}</ref><ref>{{Cite journal |last1=Wang |first1=YuanQing |last2=Meng |first2=Jin |last3=Beard |first3=Christopher K. |last4=Li |first4=Qian |last5=Ni |first5=XiJun |last6=Gebo |first6=Daniel L. |last7=Bai |first7=Bin |last8=Jin |first8=Xun |last9=Li |first9=Ping |date=3 November 2010 |title=Early Paleogene stratigraphic sequences, mammalian evolution and its response to environmental changes in Erlian Basin, Inner Mongolia, China |journal=[[Science China Earth Sciences]] |volume=53 |issue=12 |pages=1918–1926 |doi=10.1007/s11430-010-4095-8 |bibcode=2010ScChD..53.1918W }}</ref> while more archaic mammals declined.<ref>{{Cite journal |last=Gingerich |first=Philip D. |date=May 2006 |title=Environment and evolution through the Paleocene–Eocene thermal maximum |journal=[[Trends in Ecology & Evolution]] |volume=21 |issue=5 |pages=246–253 |doi=10.1016/j.tree.2006.03.006 |pmid=16697910 }}</ref> Humid conditions caused migration of modern Asian mammals northward, dependent on the climatic belts. Uncertainty remains for the timing and tempo of migration.<ref name="Adatte2014" /> It is possible that the Indian Subcontinent acted as a diversity center from which mammalian lineages radiated into Africa and the continents of the Northern Hemisphere,<ref>{{Cite journal |last1=Sahni |first1=A. |last2=Patnaik |first2=R. |date=10 June 2022 |title=An Eocene Greenhouse Forested India: Were Biotic Radiations Triggered by Early Palaeogene Thermal Events?  |journal=Journal of the Geological Society of India |volume=98 |issue=6 |pages=753–759 |doi=10.1007/s12594-022-2064-4 |bibcode=2022JGSI...98..753S }}</ref> though some studies contradict this and instead favour an "into India" theory of mammalian dispersal.<ref>{{Cite journal |last1=Khozyem |first1=Hassan |last2=Adatte |first2=Thierry |last3=Keller |first3=Gerta |last4=Spangenberg |first4=Jorge E. |date=June 2021 |title=Organic carbon isotope records of the Paleocene- Eocene thermal maximum event in India provide new insights into mammal origination and migration |journal=[[Journal of Asian Earth Sciences]] |volume=212 |article-number=104736 |doi=10.1016/j.jseaes.2021.104736 |bibcode=2021JAESc.21204736K }}</ref> Multiple Eurasian mammal orders invaded North America, but because niche space was not saturated, these had little effect on overall community structure.<ref>{{Cite journal |last1=Fraser |first1=Danielle |last2=Lyons |first2=S. Kathleen |date=September 2020 |title=Mammal Community Structure through the Paleocene-Eocene Thermal Maximum |journal=[[The American Naturalist]] |volume=196 |issue=3 |pages=271–290 |doi=10.1086/709819 |pmid=32813992 |bibcode=2020ANat..196..271F }}</ref> Mammals exhibited significant dietary change as well; the mesonychid ''[[Dissacus|Dissacus praenuntius]]'', for example, became much more [[Osteophagy|osteophagous]] over the hyperthermal's course.<ref>{{Cite journal |last1=Schwartz |first1=Andrew |last2=DeSantis |first2=Larisa R.G. |last3=Scott |first3=Rob S. |date=October 2025 |title=Dietary change across the Paleocene-Eocene Thermal Maximum in the mesonychid Dissacus praenuntius |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=675 |article-number=113089 |doi=10.1016/j.palaeo.2025.113089 |bibcode=2025PPP...67513089S }}</ref>

[[Sauropsida|Sauropsids]] such as [[Squamata|squamates]] also show evidence of faunal turnover.<ref>{{cite book |last1=Augé |first1=Marc |title=Causes and consequences of globally warm climates in the early Paleogene |chapter=Lacertilian faunal change across the Paleocene-Eocene boundary in Europe |date=2003 |doi=10.1130/0-8137-2369-8.441 |isbn=978-0-8137-2369-3 }}</ref> Dispersal events among lizards during the PETM were mainly intracontinental, not intercontinental.<ref>{{Cite journal |last=Smith |first=Krister T. |date=9 March 2010 |title=A new lizard assemblage from the earliest eocene (Zone Wa0) of the bighorn basin, wyoming, USA: Biogeography during the warmest interval of the cenozoic |journal=[[Journal of Systematic Palaeontology]] |volume=7 |issue=3 |pages=299–358 |doi=10.1017/S1477201909002752 }}</ref> [[Snake|Snakes]] in the genus ''Cheilophis'' experienced a notable dispersal eventsduring the PETM.<ref>{{Cite journal |last1=Georgalis |first1=Georgios L. |last2=Mennecart |first2=Bastien |date=25 June 2025 |title=Another wanderer from the Paleocene-Eocene Thermal Maximum? A new species of the North American snake genus Cheilophis Gilmore, 1938 from the early Eocene of France |journal=[[Comptes Rendus Palevol]] |volume=24 |issue=16 |doi=10.5852/cr-palevol2025v24a16 |doi-access=free }}</ref> [[Gastornithid|Gastornithids]] appear to have been unaffected by the PETM as far as changes in their size were concerned.<ref>{{Cite journal |last1=Buffetaut |first1=Eric |last2=Angst |first2=Delphine |date=November 2014 |title=Stratigraphic distribution of large flightless birds in the Palaeogene of Europe and its palaeobiological and palaeogeographical implications |journal=[[Earth-Science Reviews]] |volume=138 |pages=394–408 |doi=10.1016/j.earscirev.2014.07.001 |bibcode=2014ESRv..138..394B }}</ref>

The diversity of insect herbivory, as measured by the amount and diversity of damage to plants caused by insects, increased during the PETM in correlation with global warming.<ref>{{cite journal |last1=Currano |first1=Ellen C. |last2=Wilf |first2=Peter |last3=Wild |first3=Scott L. |last4=Labandeira |first4=Conrad C. |last5=Lovecock |first5=Elizabeth C. |last6=Royer |first6=Dana L. |date=12 February 2008 |title=Sharply increased insect herbivory during the Paleocene–Eocene Thermal Maximum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=105 |issue=6 |pages=1960–1964 |pmid=18268338 |pmc=2538865 |doi=10.1073/pnas.0708646105 |doi-access=free}}</ref> The ant genus ''[[Gesomyrmex]]'' radiated across Eurasia during the PETM.<ref>{{cite journal |last1=Aria |first1=Cédric |last2=Jouault |first2=Corentin |last3=Perrichot |first3=Vincent |last4=Nel |first4=André |date=2 February 2023 |title=The megathermal ant genus Gesomyrmex (Formicidae: Formicinae), palaeoindicator of wide latitudinal biome homogeneity during the PETM |journal=[[Geological Magazine]] |volume=160 |issue=1 |pages=187–197 |doi=10.1017/S0016756822001248 |doi-access=free |bibcode=2023GeoM..160..187A }}</ref> As with mammals, soil-dwelling invertebrates are observed to have dwarfed during the PETM.<ref>{{cite journal |last1=Smith |first1=Jon J. |last2=Hasiotis |first2=Stephen T. |last3=Kraus |first3=Mary J. |last4=Woody |first4=Daniel T. |date=20 October 2009 |title=Transient dwarfism of soil fauna during the Paleocene–Eocene Thermal Maximum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=106 |issue=42 |pages=17655–17660 |doi=10.1073/pnas.0909674106 |pmid=19805060 |pmc=2757401 |doi-access=free |bibcode=2009PNAS..10617655S }}</ref>

A profound change in terrestrial vegetation across the globe is associated with the PETM. Across all regions, floras from the latest Palaeocene are highly distinct from those of the PETM and the Early Eocene.<ref>{{cite journal |last1=Korasidis |first1=Vera A. |last2=Wing |first2=Scott L. |last3=Shields |first3=Christine A. |last4=Kiehl |first4=Jeffrey T. |date=9 April 2022 |title=Global Changes in Terrestrial Vegetation and Continental Climate During the Paleocene-Eocene Thermal Maximum |journal=[[Paleoceanography and Paleoclimatology]] |volume=37 |issue=4 |pages=1–21 |article-number=e2021PA004325 |doi=10.1029/2021PA004325 |doi-access=free |bibcode=2022PaPa...37.4325K}}</ref> The Arctic became dominated by palms and broadleaf forests.<ref>{{Cite journal |last1=Willard |first1=Debra A. |last2=Donders |first2=Timme H. |last3=Reichgelt |first3=Tammo |last4=Greenwood |first4=David R. |last5=Sangiorgi |first5=Francesca |last6=Peterse |first6=Francien |last7=Nierop |first7=Klaas G.J. |last8=Frieling |first8=Joost |last9=Schouten |first9=Stefan |last10=Sluijs |first10=Appy |date=July 2019 |title=Arctic vegetation, temperature, and hydrology during Early Eocene transient global warming events |journal=[[Global and Planetary Change]] |volume=178 |pages=139–152 |doi=10.1016/j.gloplacha.2019.04.012 |bibcode=2019GPC...178..139W |doi-access=free}}</ref> The Gulf coast of central Texas was covered in tropical rainforests and tropical seasonal forests.<ref>{{Cite journal |last1=Wagner |first1=Jennifer D. |last2=Peppe |first2=Daniel J. |last3=O'Keefe |first3=Jennifer M. K. |last4=Denison |first4=Christopher N. |date=31 October 2023 |title=Plant community change across the Paleocene–Eocene boundary in the Gulf Coastal Plain, Central Texas |journal=[[PALAIOS]] |volume=38 |issue=10 |pages=436–451 |doi=10.2110/palo.2022.008 |bibcode=2023Palai..38..436W }}</ref>

