View source for Archean

{{Short description|Geologic eon, 4031–2500 million years ago}}
{{Distinguish|Archaea}}
{{use dmy dates|date=July 2020}}
{{Infobox geologic timespan
| name                         = {{Color|White|Archean}}
| color                        = Archean
| top_bar                      = all time
| time_start                   = {{period start|archean}}
| time_start_uncertainty       = {{period start error|archean|sign=no}}
| time_end                     = {{period end|archean}}
| image_art                    = File:Archean.png
| caption_art                  = Artist's impression of an Archean landscape
<!--Chronology-->  
| timeline                     = Eons
| former_subdivisions          =
| formerly_part_of             =
| partially_contained_in       =
| partially_contains           =
<!--Etymology-->
| chrono_name                  =
| strat_name                   =
| name_formality               = Formal
| name_accept_date             =
| alternate_spellings          = Archaean, Archæan
| synonym1                     = Eozoic
| synonym1_coined              = [[John William Dawson|J.W. Dawson]], 1865
| former_names                 =
| proposed_names               =
<!--Usage Information--> 
| celestial_body               = earth
| usage                        = Global ([[International Commission on Stratigraphy|ICS]])
| timescales_used              = ICS Time Scale
| formerly_used_by             =
| not_used_by                  =
<!--Definition-->
| chrono_unit                  = Eon
| strat_unit                   = Eonothem
| proposed_by                  = 
| timespan_formality           = Formal
| lower_boundary_def           = Ten oldest U-Pb zircon ages
| lower_gssa_location          = Along the Acasta River, [[Northwest Territories]], [[Canada]]
| lower_gssa_coords            = {{Coord|65.1738|N|115.5538|W|display=inline}}
| lower_gssa_accept_date       = {{period start ratification|archean}}
| upper_boundary_def           = Defined Chronometrically
| upper_gssa_accept_date       = {{period end ratification|archean}}
}}
The '''Archean''' ({{IPAc-en|ipa|ɑr|ˈ|k|iː|ə|n}} {{Respell|ar|KEE|ən}}, also spelled '''Archaean''' or '''Archæan'''), in older sources sometimes called the '''Archaeozoic''', is the second of the four [[geologic eon]]s of [[Earth]]'s [[history of Earth|history]], preceded by the [[Hadean Eon]] and followed by the [[Proterozoic]] and the [[Phanerozoic]]. The Archean represents the time period from {{Ma|4031|2500|Ma}} (million years ago). The [[Late Heavy Bombardment]] is hypothesized to overlap with the beginning of the Archean. The [[Pongola glaciation|oldest known glaciation]] occurred in the middle of the eon.

The Earth during the Archean was mostly a [[ocean world|water world]]: there was [[continental crust]], but much of it was under an [[ocean]] deeper than today's oceans. Except for some rare relict crystals ([[Hadean zircon]]), today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent tectonic activity. The [[Earth's atmosphere]] was also vastly different in [[atmospheric chemistry|composition]] from today's: the [[prebiotic atmosphere]] was a [[reducing atmosphere]] rich in [[atmospheric methane|methane]] and lacking free [[oxygen]].

The [[earliest known life forms|earliest known life]], mostly represented by shallow-water [[microbial mat]]s called [[stromatolite]]s, started in the Archean and remained simple [[prokaryote]]s ([[archaea]] and [[bacteria]]) throughout the eon. The earliest [[photosynthetic]] processes, especially those by early [[cyanobacteria]], appeared in the mid/late Archean and led to [[Great Oxidation Event|a permanent chemical change]] in the ocean and the atmosphere after the Archean.

==Etymology and changes in classification==
The word ''Archean'' is derived from the Greek word {{lang|grc-Latn|arkhē}} ({{wikt-lang|grc|ἀρχή}}), meaning 'beginning, origin'.<ref>{{OEtymD|Archaean}}</ref> The [[Pre-Cambrian]] had been believed to be without life (azoic); however, fossils were found in deposits that were judged to belong to the [[Azoic age]]. Before the Hadean Eon was recognized, the Archean spanned Earth's early history from its formation about 4,540&nbsp;Ma until 2,500&nbsp;Ma.