Freshwater animals suffered mass mortality due to toxigenic cyanobacterial blooms enkindled by the extreme heat.<ref>{{Cite journal |last1=Chen |first1=Zuoling |last2=Ding |first2=Zhongli |last3=Sun |first3=Jimin |last4=Yang |first4=Shiling |last5=Ni |first5=Xijun |last6=Wang |first6=Xu |last7=Wang |first7=Yongli |last8=Zhang |first8=Jiangyong |last9=He |first9=Wei |date=August 2023 |title=Freshwater ecosystem collapse and mass mortalities at the Paleocene-Eocene thermal maximum |journal=[[Global and Planetary Change]] |volume=227 |article-number=104175 |doi=10.1016/j.gloplacha.2023.104175 |bibcode=2023GPC...22704175C }}</ref>

===Geologic effects===
Sediment deposition changed significantly at many [[outcrop]]s and in many drill cores spanning this time interval.<ref name="ScientificDrilling">{{cite journal |last1=Clyde |first1=William C. |last2=Gingerich |first2=Philip D. |last3=Wing |first3=S. L. |last4=Röhl |first4=Ursula |last5=Westerhold |first5=T. |last6=Bowen |first6=G. |last7=Johnson |first7=K. |last8=Baczynski |first8=A. A. |last9=Diefendorf |first9=A. |last10=McInerney |first10=F. |last11=Schnurrenberger |first11=D. |last12=Noren |first12=A. |last13=Brady |first13=K. |date=5 November 2013 |title=Bighorn Basin Coring Project (BBCP): a continental perspective on early Paleogene hyperthermals |journal=Scientific Drilling |volume=16 |pages=21–31 |doi=10.5194/sd-16-21-2013 |bibcode=2013SciDr..16...21C |doi-access=free |hdl=2440/83200 |hdl-access=free}}</ref> During the PETM, sediments are enriched with [[kaolinite]] from a [[detrital]] source due to [[denudation]] (initial processes such as [[volcanoes]], [[earthquakes]], and [[plate tectonics]]).<ref>{{Cite journal |last1=Gibson |first1=Thomas G. |last2=Bybell |first2=Laurel M. |last3=Owens |first3=James P. |date=August 1993 |title=Latest Paleocene lithologic and biotic events in neritic deposits of southwestern New Jersey |journal=[[Paleoceanography and Paleoclimatology]] |volume=8 |issue=4 |pages=495–514 |doi=10.1029/93PA01367 |bibcode=1993PalOc...8..495G }}</ref><ref>{{cite journal |last1=Bolle |first1=Marie-Pierre |last2=Pardo |first2=Alfonso |last3=Adatte |first3=Thierry |last4=Tantawy |first4=Abdel Aziz |last5=Hinrichs |first5=kai-Uwe |last6=Von Salis |first6=Katharina |last7=Burns |first7=Steve |date=6 August 2009 |title=Climatic evolution on the southern and northern margins of the Tethys from the Paleocene to the early Eocene |journal=GFF |volume=122 |issue=1 |pages=31–32 |doi=10.1080/11035890001221031 }}</ref><ref>{{cite journal |last1=Clechenko |first1=Elizabeth R. |last2=kelly |first2=D. Clay |last3=Harrington |first3=Guy J. |last4=Stiles |first4=Cynthia A. |date=March 2007 |title=Terrestrial records of a regional weathering profile at the Paleocene-Eocene boundary in the Williston Basin of North Dakota |journal=[[Geological Society of America Bulletin]] |volume=119 |issue=3–4 |pages=428–442 |doi=10.1130/B26010.1 |bibcode=2007GSAB..119..428C}}</ref> Increased precipitation and enhanced erosion of older kaolinite-rich soils and sediments may have been responsible for this.<ref>{{cite journal |last1=Wang |first1=Chaowen |last2=Adriaens |first2=Rieko |last3=Hong |first3=Hanlie |last4=Elsen |first4=Jan |last5=Vandenberghe |first5=Noël |last6=Lourens |first6=Lucas J. |last7=Gingerich |first7=Philip D. |last8=Abels |first8=Hemmo A. |date=July 2017 |title=Clay mineralogical constraints on weathering in response to early Eocene hyperthermal events in the Bighorn Basin, Wyoming (Western Interior, USA) |journal=[[Geological Society of America Bulletin]] |volume=129 |issue=7–8 |pages=997–1011 |doi=10.1130/B31515.1 |bibcode=2017GSAB..129..997W |hdl=1874/362201 |hdl-access=free}}</ref><ref>{{cite journal |last1=John |first1=Cédric M. |last2=Banerjee |first2=Neil R. |last3=Longstaffe |first3=Fred John |last4=Sica |first4=Cheyenne |last5=Law |first5=Kimberley R. |last6=Zachos |first6=James C. |date=July 2012 |title=Clay assemblage and oxygen isotopic constraints on the weathering response to the Paleocene-Eocene thermal maximum, East Coast of North America |journal=[[Geology (journal)|Geology]] |volume=40 |issue=7 |pages=591–594 |doi=10.1130/G32785.1 |bibcode=2012Geo....40..591J }}</ref><ref>{{cite journal |last1=Mason |first1=T. G. |last2=Bybell |first2=Laurel M. |last3=Mason |first3=D. B. |date=July 2000 |title=Stratigraphic and climatic implications of clay mineral changes around the Paleocene/Eocene boundary of the northeastern US margin |journal=[[Sedimentary Geology (journal)|Sedimentary Geology]] |volume=134 |issue=1–2 |pages=65–92 |doi=10.1016/S0037-0738(00)00014-2 |bibcode=2000SedG..134...65G}}</ref> Increased weathering from the enhanced runoff formed thick paleosoil enriched with [[Concretion|carbonate nodules]] (''Microcodium'' like), and this suggests a [[semi-arid climate]].<ref name=Adatte2014 /> Unlike during lesser, more gradual hyperthermals, [[glauconite]] authigenesis was inhibited.<ref>{{cite journal |last1=Choudhury |first1=Tathagata Roy |last2=Khanolkar |first2=Sonal |last3=Banerjee |first3=Santanu |date=July 2022 |title=Glauconite authigenesis during the warm climatic events of Paleogene: Case studies from shallow marine sections of Western India |journal=[[Global and Planetary Change]] |volume=214 |article-number=103857 |doi=10.1016/j.gloplacha.2022.103857 |bibcode=2022GPC...21403857R }}</ref>

The sedimentological effects of the PETM lagged behind the carbon isotope shifts.<ref>{{cite journal |last1=Manners |first1=Hayley R. |last2=Grimes |first2=Stephen T. |last3=Sutton |first3=Paul A. |last4=Domingo |first4=Laura |last5=Leng |first5=Melanie J. |last6=Twitchett |first6=Richard J. |last7=Hart |first7=Malcolm B. |last8=Jones |first8=Tom Dunkley |last9=Pancost |first9=Richard D. |last10=Duller |first10=Robert |last11=Lopez-Martinez |first11=Nieves |date=15 August 2013 |title=Magnitude and profile of organic carbon isotope records from the Paleocene–Eocene Thermal Maximum: Evidence from northern Spain |journal=[[Earth and Planetary Science Letters]] |volume=376 |pages=220–230 |doi=10.1016/j.epsl.2013.06.016 |bibcode=2013E&PSL.376..220M}}</ref> In the Tremp-Graus Basin of northern Spain, fluvial systems grew and rates of deposition of alluvial sediments increased with a lag time of around 3,800 years after the PETM.<ref>{{cite journal |last1=Pujalte |first1=Victoriano |last2=Schmitz |first2=Birger |last3=Payros |first3=Aitor |date=March 2022 |title=A rapid sedimentary response to the Paleocene-Eocene Thermal Maximum hydrological change: New data from alluvial units of the Tremp-Graus Basin (Spanish Pyrenees) |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=589 |article-number=110818 |doi=10.1016/j.palaeo.2021.110818 |doi-access=free |bibcode=2022PPP...58910818P |hdl=10810/57467 |hdl-access=free}}</ref>

At some marine locations (mostly deep-marine), sedimentation rates must have decreased across the PETM, presumably because of carbonate dissolution on the seafloor; at other locations (mostly shallow-marine), sedimentation rates must have increased across the PETM, presumably because of enhanced delivery of riverine material during the event.<ref name=Giusberti2007>{{cite journal |author=Giusberti, L. |author2=Rio, D. |author3=Agnini, C. |author4=Backman, J. |author5=Fornaciari, E. |author6=Tateo, F. |author7= Oddone, M. |title=Mode and tempo of the Paleocene-Eocene thermal maximum in an expanded section from the Venetian pre-Alps |year=2007 |journal=[[Geological Society of America Bulletin]]| volume=119 |issue=3–4 |pages=391–412 |doi=10.1130/B25994.1 |bibcode=2007GSAB..119..391G}}</ref>

==Possible causes==
Discriminating between different possible causes of the PETM is difficult. Temperatures were rising globally at a steady pace, and a mechanism must be invoked to produce an instantaneous spike which may have been accentuated or catalyzed by positive feedback (or activation of "tipping points"<ref name="Kender2021"/>). The biggest aid in disentangling these factors comes from a consideration of the carbon isotope mass balance. We know the entire [[wikt:exogenic|exogenic]] [[carbon cycle]] (i.e. the carbon contained within the oceans and atmosphere, which can change on short timescales) underwent a −0.2&nbsp;% to −0.3&nbsp;% perturbation in {{delta|13|C}}, and by considering the isotopic signatures of other carbon reserves, can consider what mass of the reserve would be necessary to produce this effect. The assumption underpinning this approach is that the mass of exogenic carbon was the same in the [[Paleogene]] as it is today – something which is very difficult to confirm.