Instead of being based on [[stratigraphy]], the beginning and end of the Archean Eon are defined [[Chronometric dating|chronometrically]]. The eon's lower boundary or starting point of 4,031±3&nbsp;Ma is officially recognized by the [[International Commission on Stratigraphy]],<ref name="GSSP"/> which is the age of the oldest known intact rock formations on Earth. Evidence of rocks from the preceding Hadean Eon are therefore restricted by definition to non-rock and non-terrestrial sources such as individual mineral grains and lunar samples--

==Geology==
When the Archean began, the Earth's [[Earth's internal heat budget|heat flow]] was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500&nbsp;{{tooltip|2=Megaannum, or 1 million years|Ma}}). The extra heat was partly remnant heat from [[Accretion (astrophysics)|planetary accretion]], from the formation of the [[structure of the Earth|metallic core]], and partly arose from the decay of [[radioactive]] elements. As a result, the Earth's mantle was significantly hotter than today.<ref>{{cite journal |last1=Galer |first1=Stephen J. G. |last2=Mezger |first2=Klaus |date=1 December 1998 |title=Metamorphism, denudation and sea level in the Archean and cooling of the Earth |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926898000837 |journal=[[Precambrian Research]] |volume=92 |issue=4 |pages=389–412 |doi=10.1016/S0301-9268(98)00083-7 |bibcode=1998PreR...92..389G |access-date=24 November 2022|url-access=subscription }}</ref>
[[File:Evolution of Earth's radiogenic heat.svg|thumb|upright=1.5|center|The evolution of Earth's [[Radiogenic nuclide|radiogenic heat]] flow over time]]

Although a few mineral grains have survived from the [[Hadean]], the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in [[Geology of Greenland|Greenland]], [[Geology of Siberia|Siberia]], the [[Canadian Shield]], [[Geology of Montana|Montana]], [[Geology of Wyoming|Wyoming]] (exposed parts of the [[Wyoming craton|Wyoming Craton]]), [[Geology of Minnesota|Minnesota]] (Minnesota River Valley), the [[Baltic Shield]], the [[Geology of Bulgaria|Rhodope Massif]], [[Geology of Scotland|Scotland]], [[Geological history of India|India]], [[Geology of Brazil|Brazil]], western [[Geology of Australia|Australia]], and southern [[Geology of Africa|Africa]].{{Citation needed|date=May 2020}} [[Granitic]] rocks predominate throughout the crystalline remnants of the surviving Archean crust. These include great melt sheets and voluminous [[intrusive rock|plutonic]] masses of [[granite]], [[diorite]], [[layered intrusion]]s, [[anorthosite]]s and [[monzonite]]s known as [[sanukitoid]]s. Archean rocks are often heavily metamorphosed deep-water sediments, such as [[graywacke]]s, [[mudstone]]s, volcanic sediments, and [[banded iron formation]]s. [[Volcanic]] activity was considerably higher than today, with numerous lava eruptions, including unusual types such as [[komatiite]].<ref name=Dostal2008>{{cite journal |vauthors=Dostal J |year=2008 |title=Igneous Rock Associations 10. Komatiites |journal=Geoscience Canada |volume=35 |issue=1 |url=https://journals.lib.unb.ca/index.php/GC/article/view/11074/11722}}</ref> [[Carbonate]] rocks are rare, indicating that the oceans were more acidic, due to dissolved [[carbon dioxide]], than during the Proterozoic.<ref>{{cite book |last1=Cooper |first1=John D. |last2=Miller |first2=Richard H. |last3=Patterson |first3=Jacqueline |date=1986 |title=A Trip Through Time: Principles of historical geology |publisher=Merrill Publishing Company |location=Columbus |isbn=978-0-675-20140-7 |page=[https://archive.org/details/tripthroughtimep0000coop/page/180 180] |url=https://archive.org/details/tripthroughtimep0000coop |url-access=registration}}</ref> [[Greenstone belt]]s are typical Archean formations, consisting of alternating units of metamorphosed [[mafic]] igneous and sedimentary rocks, including [[Archean felsic volcanic rocks]]. The metamorphosed igneous rocks were derived from volcanic [[island arc]]s, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a [[forearc]] basin. Greenstone belts, which include both types of metamorphosed rock, represent [[suture (geology)|sutures]] between the protocontinents.<ref name=Stanley1999>{{cite book |last=Stanley |first=Steven M. |year=1999 |title=Earth System History |publisher=W.H. Freeman and Company |location=New York |isbn=978-0-7167-2882-5}}</ref>{{rp|pages=302–303}}