===Eruption of large kimberlite field===
Although the cause of the initial warming has been attributed to a massive injection of carbon ({{CO2}} and/or CH<sub>4</sub>) into the atmosphere, the source of the carbon has yet to be found. The emplacement of a large cluster of [[kimberlite]] pipes at ~56 Ma in the [[Lac de Gras]] region of northern Canada may have provided the carbon that triggered early warming in the form of exsolved magmatic {{CO2}}. Calculations indicate that the estimated 900–1,100 Pg<ref>{{cite journal |last1=Carozza |first1=D. A. |last2=Mysak |first2=L. A. |last3=Schmidt |first3=G. A. |title=Methane and environmental change during the Paleocene-Eocene thermal maximum (PETM): Modeling the PETM onset as a two-stage event |journal=[[Geophysical Research Letters]] |date=2011 |volume=38 |issue=5 |pages=L05702 |doi=10.1029/2010GL046038 |bibcode=2011GeoRL..38.5702C |doi-access=free }}</ref> of carbon required for the initial approximately 3&nbsp;°C of ocean water warming associated with the Paleocene-Eocene thermal maximum could have been released during the emplacement of a large kimberlite cluster.<ref>{{cite journal |last1=Patterson |first1=M. V. |last2=Francis |first2=D. |title=Kimberlite eruptions as triggers for early Cenozoic hyperthermals |journal=[[Geochemistry, Geophysics, Geosystems]] |date=2013 |volume=14 |issue=2 |pages=448–456 |doi=10.1002/ggge.20054 |bibcode=2013GGG....14..448P |doi-access=free}}</ref> The transfer of warm surface ocean water to intermediate depths led to thermal dissociation of seafloor methane hydrates, providing the isotopically depleted carbon that produced the carbon isotopic excursion. The coeval ages of two other kimberlite clusters in the Lac de Gras field and two other early Cenozoic hyperthermals indicate that {{CO2}} degassing during kimberlite emplacement is a plausible source of the {{CO2}} responsible for these sudden global warming events.

===Volcanic activity===
[[Image:Wfm ardnamurchan landsat.jpg|thumb|upright=1.35|Satellite photo of [[Ardnamurchan]] – with clearly visible circular shape, which is the 'plumbings of an ancient volcano']]

==== North Atlantic Igneous Province ====
One of the leading candidates for the cause of the observed carbon cycle disturbances and global warming is volcanic activity associated with the [[North Atlantic Igneous Province]] (NAIP),<ref name = "JonesSM2019" /> which is believed to have released more than 10,000 gigatons of carbon during the PETM based on the relatively isotopically heavy values of the initial carbon addition.<ref name="Gutjahr2017" /> [[Mercury (element)|Mercury]] anomalies during the PETM point to massive volcanism during the event,<ref>{{Cite journal |last1=Jones |first1=Morgan T. |last2=Percival |first2=Lawrence M. E. |last3=Stokke |first3=Ella W. |last4=Frieling |first4=Joost |last5=Mather |first5=Tamsin A. |last6=Riber |first6=Lars |last7=Schubert |first7=Brian A. |last8=Schultz |first8=Bo |last9=Tegner |first9=Christian |last10=Planke |first10=Sverre |last11=Svensen |first11=Henrik H. |date=6 February 2019 |title=Mercury anomalies across the Palaeocene–Eocene Thermal Maximum |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=217–236 |doi=10.5194/cp-15-217-2019 |doi-access=free |bibcode=2019CliPa..15..217J |hdl=10852/73789 |hdl-access=free }}</ref> supplemented by tellurium anomalies.<ref>{{Cite journal |last1=Baumann |first1=Nils Björn |last2=Regelous |first2=Marcel |last3=Adatte |first3=Thierry |last4=Regelous |first4=Anette |last5=Haase |first5=Karsten M. |last6=Khozyem |first6=Hassan |date=22 February 2025 |title=Tellurium and Mercury in Late Cretaceous to Early Eocene Sediments as Proxies for Volcanic Activity in the Deccan and North Atlantic Large Igneous Provinces |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=26 |issue=2 |article-number=e2024GC011967 |doi=10.1029/2024GC011967 |bibcode=2025GGG....2611967B |doi-access=free }}</ref><ref>{{Cite journal |last1=Baumann |first1=N. B. |last2=Regelous |first2=M. |last3=Adatte |first3=T. |last4=Thibault |first4=N. R. |last5=Regelous |first5=A. |last6=Schultz |first6=B. P. |last7=Fantasia |first7=A. |last8=Madsen |first8=H. |last9=Haase |first9=K. M. |date=15 December 2024 |title=Linking the PETM and North Atlantic volcanism using tellurium in sediments |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=656 |article-number=112575 |doi=10.1016/j.palaeo.2024.112575 |bibcode=2024PPP...65612575B |doi-access=free }}</ref> On top of that, increases in ∆<sup>199</sup>Hg show intense volcanism was concurrent with the beginning of the PETM.<ref>{{cite journal |last1=Jin |first1=Simin |last2=Kemp |first2=David B. |last3=Yin |first3=Runsheng |last4=Sun |first4=Ruyang |last5=Shen |first5=Jun |last6=Jolley |first6=David W. |last7=Vieira |first7=Manuel |last8=Huang |first8=Chunju |date=15 January 2023 |title=Mercury isotope evidence for protracted North Atlantic magmatism during the Paleocene-Eocene Thermal Maximum |journal=[[Earth and Planetary Science Letters]] |volume=602 |article-number=117926 |doi=10.1016/j.epsl.2022.117926 |doi-access=free |bibcode=2023E&PSL.60217926J}}</ref> [[Osmium]] isotopic anomalies in Arctic Ocean sediments dating to the PETM have been interpreted as evidence of a volcanic cause of this hyperthermal.<ref>{{cite journal |last1=Dickson |first1=Alexander J. |last2=Cohen |first2=Anthony S. |last3=Coe |first3=Angela L. |last4=Davies |first4=Marc |last5=Shcherbinina |first5=Ekaterina A. |last6=Gavrilov |first6=Yuri O. |date=15 November 2015 |title=Evidence for weathering and volcanism during the PETM from Arctic Ocean and Peri-Tethys osmium isotope records |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=438 |pages=300–307 |doi=10.1016/j.palaeo.2015.08.019 |doi-access=free |bibcode=2015PPP...438..300D}}</ref>

Intrusions of hot magma into carbon-rich sediments may have triggered the degassing of isotopically light methane in sufficient volumes to cause global warming and the observed isotope anomaly. This hypothesis is documented by the presence of extensive intrusive sill complexes and thousands of kilometer-sized [[hydrothermal vent]] complexes in sedimentary basins on the mid-Norwegian margin and west of Shetland.<ref name="Svensen2004">{{cite journal |author=Svensen, H. |author2=Planke, S. |author3=Malthe-Sørenssen, A. |author4=Jamtveit, B. |author5=Myklebust, R. |author6=Eidem, T. |author7=Rey, S. S. |year=2004 |title=Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. |journal=[[Nature (journal)|Nature]] |volume=429 |pages=542–545 |doi=10.1038/nature02566 |pmid=15175747 |issue=6991 |bibcode=2004Natur.429..542S }}</ref><ref name="Storey2007">{{cite journal |author=Storey, M. |author2=Duncan, R.A. |author3=Swisher III, C.C. |year=2007 |title=Paleocene-Eocene Thermal Maximum and the Opening of the Northeast Atlantic |journal=[[Science (journal)|Science]] |volume=316 |issue=5824 |pages=587–9 |doi=10.1126/science.1135274 |pmid=17463286 |bibcode=2007Sci...316..587S }}</ref><ref>{{Cite journal |last1=Frieling |first1=Joost |last2=Svensen |first2=Henrik H. |last3=Planke |first3=Sverre |last4=Cramwinckel |first4=Margot J. |last5=Selnes |first5=Haavard |last6=Sluijs |first6=Appy |date=25 October 2016 |title=Thermogenic methane release as a cause for the long duration of the PETM |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=113 |issue=43 |pages=12059–12064 |doi=10.1073/pnas.1603348113 |doi-access=free |pmc=5087067 |pmid=27790990 |bibcode=2016PNAS..11312059F }}</ref> This hydrothermal venting occurred at shallow depths, enhancing its ability to vent gases into the atmosphere and influence the global climate.<ref>{{Cite journal |last1=Berndt |first1=Christian |last2=Planke |first2=Sverre |last3=Alvarez Zarikian |first3=Carlos A. |last4=Frieling |first4=Joost |last5=Jones |first5=Morgan T. |last6=Millett |first6=John M. |last7=Brinkhuis |first7=Henk |last8=Bünz |first8=Stefan |last9=Svensen |first9=Henrik H. |last10=Longman |first10=Jack |last11=Scherer |first11=Reed P. |last12=Karstens |first12=Jens |last13=Manton |first13=Ben |last14=Nelissen |first14=Mei |last15=Reed |first15=Brandon |date=3 August 2023 |title=Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum |journal=[[Nature Geoscience]] |volume=16 |issue=9 |pages=803–809 |doi=10.1038/s41561-023-01246-8 |doi-access=free |bibcode=2023NatGe..16..803B |hdl=10037/29764 |hdl-access=free }}</ref> Volcanic eruptions of a large magnitude can impact global climate, reducing the amount of solar radiation reaching the Earth's surface, lowering temperatures in the troposphere, and changing atmospheric circulation patterns. Large-scale volcanic activity may last only a few days, but the massive outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sulfate aerosols, sub-micron droplets containing about 75 percent sulfuric acid. Following eruptions, these aerosol particles can linger as long as three to four years in the stratosphere.<ref name="Wolfe2000">{{cite web |author=Jason Wolfe |date=5 September 2000 |url=http://earthobservatory.nasa.gov/Features/Volcano/ |title=Volcanoes and Climate Change |publisher=NASA |work=Earth Observatory |access-date=19 February 2009 |archive-url=https://web.archive.org/web/20170711231320/https://earthobservatory.nasa.gov/Features/Volcano/ |archive-date=11 July 2017 |url-status=live}}</ref> Furthermore, phases of volcanic activity could have triggered the release of methane clathrates and other potential feedback loops.<ref name="Panchuk2008" /><ref name="Gutjahr2017" /><ref name="Kender2021" /> NAIP volcanism influenced the climatic changes of the time not only through the addition of greenhouse gases but also by changing the bathymetry of the North Atlantic.<ref name=":1">{{Cite journal |last1=Jones |first1=Morgan T. |last2=Stokke |first2=Ella W. |last3=Rooney |first3=Alan D. |last4=Frieling |first4=Joost |last5=Pogge von Strandmann |first5=Philip A. E. |last6=Wilson |first6=David J. |last7=Svensen |first7=Henrik H. |last8=Planke |first8=Sverre |last9=Adatte |first9=Thierry |last10=Thibault |first10=Nicolas |last11=Vickers |first11=Madeleine L. |last12=Mather |first12=Tamsin A. |last13=Tegner |first13=Christian |last14=Zuchuat |first14=Valentin |last15=Schultz |first15=Bo P. |date=8 July 2023 |title=Tracing North Atlantic volcanism and seaway connectivity across the Paleocene–Eocene Thermal Maximum (PETM) |journal=[[Climate of the Past]] |volume=19 |issue=8 |pages=1623–1652 |doi=10.5194/cp-19-1623-2023 |doi-access=free |bibcode=2023CliPa..19.1623J |hdl=10852/105019 |hdl-access=free }}</ref> The connection between the North Sea and the North Atlantic through the Faroe-Shetland Basin was severely restricted,<ref>{{Cite journal |last1=Hartley |first1=Ross A. |last2=Roberts |first2=Gareth G. |last3=White |first3=Nicky |last4=Richardson |first4=Chris |date=10 July 2011 |title=Transient convective uplift of an ancient buried landscape |journal=[[Nature Geoscience]] |volume=4 |issue=8 |pages=562–565 |doi=10.1038/ngeo1191 |bibcode=2011NatGe...4..562H }}</ref><ref>{{Cite journal |last1=White |first1=Nicky |last2=Lovell |first2=Bryan |date=26 June 1997 |title=Measuring the pulse of a plume with the sedimentary record |journal=[[Nature (journal)|Nature]] |volume=387 |issue=6636 |pages=888–891 |doi=10.1038/43151 |bibcode=1997Natur.387..888W |doi-access=free }}</ref><ref>{{Cite journal |last1=Shaw Champion |first1=M. E. |last2=White |first2=N. J. |last3=Jones |first3=S. M. |last4=Lovell |first4=J. P. B. |date=9 January 2008 |title=Quantifying transient mantle convective uplift: An example from the Faroe-Shetland basin |journal=[[Tectonics (journal)|Tectonics]] |volume=27 |issue=1 |pages=1–18 |article-number=2007TC002106 |doi=10.1029/2007TC002106 |doi-access=free |bibcode=2008Tecto..27.1002C }}</ref> as was its connection to it by way of the [[English Channel]].<ref name=":1" />