[[Plate tectonics]] likely started vigorously in the [[Hadean]], but slowed down in the Archean.<ref name=Korenaga2021>{{cite journal |last=Korenaga |first=J |year=2021 |title=Was There Land on the Early Earth? |journal=Life |doi=10.3390/life11111142 |doi-access=free |pmid=34833018 |pmc=8623345 |volume=11 |issue=11 |page=1142|bibcode=2021Life...11.1142K }}</ref><ref>{{cite journal |last=Korenaga |first=J |year=2021 |title=Hadean geodynamics and the nature of early continental crust |journal=[[Precambrian Research]] |doi=10.1016/j.precamres.2021.106178 |bibcode=2021PreR..35906178K |s2cid=233441822 |volume=359 |article-number=106178}}</ref> The slowing of plate tectonics was probably due to an increase in the viscosity of the [[mantle (geology)|mantle]] due to outgassing of its water.<ref name=Korenaga2021/> Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely.<ref>{{cite journal |last1=Bada |first1=J. L. |last2=Korenaga |first2=J. |year=2018 |title=Exposed areas above sea level on Earth >3.5 Gyr ago: Implications for prebiotic and primitive biotic chemistry |journal=Life |doi=10.3390/life8040055 |doi-access=free |pmid=30400350 |pmc=6316429 |volume=8 |issue=4 |page=55|bibcode=2018Life....8...55B }}</ref> Only at the end of the Archean did the continents likely emerge from the ocean.<ref name="BindemanEtAl2018">{{cite journal |last1=Bindeman |first1=I. N. |last2=Zakharov |first2=D. O. |last3=Palandri |first3=J. |last4=Greber |first4=N. D. |last5=Dauphas |first5=N. |last6=Retallack |first6=Gregory J. |last7=Hofmann |first7=A. |last8=Lackey |first8=J. S. |last9=Bekker |first9=A. |date=23 May 2018 |url=https://www.nature.com/articles/s41586-018-0131-1 |title=Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago |journal=[[Nature (journal)|Nature]] |doi=10.1038/s41586-018-0131-1 |pmid=29795252 |bibcode=2018Natur.557..545B |s2cid=43921922 |volume=557 |issue=7706 |pages=545–548 |access-date=25 December 2023|url-access=subscription }}</ref> The emergence of continents towards the end of the Archaean initiated continental weathering that left its mark on the oxygen isotope record by enriching seawater with isotopically light oxygen.<ref>{{Cite journal |last1=Johnson |first1=Benjamin W. |last2=Wing |first2=Boswell A. |date=2 March 2020 |title=Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean |url=https://www.nature.com/articles/s41561-020-0538-9 |journal=[[Nature Geoscience]] |language=en |volume=13 |issue=3 |pages=243–248 |doi=10.1038/s41561-020-0538-9 |bibcode=2020NatGe..13..243J |s2cid=211730235 |issn=1752-0908 |access-date=25 December 2023|url-access=subscription }}</ref>