Later phases of NAIP volcanic activity may have caused the other hyperthermal events of the Early Eocene as well, such as ETM2.<ref name="Panchuk2008" />

==== Other volcanic activity ====
It has also been suggested that volcanic activity around the Caribbean may have disrupted the circulation of oceanic currents, amplifying the magnitude of climate change.<ref name="Bralower1997">{{cite journal |author=Bralower, T.J. |author2=Thomas, D.J. |author3=Zachos, J.C. |author4=Hirschmann, M.M. |author5=Röhl, U. |author6=Sigurdsson, H. |author7=Thomas, E. |author8=Whitney, D.L. |year=1997 |title=High-resolution records of the late Paleocene thermal maximum and circum-Caribbean volcanism: Is there a causal link? |journal=[[Geology (journal)|Geology]] |volume=25 |issue=11 |pages=963–966 |bibcode=1997Geo....25..963B |doi=10.1130/0091-7613(1997)025<0963:HRROTL>2.3.CO;2}}</ref>

===Orbital forcing===
The presence of later (smaller) warming events of a global scale, such as the Elmo horizon (aka [[Eocene Thermal Maximum 2|ETM2]]), has led to the hypothesis that the events repeat on a regular basis, driven by maxima in the 400,000 and 100,000 year [[Milankovic cycles|eccentricity cycles]] in the [[Earth's orbit]].<ref name="PiedrahitaEtAl2022">{{cite journal |last1=Piedrahita |first1=Victor A. |last2=Galeotti |first2=Simone |last3=Zhao |first3=Xiang |last4=Roberts |first4=Andrew P. |last5=Rohling |first5=Eelco J. |last6=Heslop |first6=David |last7=Florindo |first7=Fabio |last8=Grant |first8=Katharine M. |last9=Rodríguez-Sanz |first9=Laura |last10=Reghellin |first10=Daniele |last11=Zeebe |first11=Richard E. |date=15 November 2022 |title=Orbital phasing of the Paleocene-Eocene Thermal Maximum |journal=[[Earth and Planetary Science Letters]] |volume=598 |article-number=117839 |doi=10.1016/j.epsl.2022.117839 |bibcode=2022E&PSL.59817839P |doi-access=free }}</ref> Cores from Howard's Tract, [[Maryland]] indicate the PETM occurred as a result of an extreme in axial precession during an orbital eccentricity maximum.<ref>{{cite journal |last1=Lee |first1=Mingsong |last2=Bralower |first2=Timothy J. |last3=Kump |first3=Lee R. |last4=Self-Trail |first4=Jean M. |last5=Zachos |first5=James C. |last6=Rush |first6=William D. |last7=Robinson |first7=Marci M. |date=24 September 2022 |title=Astrochronology of the Paleocene-Eocene Thermal Maximum on the Atlantic Coastal Plain |journal=[[Nature Communications]] |volume=13 |issue=1 |page=5618 |doi=10.1038/s41467-022-33390-x |pmid=36153313 |pmc=9509358 |bibcode=2022NatCo..13.5618L}}</ref> The current warming period is expected to last another 50,000 years due to a minimum in the eccentricity of the Earth's orbit. Orbital increase in insolation (and thus temperature) would force the system over a threshold and unleash positive feedbacks.<ref name=Lourens2005>{{cite journal |author=Lourens, L.J. |author2=Sluijs, A. |author3=Kroon, D. |author4=Zachos, J.C. |author5=Thomas, E. |author6=Röhl, U. |author7=Bowles, J. |author8=Raffi, I. |year=2005 |title=Astronomical pacing of late Palaeocene to early Eocene global warming events |journal=[[Nature (journal)|Nature]] |volume=435 |issue=7045 |pages=1083–1087 |doi=10.1038/nature03814 |pmid=15944716 |bibcode=2005Natur.435.1083L |hdl=1874/11299 |hdl-access=free}}</ref> The orbital forcing hypothesis has been challenged by a study finding the PETM to have coincided with a minimum in the ~400 kyr eccentricity cycle, inconsistent with a proposed orbital trigger for the hyperthermal.<ref name="CramerEtAl2003PP">{{cite journal |last1=Cramer |first1=Benjamin S. |last2=Wright |first2=James D. |last3=Kent |first3=Dennis V. |last4=Aubry |first4=Marie-Pierre |date=18 December 2003 |title=Orbital climate forcing of δ13C excursions in the late Paleocene–early Eocene (chrons C24n–C25n) |journal=[[Paleoceanography and Paleoclimatology]] |volume=18 |issue=4 |page=1097 |doi=10.1029/2003PA000909 |bibcode=2003PalOc..18.1097C |doi-access=free}}</ref>

===Comet impact===
One theory holds that a <sup>12</sup>C-rich comet struck the earth and initiated the warming event. A cometary impact coincident with the P/E boundary can also help explain some enigmatic features associated with this event, such as the iridium anomaly at [[Zumaia]], the abrupt appearance of a localized kaolinitic clay layer with abundant magnetic nanoparticles, and especially the nearly simultaneous onset of the carbon isotope excursion and the thermal maximum.

A key feature and testable prediction of a comet impact is that it should produce virtually instantaneous environmental effects in the atmosphere and surface ocean with later repercussions in the deeper ocean.<ref name=Kent2003>{{cite journal |author=Kent, D.V. |author2=Cramer, B.S. |author3=Lanci, L. |author4=Wang, D. |author5=Wright, J.D. |author6=Van Der Voo, R. |title=A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion |year=2003 |journal=[[Earth and Planetary Science Letters]] |volume=211 |issue=1–2 |pages=13–26 |doi=10.1016/S0012-821X(03)00188-2 |bibcode=2003E&PSL.211...13K}}</ref> Even allowing for feedback processes, this would require at least 100 gigatons of extraterrestrial carbon.<ref name=Kent2003/> Such a catastrophic impact should have left its mark on the globe. A clay layer of {{cvt|5-20|m|ft}} thickness on the coastal shelf of New Jersey contained unusual amounts of magnetite, but it was found to have formed 9-18 kyr too late for these magnetic particles to have been a result of a comet's impact, and the particles had a crystal structure which was a signature of [[magnetotactic bacteria]] rather than an extraterrestrial origin.<ref name=Kopp2007>{{cite journal |author=Kopp, R.E. |author2=Raub, T. |author3=Schumann, D. |author4=Vali, H. |author5=Smirnov, A.V. |author6=Kirschvink, J.L. |title=Magnetofossil spike during the Paleocene-Eocene thermal maximum: Ferromagnetic resonance, rock magnetic, and electron microscopy evidence from Ancora, New Jersey, United States |year=2007 |journal=[[Paleoceanography and Paleoclimatology]]| volume=22 |issue=4 |pages=PA4103 |doi=10.1029/2007PA001473 |bibcode=2007PalOc..22.4103K |doi-access=free}}</ref> However, recent analyses have shown that isolated particles of non-biogenic origin make up the majority of the magnetic particles in the clay sample.<ref name=Wang2012>{{cite journal |author=Wang, H. |author2=Kent, Dennis V. |author3=Jackson, Michael J. |title=Evidence for abundant isolated magnetic nanoparticles at the Paleocene–Eocene boundary |year=2012 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |doi=10.1073/pnas.1205308110 |volume=110 |issue=2 |pages=425–430 |bibcode=2013PNAS..110..425W |pmid=23267095 |pmc=3545797| doi-access=free}}</ref>

A 2016 report in ''[[Science Magazine|Science]]'' describes the discovery of impact ejecta from three marine P-E boundary sections from the Atlantic margin of the eastern U.S., indicating that an extraterrestrial impact occurred during the carbon isotope excursion at the P-E boundary.<ref name="SchallerFung2016">{{cite journal |author=Schaller, M. F. |author2=Fung, M. K. |author3=Wright, J. D. |author4=Katz, M. E. |author5=Kent, D. V. |title=Impact ejecta at the Paleocene-Eocene boundary |year=2016 |journal=[[Science (journal)|Science]] |volume=354 |issue=6309 |pages=225–229 |doi=10.1126/science.aaf5466 |pmid=27738171 |bibcode=2016Sci...354..225S }}</ref><ref>{{cite news |last1=Timmer |first1=John |title=Researchers push argument that comet caused ancient climate change |url=https://arstechnica.com/science/2016/10/researchers-push-argument-that-comet-caused-ancient-climate-change/ |work=Ars Technica |date=13 October 2016 }}</ref> The silicate glass spherules found were identified as [[microtektite]]s and microkrystites.<ref name="SchallerFung2016"/>

===Burning of peat===
The combustion of prodigious quantities of [[peat]] was once postulated, because there was probably a greater mass of carbon stored as living terrestrial biomass during the Paleocene than there is today since plants in fact grew more vigorously during the period of the PETM. This theory was refuted, because in order to produce the {{delta|13|C}} excursion observed, over 90 percent of the Earth's biomass would have to have been combusted. However, the Paleocene is also recognized as a time of significant peat accumulation worldwide. A comprehensive search failed to find evidence for the combustion of fossil organic matter, in the form of soot or similar particulate carbon.<ref>{{cite journal |author=Moore, E. |last2=Kurtz, Andrew C. |year=2008 |title=Black carbon in Paleocene-Eocene boundary sediments: A test of biomass combustion as the PETM trigger |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=267 |issue=1–2 |pages=147–152 |doi=10.1016/j.palaeo.2008.06.010 |bibcode=2008PPP...267..147M}}</ref> Global records of polycyclic aromatic hydrocarbons during the PETM are highly heterogeneous and do not support a global peat burning event.<ref>{{Cite journal |last1=Chen |first1=Zuoling |last2=Li |first2=Chenyan |last3=Yang |first3=Shiling |last4=Lü |first4=Peizong |last5=Tang |first5=Zihua |last6=Sun |first6=Jimin |last7=Ni |first7=Xijun |last8=Fang |first8=Linhao |last9=Ding |first9=Zhongli |date=28 January 2025 |title=Two Sites in East Asia Add to Spatiotemporal Heterogeneity of Wildfire Activity Across the Paleocene–Eocene Thermal Maximum |journal=[[Geophysical Research Letters]] |volume=52 |issue=2 |article-number=e2024GL113829 |doi=10.1029/2024GL113829 |bibcode=2025GeoRL..5213829C |doi-access=free }}</ref> Nonetheless, release of carbon from peat has been considered a possible feedback mechanism that helped to create the conditions of the PETM.<ref>{{Cite journal |last1=Fokkema |first1=Chris D. |last2=Agterhuis |first2=Tobias |last3=Gerritsma |first3=Danielle |last4=de Goeij |first4=Myrthe |last5=Liu |first5=Xiaoqing |last6=de Regt |first6=Pauline |last7=Rice |first7=Addison |last8=Vennema |first8=Laurens |last9=Agnini |first9=Claudia |last10=Bijl |first10=Peter K. |last11=Frieling |first11=Joost |last12=Huber |first12=Matthew |last13=Peterse |first13=Francien |last14=Sluijs |first14=Appy |date=12 June 2024 |title=Polar amplification of orbital-scale climate variability in the early Eocene greenhouse world |journal=[[Climate of the Past]] |volume=20 |issue=6 |pages=1303–1325 |doi=10.5194/cp-20-1303-2024 |bibcode=2024CliPa..20.1303F |doi-access=free |hdl=11577/3517930 |hdl-access=free }}</ref>

===Enhanced respiration===
Respiration rates of organic matter increase when temperatures rise. One feedback mechanism proposed to explain the rapid rise in carbon dioxide levels is a sudden, speedy rise in terrestrial respiration rates concordant with global temperature rise initiated by any of the other causes of warming.<ref>{{cite journal |last1=Bowen |first1=Gabriel J. |date=October 2013 |title=Up in smoke: A role for organic carbon feedbacks in Paleogene hyperthermals |journal=[[Global and Planetary Change]] |volume=109 |pages=18–29 |doi=10.1016/j.gloplacha.2013.07.001 |bibcode=2013GPC...109...18B}}</ref> Mathematical modelling supports increased organic matter oxidation as a viable explanation for observed isotopic excursions in carbon during the PETM's onset.<ref>{{Cite journal |last1=Cui |first1=Ying |last2=Schubert |first2=Brian A. |date=November 2018 |title=Towards determination of the source and magnitude of atmospheric pCO2 change across the early Paleogene hyperthermals |journal=[[Global and Planetary Change]] |volume=170 |pages=120–125 |doi=10.1016/j.gloplacha.2018.08.011}}</ref>

=== Terrestrial methane release ===
Release of methane from wetlands was a contributor to the PETM warming. Evidence for this comes from a {{delta|13|C}} decrease in hopanoids from mire sediments, likely reflecting increased wetland methanogenesis deeper within the mires.<ref>{{Cite journal |last1=Pancost |first1=Richard D. |last2=Steart |first2=David S. |last3=Handley |first3=Luke |last4=Collinson |first4=Margaret E. |last5=Hooker |first5=Jerry J. |last6=Scott |first6=Andrew C. |last7=Grassineau |first7=Nathalie V. |last8=Glasspool |first8=Ian J. |date=September 2007 |title=Increased terrestrial methane cycling at the Palaeocene–Eocene thermal maximum |journal=[[Nature (journal)|Nature]] |volume=449 |issue=7160 |pages=332–335 |doi=10.1038/nature06012 |pmid=17882218 |bibcode=2007Natur.449..332P }}</ref>

===Methane clathrate release===
Methane hydrate dissolution has been invoked as a highly plausible causal mechanism for the carbon isotope excursion and warming observed at the PETM.<ref>{{cite journal |last1=Dickens |first1=G. R. |date=5 August 2011 |title=Down the Rabbit Hole: toward appropriate discussion of methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events |journal=[[Climate of the Past]] |volume=7 |issue=3 |pages=831–846 |doi=10.5194/cp-7-831-2011 |bibcode=2011CliPa...7..831D |doi-access=free }}</ref> The most obvious feedback mechanism that could amplify the initial perturbation is that of [[methane clathrate]]s. Under certain temperature and pressure conditions, methane – which is being produced continually by decomposing microbes in sea bottom sediments – is stable in a complex with water, which forms ice-like cages trapping the methane in solid form. As temperature rises, the pressure required to keep this clathrate configuration stable increases, so shallow clathrates dissociate, releasing methane gas to make its way into the atmosphere. Since biogenic clathrates have a {{delta|13|C}} signature of −60&nbsp;‰ (inorganic clathrates are the still rather large −40&nbsp;‰), relatively small masses can produce large {{delta|13|C}} excursions. Further, methane is a potent [[greenhouse gas]] as it is released into the atmosphere, so it causes warming, and as the ocean transports this warmth to the bottom sediments, it destabilizes more clathrates.<ref name=Dickens1/>

In order for the clathrate hypothesis to be applicable to PETM, the oceans must show signs of having been warmer slightly before the carbon isotope excursion, because it would take some time for the methane to become mixed into the system and {{delta|13|C}}-reduced carbon to be returned to the deep ocean sedimentary record. Up until the 2000s, the evidence suggested that the two peaks were in fact simultaneous, weakening the support for the methane theory. In 2002, a short gap between the initial warming and the {{delta|13|C}} excursion was detected.<ref name=Thomas2002>{{cite journal |last1=Thomas |first1=Deborah J. |last2=Zachos |first2=James C. |last3=Bralower |first3=Timothy J. |last4=Thomas |first4=Ellen |last5=Bohaty |first5=Steven |title=Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum |journal=Geology |date=2002 |volume=30 |issue=12 |page=1067 |doi=10.1130/0091-7613(2002)030<1067:WTFFTF>2.0.CO;2 |bibcode=2002Geo....30.1067T }}</ref> In 2007, chemical markers of surface temperature ([[TEX86|TEX<sub>86</sub>]]) had also indicated that warming occurred around 3,000 years before the carbon isotope excursion, although this did not seem to hold true for all cores.<ref name=Sluijs2007/> However, research in 2005 found no evidence of this time gap in the deeper (non-surface) waters.<ref name=Tripati2005>{{cite journal| author = Tripati, A. |author2=Elderfield, H. |year=2005 |title=Deep-Sea Temperature and Circulation Changes at the Paleocene-Eocene Thermal Maximum |journal=[[Science (journal)|Science]] |volume=308 |issue=5730 |pages=1894–1898 |doi=10.1126/science.1109202 |pmid=15976299 |bibcode=2005Sci...308.1894T }}</ref> Moreover, the small apparent change in TEX<sub>86</sub> that precede the {{delta|13|C}} anomaly can easily (and more plausibly) be ascribed to local variability (especially on the Atlantic coastal plain, e.g. Sluijs, et al., 2007) as the TEX<sub>86</sub> paleo-thermometer is prone to significant biological effects. 

The {{delta|18|O}} of benthic or planktonic forams does not show any pre-warming in any of these localities, and in an ice-free world, it is generally a much more reliable indicator of past ocean temperatures. Analysis of these records reveals another interesting fact: planktonic (floating) forams record the shift to lighter isotope values earlier than benthic (bottom dwelling) forams.<ref>{{cite journal |last1=Kelly |first1=D. Clay |date=28 December 2002 |title=Response of Antarctic (ODP Site 690) planktonic foraminifera to the Paleocene–Eocene thermal maximum: Faunal evidence for ocean/climate change |journal=[[Paleoceanography and Paleoclimatology]] |volume=17 |issue=4 |pages=23-1-23-13 |doi=10.1029/2002PA000761 |bibcode=2002PalOc..17.1071K |doi-access=free }}</ref> The lighter (lower {{delta|13|C}}) methanogenic carbon can only be incorporated into foraminifer shells after it has been oxidised. A gradual release of the gas would allow it to be oxidised in the deep ocean, which would make benthic foraminifera show lighter values earlier. The fact that the planktonic foraminifera are the first to show the signal suggests that the methane was released so rapidly that its oxidation used up all the oxygen at depth in the water column, allowing some methane to reach the atmosphere unoxidised, where atmospheric oxygen would react with it. This observation also allows us to constrain the duration of methane release to under around 10,000 years.<ref name=Thomas2002/>

However, there are several major problems with the methane hydrate dissociation hypothesis. The most parsimonious interpretation for surface-water foraminifera to show the {{delta|13|C}} excursion before their benthic counterparts (as in the Thomas et al. paper) is that the perturbation occurred from the top down, and not the bottom up. If the anomalous {{delta|13|C}} (in whatever form: CH<sub>4</sub> or {{CO2}}) entered the atmospheric carbon reservoir first, and then diffused into the surface ocean waters, which mix with the deeper ocean waters over much longer time-scales, we would expect to observe the planktonics shifting toward lighter values before the benthics.<ref>{{Cite journal |last1=Zachos |first1=James C |last2=Bohaty |first2=Steven M |last3=John |first3=Cedric M |last4=McCarren |first4=Heather |last5=Kelly |first5=Daniel C |last6=Nielsen |first6=Tina |date=15 July 2007 |title=The Palaeocene–Eocene carbon isotope excursion: constraints from individual shell planktonic foraminifer records |journal=[[Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences]] |volume=365 |issue=1856 |pages=1829–1842 |doi=10.1098/rsta.2007.2045 |pmid=17513259 |bibcode=2007RSPTA.365.1829Z }}</ref>

An additional critique of the methane clathrate release hypothesis is that the warming effects of large-scale methane release would not be sustainable for more than a millennium. Thus, exponents of this line of criticism suggest that methane clathrate release could not have been the main driver of the PETM, which lasted for 50,000 to 200,000 years.<ref name="HigginsSchrag2006">{{cite journal |last1=Higgins |first1=John A. |last2=Schrag |first2=Daniel P. |date=30 May 2006 |title=Beyond methane: Towards a theory for the Paleocene–Eocene Thermal Maximum |journal=[[Earth and Planetary Science Letters]] |volume=245 |issue=3–4 |pages=523–537 |doi=10.1016/j.epsl.2006.03.009 |bibcode=2006E&PSL.245..523H}}</ref>

There has been some debate about whether there was a large enough amount of methane hydrate to be a major carbon source; a 2011 paper proposed that was the case.<ref name=Gu2011>{{cite journal |author=Gu, Guangsheng |author2=Dickens, G.R. |author3=Bhatnagar, G. |author4=Colwell, F.S. |author5=Hirasaki, G.J. |author6= Chapman, W.G. |year=2011 |title=Abundant Early Palaeogene marine gas hydrates despite warm deep-ocean temperatures |journal=[[Nature Geoscience]] |volume=4 |issue=12 |pages =848–851 |doi=10.1038/ngeo1301 |bibcode= 2011NatGe...4..848G}}</ref> The present-day global methane hydrate reserve was once considered to be between 2,000 and 10,000 Gt C (billions of tons of [[carbon]]), but is now estimated between 1500 and 2000 Gt C.<ref name="IPCC AR6 WG1 Ch.5">{{cite book |title=Climate Change 2021 – the Physical Science Basis |chapter=Global Carbon and Other Biogeochemical Cycles and Feedbacks |date=2023 |pages=673–816 |doi=10.1017/9781009157896.007 |isbn=978-1-009-15789-6 |author1=[[Intergovernmental Panel on Climate Change]] }}</ref> However, because the global ocean bottom temperatures were ~6&nbsp;°C higher than today, which implies a much smaller volume of sediment hosting gas hydrate than today, the global amount of hydrate before the PETM has been thought to be much less than present-day estimates.<ref name="HigginsSchrag2006" /> One study, however, suggests that because seawater oxygen content was lower, sufficient methane clathrate deposits could have been present to make them a viable mechanism for explaining the isotopic changes.<ref>{{Cite journal |last1=Buffett |first1=Bruce |last2=Archer |first2=David |date=15 November 2004 |title=Global inventory of methane clathrate: sensitivity to changes in the deep ocean |journal=[[Earth and Planetary Science Letters]] |volume=227 |issue=3 |pages=185–199 |doi=10.1016/j.epsl.2004.09.005 |bibcode=2004E&PSL.227..185B }}</ref> In a 2006 study, scientists regarded the source of carbon for the PETM to be a mystery.<ref name=Pagani_Science_2006>{{cite journal |last=Pagani |first=Mark |author2=Caldeira, K. |author3=Archer, D. |author4=Zachos, J.C. |title=An Ancient Carbon Mystery |journal=[[Science (journal)|Science]] |date=8 December 2006 |volume=314 |issue=5805 |pages=1556–7 |doi=10.1126/science.1136110 |pmid=17158314 }}</ref> A 2011 study, using numerical simulations suggests that enhanced organic carbon [[sedimentation]] and [[methanogenesis]] could have compensated for the smaller volume of hydrate stability.<ref name=Gu2011/> A 2016 study based on reconstructions of atmospheric {{CO2}} content during the PETM's carbon isotope excursions (CIE), using triple oxygen isotope analysis, suggests a massive release of seabed methane into the atmosphere as the driver of climatic changes. The authors also state that a massive release of methane hydrates through thermal dissociation of methane hydrate deposits has been the most convincing hypothesis for explaining the CIE ever since it was first identified, according to them.<ref>{{cite journal |last1=Gehler |first1=Alexander |last2=Gingerich |first2=Philip D. |last3=Pack |first3=Andreas |year=2015 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |title=Temperature and atmospheric CO2 concentration estimates through the PETM using triple oxygen isotope analysis of mammalian bioapatite |volume=113 |issue=8 |pages=7739–7744 |doi=10.1073/pnas.1518116113| pmid=27354522 |pmc=4948332 |bibcode=2016PNAS..113.7739G |doi-access=free}}</ref> 

In 2019, a study suggested that there was a global warming of around 2 degrees several millennia before PETM, and that this warming had eventually destabilized methane hydrates and caused the increased carbon emission during PETM, as evidenced by the large increase in [[barium]] ocean concentrations (since PETM-era hydrate deposits would have been also been rich in barium, and would have released it upon their meltdown).<ref>{{Cite journal |last1=Frieling |first1=J. |last2=Peterse |first2=F. |last3=Lunt |first3=D. J. |last4=Bohaty |first4=S. M. |last5=Sinninghe Damsté |first5=J. S. |last6=Reichart |first6=G. -J. |last7=Sluijs |first7=A. |date=18 March 2019 |title=Widespread Warming Before and Elevated Barium Burial During the Paleocene-Eocene Thermal Maximum: Evidence for Methane Hydrate Release? |journal=[[Paleoceanography and Paleoclimatology]] |volume=34 |issue=4 |pages=546–566 |doi=10.1029/2018PA003425 |pmid=31245790 |pmc=6582550 |bibcode=2019PaPa...34..546F}}</ref> In 2022, a foraminiferal records study had reinforced this conclusion, suggesting that the release of CO<sup>2</sup> before PETM was comparable to the current anthropogenic emissions in its rate and scope, to the point that there was enough time for a recovery to background levels of warming and [[ocean acidification]] in the centuries to millennia between the so-called pre-onset excursion (POE) and the main event (carbon isotope excursion, or CIE).<ref name="Kender2021">{{cite journal |last1=Kender |first1=Sev |last2=Bogus |first2=Kara |last3=Pedersen |first3=Gunver K. |last4=Dybkjær |first4=Karen |last5=Mather |first5=Tamsin A. |last6=Mariani |first6=Erica |last7=Ridgwell |first7=Andy |last8=Riding |first8=James B. |last9=Wagner |first9=Thomas |last10=Hesselbo |first10=Stephen P. |last11=Leng |first11=Melanie J. |title=Paleocene/Eocene carbon feedbacks triggered by volcanic activity |journal=[[Nature Communications]] |date=31 August 2021 |volume=12 |issue=1 |page=5186 |doi=10.1038/s41467-021-25536-0 |pmid=34465785 |pmc=8408262 |bibcode=2021NatCo..12.5186K |hdl=10871/126942 |hdl-access=free }}</ref> A 2021 paper had further indicated that while PETM began with a significant intensification of volcanic activity and that lower-intensity volcanic activity sustained elevated carbon dioxide levels, "at least one other carbon reservoir released significant greenhouse gases in response to initial warming".<ref>{{cite journal |last1=Babila |first1=Tali L. |last2=Penman |first2=Donald E. |last3=Standish |first3=Christopher D. |last4=Doubrawa |first4=Monika |last5=Bralower |first5=Timothy J. |last6=Robinson |first6=Marci M. |last7=Self-Trail |first7=Jean M. |last8=Speijer |first8=Robert P. |last9=Stassen |first9=Peter |last10=Foster |first10=Gavin L. |last11=Zachos |first11=James C. |title=Surface ocean warming and acidification driven by rapid carbon release precedes Paleocene-Eocene Thermal Maximum |journal=[[Science Advances]] |date=16 March 2022 |volume=8 |issue=11 |at=eabg1025 |doi=10.1126/sciadv.abg1025 |doi-access=free |pmid=35294237 |pmc=8926327 |bibcode=2022SciA....8G1025B |hdl=20.500.12942/694229 |hdl-access=free}}</ref>

It was estimated in 2001 that it would take around 2,300 years for an increased temperature to diffuse warmth into the sea bed to a depth sufficient to cause a release of clathrates, although the exact time-frame is highly dependent on a number of poorly constrained assumptions.<ref name=Katz2001>{{cite journal |author=Katz, M.E. |author2=Cramer, B.S. |author3=Mountain, G.S. |author4=Katz, S. |author5=Miller, K.G. |year=2001 |title=Uncorking the bottle: What triggered the Paleocene/Eocene thermal maximum methane release |journal=[[Paleoceanography and Paleoclimatology]] |volume=16 |issue=6 |page=667 |doi-access=free |doi=10.1029/2000PA000615 |bibcode=2001PalOc..16..549K |citeseerx=10.1.1.173.2201}}</ref> [[Ocean warming]] due to flooding and pressure changes due to a sea-level drop may have caused clathrates to become unstable and release methane. This can take place over as short of a period as a few thousand years. The reverse process, that of fixing methane in clathrates, occurs over a larger scale of tens of thousands of years.<ref>{{Cite journal |last=MacDonald |first=Gordon J. |title=Role of methane clathrates in past and future climates |year=1990 |journal=Climatic Change |volume=16 |issue=3 |pages=247–281 |doi=10.1007/BF00144504 |bibcode=1990ClCh...16..247M }}</ref>

===Ocean circulation===
The large scale patterns of ocean circulation are important when considering how heat was transported through the oceans. Our understanding of these patterns is still in a preliminary stage. Models show that there are possible mechanisms to quickly transport heat to the shallow, clathrate-containing ocean shelves, given the right bathymetric profile, but the models cannot yet match the distribution of data we observe. "Warming accompanying a south-to-north switch in deepwater formation would produce sufficient warming to destabilize seafloor gas hydrates over most of the world ocean to a water depth of at least 1900 m." This destabilization could have resulted in the release of more than 2000 gigatons of methane gas from the clathrate zone of the ocean floor.<ref name=Bice2002>{{cite journal |author=Bice, K.L. |author2=Marotzke, J. |year=2002 |title=Could changing ocean circulation have destabilized methane hydrate at the Paleocene/Eocene boundary |journal=[[Paleoceanography and Paleoclimatology]] |volume=17 |issue=2 |page=1018 |bibcode=2002PalOc..17.1018B |doi=10.1029/2001PA000678 |doi-access=free |hdl=11858/00-001M-0000-0014-3AC0-A |hdl-access=free }}</ref> The timing of changes in ocean circulation with respect to the shift in carbon isotope ratios has been argued to support the proposition that warmer deep water caused methane hydrate release.<ref>{{cite journal |last1=Tripati |first1=Aradhna K. |last2=Elderfield |first2=Henry |date=14 February 2004 |title=Abrupt hydrographic changes in the equatorial Pacific and subtropical Atlantic from foraminiferal Mg/Ca indicate greenhouse origin for the thermal maximum at the Paleocene-Eocene Boundary |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=5 |issue=2 |pages=1–11 |article-number=2003GC000631 |bibcode=2004GGG.....5.2006T |doi=10.1029/2003GC000631 |doi-access=free}}</ref> However, a different study found no evidence of a change in deep water formation, instead suggesting that deepened subtropical subduction rather than subtropical deep water formation occurred during the PETM.<ref>{{cite journal |last1=Bice |first1=Karen L. |last2=Marotzke |first2=Jochem |date=15 June 2001 |title=Numerical evidence against reversed thermohaline circulation in the warm Paleocene/Eocene ocean |journal=[[Journal of Geophysical Research]] |volume=106 |issue=C6 |pages=11529–11542 |bibcode=2001JGR...10611529B |doi=10.1029/2000JC000561 |doi-access=free |hdl=11858/00-001M-0000-0014-3AC6-D |hdl-access=free}}</ref>

Arctic freshwater input into the North Pacific could serve as a catalyst for methane hydrate destabilization, an event suggested as a precursor to the onset of the PETM.<ref name="Cope, Jesse Tiner">{{cite journal |last1=Cope |first1=Jesse T. |last2=Winguth |first2=Arne |title=On the sensitivity of ocean circulation to arctic freshwater input during the Paleocene/Eocene Thermal Maximum |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=June 2011 |volume=306 |issue=1–2 |pages=82–94 |doi=10.1016/j.palaeo.2011.03.032 |bibcode=2011PPP...306...82C }}</ref>

==Recovery==
[[Proxy (climate)|Climate proxies]], such as ocean sediments (depositional rates) indicate a duration of ~83 ka, with ~33 ka in the early rapid phase and ~50 ka in a subsequent gradual phase.<ref name=McInerney2011/>

The most likely method of recovery involves an increase in biological productivity, transporting carbon to the deep ocean. This would be assisted by higher global temperatures and {{CO2}} levels, as well as an increased nutrient supply (which would result from higher continental weathering due to higher temperatures and rainfall; volcanoes may have provided further nutrients). Evidence for higher biological productivity comes in the form of bio-concentrated [[barium]].<ref name=Bains2000>{{cite journal |author=Bains, S. |author2=Norris, R.D. |author3=Corfield, R.M. |author4=Faul, K.L. |year=2000 |title=Termination of global warmth at the Palaeocene/Eocene boundary through productivity feedback |journal=[[Nature (journal)|Nature]] |volume=407 |issue=6801 |pages=171–4 |doi=10.1038/35025035 |pmid=11001051 |bibcode=2000Natur.407..171B }}</ref> However, this proxy may instead reflect the addition of barium dissolved in methane.<ref name=Dickens2003>{{Cite book| doi=10.1130/0-8137-2369-8.11| title=Special Paper 369: Causes and consequences of globally warm climates in the early Paleogene| isbn=978-0-8137-2369-3| year=2003| last1=Dickens |first1=G. R.| last2=Fewless| first2=T.| chapter=Excess barite accumulation during the Paleocene-Eocene thermal Maximum: Massive input of dissolved barium from seafloor gas hydrate reservoirs| last3=Thomas |first3=E.| last4=Bralower |first4=T. J.| volume=369| page=11 }}</ref> Diversifications suggest that productivity increased in near-shore environments, which would have been warm and fertilized by run-off, outweighing the reduction in productivity in the deep oceans.<ref name=Kelly1998>{{cite journal |author=Kelly, D.C. |author2=Bralower, T.J. |author3=Zachos, J.C. |year=1998 |title=Evolutionary consequences of the latest Paleocene thermal maximum for tropical planktonic foraminifera |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=141 |issue=1 |pages=139–161 |doi=10.1016/S0031-0182(98)00017-0| bibcode=1998PPP...141..139K}}</ref> Large deposits in the Arctic Ocean floor of the aquatic fern Azolla in the middle Eocene (the "[[Azolla Event]]") may have been a contributory factor in the early stages of the end of the PETM by sequestering carbon in buried decayed Azolla.<ref name=Speelman2009/> Another pulse of NAIP volcanic activity may have also played a role in terminating the hyperthermal via a volcanic winter.<ref name=Stokke2020/>

==Comparison with today's climate change==
Since at least 1997, the PETM has been investigated in [[geoscience]] as an analogue to understand the [[effects of global warming]] and of massive carbon inputs to the ocean and atmosphere,<ref>{{Cite journal |last1=Bowen |first1=Gabriel J. |last2=Bralower |first2=Timothy J. |last3=Delaney |first3=Margareth L. |last4=Dickens |first4=Gerald R. |last5=Kelly |first5=Daniel C. |last6=Koch |first6=Paul L. |last7=Kump |first7=Lee R. |last8=Meng |first8=Jin |last9=Sloan |first9=Lisa C. |last10=Thomas |first10=Ellen |last11=Wing |first11=Scott L. |last12=Zachos |first12=James C. |date=25 April 2006 |title=Eocene hyperthermal event offers insight into greenhouse warming |journal=[[Eos, Transactions American Geophysical Union]] |volume=87 |issue=17 |pages=165–169 |doi=10.1029/2006EO170002 |bibcode=2006EOSTr..87..165B }}</ref><ref name="UnderAGreenSky">{{cite book |last1=Ward |first1=Peter Douglas |date=17 April 2007 |title=Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future |chapter=Back to the Eocene |location=New York |publisher=HarperCollins |pages=169–192 |isbn=978-0-06-113791-4}}</ref><ref name="KiehlEtAl2018PTRS">{{cite journal |last1=Kiehl |first1=Jeffrey T. |last2=Shields |first2=Christine A. |last3=Snyder |first3=Mark A. |last4=Zachos |first4=James C. |last5=Rothstein |first5=Mathew |date=3 September 2018 |title=Greenhouse- and orbital-forced climate extremes during the early Eocene |journal=[[Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences]] |volume=376 |issue=2130 |pages=1–24 |pmid=30177566 |pmc=6127382 |doi=10.1098/rsta.2017.0085 |bibcode=2018RSPTA.37670085K |doi-access=free}}</ref> including [[ocean acidification]].<ref name=Dickens1>{{cite journal |author=Dickens, G.R. |author2=Castillo, M.M. |author3=Walker, J.C.G. |year=1997 |title=A blast of gas in the latest Paleocene; simulating first-order effects of massive dissociation of oceanic methane hydrate |journal=[[Geology (journal)|Geology]] |volume=25 |issue=3 |pages=259–262 |pmid=11541226 |doi=10.1130/0091-7613(1997)025<0259:abogit>2.3.co;2 |bibcode=1997Geo....25..259D}}</ref> A main difference is that during the PETM, the planet was ice-free, as the [[Drake Passage]] had not yet opened and the [[Central American Seaway]] had not yet closed.<ref>{{cite web |author=Gavin |date=2009-08-10 |title=PETM Weirdness |url=http://www.realclimate.org/index.php/archives/2009/08/petm-weirdness/ |work=RealClimate |access-date=2016-02-03 |archive-url=https://web.archive.org/web/20160212041029/http://www.realclimate.org/index.php/archives/2009/08/petm-weirdness |archive-date=2016-02-12 |url-status=live}}</ref> Although the PETM is now commonly held to be a "case study" for global warming and massive carbon emission,<ref name="HaynesHönisch2020" /><ref name=McInerney2011 /><ref name=Zeebe2009>{{cite journal |author=Zeebe, R. |author2=Zachos, J.C. |author3=Dickens, G.R. |title=Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming |year=2009 |journal=[[Nature Geoscience]] |volume=2 |issue=8 |pages=576–580 |doi=10.1038/ngeo578 |bibcode=2009NatGe...2..576Z |citeseerx=10.1.1.704.7960}}</ref> the cause, details, and overall significance of the event remain uncertain.{{Citation needed|date=January 2020}}

=== Rate of carbon addition ===
Carbon emissions during the PETM were more gradual relative to present-day anthropogenic emissions.<ref>{{Cite journal |last1=Harper |first1=Dustin T. |last2=Hönisch |first2=Bärbel |last3=Bowen |first3=Gabriel J. |last4=Zeebe |first4=Richard E. |last5=Haynes |first5=Laura L. |last6=Penman |first6=Donald E. |last7=Zachos |first7=James C. |date=3 September 2024 |title=Long- and short-term coupling of sea surface temperature and atmospheric CO 2 during the late Paleocene and early Eocene |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=121 |issue=36 |article-number=e2318779121 |doi=10.1073/pnas.2318779121 |pmc=11388285 |pmid=39186648 }}</ref> Model simulations of peak carbon addition to the ocean–atmosphere system during the PETM give a probable range of 0.3–1.7 petagrams of carbon per year (Pg C/yr), which is much slower than the currently observed carbon emission rate. One petagram of carbon is equivalent to a gigaton of carbon (GtC); the current rate of carbon injection into the atmosphere is over 10 GtC/yr, a rate much greater than the carbon injection rate that occurred during the PETM.<ref>{{cite journal |journal=[[Nature Geoscience]] |doi=10.1038/ngeo1179 |year=2011 |author=Ying Cui |author2=Lee R. Kump |author3=Andy J. Ridgwell |author4=Adam J. Charles |author5=Christopher K. Junium |author6=Aaron F. Diefendorf |author7=Katherine H. Freeman |author8=Nathan M. Urban |author9=Ian C. Harding |title=Slow release of fossil carbon during the Palaeocene–Eocene Thermal Maximum |volume=4 |issue=7 |pages=481–485 |bibcode=2011NatGe...4..481C}}</ref> It has been suggested that today's methane emission regime from the ocean floor is potentially similar to that during the PETM.<ref>{{cite journal |title=The interaction of climate change and methane hydrates |year=2017 |doi=10.1002/2016RG000534 |last1=Ruppel |first1=Carolyn D. |last2=Kessler |first2=John D. |journal=[[Reviews of Geophysics]] |volume=55 |issue=1 |pages=126–168 |bibcode=2017RvGeo..55..126R |doi-access=free |hdl=1912/8978 |hdl-access=free}}</ref> Because the modern rate of carbon release exceeds the PETM's, it is speculated the a PETM-like scenario is the best-case consequence of anthropogenic global warming, with a mass extinction of a magnitude similar to the [[Cretaceous–Paleogene extinction event|Cretaceous-Palaeogene extinction event]] being a worst-case scenario.<ref>{{cite journal |last1=Keller |first1=Gerta |last2=Mateo |first2=Paula |last3=Punekar |first3=Jahnavi |last4=Khozyem |first4=Hassan |last5=Gertsch |first5=Brian |last6=Spangenberg |first6=Jorge E. |last7=Bitchong |first7=Andre Mbabi |last8=Adatte |first8=Thierry |date=April 2018 |title=Environmental changes during the Cretaceous-Paleogene mass extinction and Paleocene-Eocene Thermal Maximum: Implications for the Anthropocene |journal=[[Gondwana Research]] |volume=56 |pages=69–89 |doi=10.1016/j.gr.2017.12.002 |doi-access=free |bibcode=2018GondR..56...69K }}</ref>

=== Similarity of temperatures ===
Professor of Earth and planetary sciences [[James Zachos]] notes that IPCC projections for 2300 in the 'business-as-usual' scenario could "potentially bring global temperature to a level the planet has not seen in 50 million years" – during the early Eocene.<ref>{{cite press release |publisher=University of California – Santa Cruz |date=10 September 2020 |title=High-fidelity record of Earth's climate history puts current changes in context |url=https://phys.org/news/2020-09-high-fidelity-earth-climate-history-current.html |website=phys.org }}</ref> Some have described the PETM as arguably the best ancient analog of modern climate change.<ref>{{cite web |author1=Timothy Bralower |author2=David Bice |title=Ancient Climate Events: Paleocene Eocene Thermal Maximum |work=EARTH 103: Earth in the Future |url=https://www.e-education.psu.edu/earth103/node/639 |access-date=26 September 2021}}</ref> Scientists have investigated effects of climate change on chemistry of the oceans by exploring oceanic changes during the PETM.<ref>{{cite press release |title=Scientists draw new connections between climate change and warming oceans |url=https://phys.org/news/2018-08-scientists-climate-oceans.html |website=phys.org |publisher=University of Toronto |date=1 August 2018 }}</ref><ref>{{cite journal |last1=Yao |first1=Weiqi |last2=Paytan |first2=Adina |last3=Wortmann |first3=Ulrich G. |title=Large-scale ocean deoxygenation during the Paleocene-Eocene Thermal Maximum |journal=[[Science (journal)|Science]] |date=24 August 2018 |volume=361 |issue=6404 |pages=804–806 |doi=10.1126/science.aar8658 |pmid=30026315 |bibcode=2018Sci...361..804Y |doi-access=free }}</ref>

=== Tipping points ===
A study found that the PETM shows that substantial [[Tipping points in the climate system|climate-shifting tipping points in the Earth system]] exist, which "can trigger release of additional carbon reservoirs and drive Earth's climate into a hotter state".<ref name="Kender2021" /><ref>{{cite press release |publisher=University of Exeter |date=31 August 2021 |title='Tipping points' in Earth's system triggered rapid climate change 55 million years ago, research shows |url=https://phys.org/news/2021-08-earth-triggered-rapid-climate-million.html |website=phys.org }}</ref>

=== Climate sensitivity ===
Whether [[climate sensitivity]] was lower or higher during the PETM than today remains under debate. A 2022 study found that the Eurasian [[Inland sea (geology)|Epicontinental Sea]] acted as a major carbon sink during the PETM due to its high biological productivity and helped to slow and mitigate the warming, and that the existence of many large epicontinental seas at that time made the Earth's climate less sensitive to forcing by greenhouse gases relative to today, when much fewer epicontinental seas exist.<ref>{{cite journal |last1=Kaya |first1=Mustafa Y. |last2=Dupont-Nivet |first2=Guillaume |last3=Frieling |first3=Joost |last4=Fioroni |first4=Chiara |last5=Rohrmann |first5=Alexander |last6=Altıner |first6=Sevinç Özkan |last7=Vardar |first7=Ezgi |last8=Tanyaş |first8=Hakan |last9=Mamtimin |first9=Mehmut |last10=Zhaojie |first10=Guo |date=31 May 2022 |title=The Eurasian epicontinental sea was an important carbon sink during the Palaeocene-Eocene thermal maximum |journal=[[Communications Earth & Environment]] |volume=3 |issue=1 |page=124 |bibcode=2022ComEE...3..124K |doi=10.1038/s43247-022-00451-4 |doi-access=free |hdl=11380/1278518 |hdl-access=free}}</ref> Other research, however, suggests that climate sensitivity was higher during the PETM than today,<ref>{{Cite journal |last1=Shaffer |first1=Gary |last2=Huber |first2=Matthew |last3=Rondanelli |first3=Roberto |last4=Pepke Pedersen |first4=Jens Olaf |date=23 June 2016 |title=Deep time evidence for climate sensitivity increase with warming |journal=[[Geophysical Research Letters]] |volume=43 |issue=12 |pages=6538–6545 |doi=10.1002/2016GL069243 |bibcode=2016GeoRL..43.6538S |doi-access=free}}</ref> meaning that sensitivity to greenhouse gas release increases the higher their concentration in the atmosphere.<ref>{{cite journal |last1=Tierney |first1=Jessica E. |last2=Zhu |first2=Jiang |last3=Li |first3=Mingsong |last4=Ridgwell |first4=Andy |last5=Hakim |first5=Gregory J. |last6=Poulsen |first6=Christopher J. |last7=Whiteford |first7=Ross D. M. |last8=Rae |first8=James W. B. |last9=Kump |first9=Lee R. |date=10 October 2022 |title=Spatial patterns of climate change across the Paleocene–Eocene Thermal Maximum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=119 |issue=42 |article-number=e2205326119 |doi=10.1073/pnas.2205326119 |doi-access=free |pmid=36215472 |pmc=9586325 |bibcode=2022PNAS..11905326T }}</ref>

==See also==
{{Div col|colwidth=20em}}
*[[Abrupt climate change]]
*[[Azolla event]]
*[[Canfield ocean]]
*[[Clathrate gun hypothesis]]
*[[Climate sensitivity]]
*[[Eocene]]
*[[Eocene Thermal Maximum 2]]
*[[Paleocene]]
*[[Paleogene]]
*[[Runaway greenhouse effect]]
{{div col end}}

==References==
{{Reflist|colwidth=30em}}

==Further reading==
* {{cite journal |last1=Jardine |first1=Phil |title=Patterns in Palaeontology: The Paleocene–Eocene Thermal Maximum |journal=Palaeontology Online |date=2011 |volume=1 |issue=5 |pages=2–7 |url=https://www.palaeontologyonline.com/?p=1132 }}

==External links==
*[http://www.bbc.co.uk/programmes/b08hpmmf BBC Radio 4, ''In Our Time'', The Paleocene–Eocene Thermal Maximum, 16 March 2017]
*[https://www.youtube.com/watch?v=81Zb0pJa3Hg Global Warming 56 Million Years Ago: What it Means for Us] (Video)
{{global warming}}

{{DEFAULTSORT:Paleocene-Eocene Thermal Maximum}}
[[Category:History of climate variability and change]]
[[Category:Paleocene]]
[[Category:Eocene]]
[[Category:Paleogene]]
[[Category:Paleoclimatology]]