Due to recycling and metamorphosis of the Archean crust, there is a lack of extensive geological evidence for specific continents. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called [[Ur (continent)|Ur]] as of 3,100&nbsp;Ma.<ref>{{cite journal |vauthors=Rogers JJ |year=1996 |title=A history of continents in the past three billion years |journal=Journal of Geology |doi=10.1086/629803 |jstor=30068065 |bibcode=1996JG....104...91R |s2cid=128776432 |volume=104 |issue=1 |pages=91–107}}</ref> Another hypothesis, which conflicts with the first, is that rocks from western Australia and southern Africa were assembled in a continent called [[Vaalbara]] as far back as 3,600&nbsp;Ma.<ref>{{cite journal |vauthors=Cheney ES |year=1996 |title=Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia |journal=Precambrian Research |doi=10.1016/0301-9268(95)00085-2 |bibcode=1996PreR...79....3C |volume=79 |issue=1–2 |pages=3–24}}</ref> Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled.<ref name=Korenaga2021/>

By the [[Neoarchean]], plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of [[slab detachment]] resulting from a hotter mantle, [[rheology|rheologically]] weaker plates, and increased tensile stresses on [[Subduction|subducting]] plates due to their crustal material metamorphosing from [[basalt]] into [[eclogite]] as they sank.<ref name=MartyDauphas2003>{{cite journal |last1=Marty |first1=Bernard |last2=Dauphas |first2=Nicolas |date=15 February 2003 |title=The nitrogen record of crust–mantle interaction and mantle convection from Archean to Present |journal=[[Earth and Planetary Science Letters]] |doi=10.1016/S0012-821X(02)01108-1 |volume=206 |issue=3–4 |pages=397–410 |bibcode=2003E&PSL.206..397M |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X02011081 |access-date=16 November 2022|url-access=subscription }}</ref><ref name=HallaEtAl2009>{{cite journal |last1=Halla |first1=Jaana |last2=Van Hunen |first2=Jeroen |last3=Heilimo |first3=Esa |last4=Hölttä |first4=Pentti |date=October 2009 |title=Geochemical and numerical constraints on Neoarchean plate tectonics |journal=[[Precambrian Research]] |doi=10.1016/j.precamres.2009.07.008 |volume=174 |issue=1–2 |pages=155–162 |bibcode=2009PreR..174..155H |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926809001661#! |access-date=12 November 2022|url-access=subscription }}</ref> There are well-preserved [[sedimentary basin]]s, and evidence of [[volcanic arc]]s, intracontinental [[rift]]s, continent-continent collisions and widespread globe-spanning [[Orogeny|orogenic events]] suggesting the assembly and destruction of one and perhaps several [[supercontinent]]s. Evidence from banded iron formations, [[chert]] beds, chemical sediments and [[Basalt|pillow basalts]] demonstrates that liquid water was prevalent and deep oceanic basins already existed.

Asteroid impacts were frequent in the early Archean.<ref>{{cite journal |last1=Borgeat |first1=Xavier |last2=Tackley |first2=Paul J. |date=12 July 2022 |title=Hadean/Eoarchean tectonics and mantle mixing induced by impacts: a three-dimensional study |journal=Progress in Earth and Planetary Science |doi=10.1186/s40645-022-00497-0 |s2cid=243973728 |volume=9 |issue=1 |page=38 |bibcode=2022PEPS....9...38B |hdl=20.500.11850/559385 |hdl-access=free |doi-access=free }}</ref> Evidence from [[spherule]] layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with a diameter greater than {{convert|10|km|sigfig=1|sp=us}} every 15 million years. This is about the size of the [[Chicxulub crater|Chicxulub]] impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.<ref name=MarchiEtal2021>{{cite journal |last1=Marchi |first1=S. |last2=Drabon |first2=N. |last3=Schulz |first3=T. |last4=Schaefer |first4=L. |last5=Nesvorny |first5=D. |last6=Bottke |first6=W. F. |last7=Koeberl |first7=C. |last8=Lyons |first8=T. |date=November 2021 |title=Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth |url=https://www.nature.com/articles/s41561-021-00835-9 |journal=[[Nature Geoscience]] |doi=10.1038/s41561-021-00835-9 |bibcode=2021NatGe..14..827M |s2cid=239055744 |volume=14 |issue=11 |pages=827–831 |access-date=25 December 2023|url-access=subscription }}</ref>

==Environment==
[[File:NASA-EarlyEarth-PaleOrangeDot-20190802.jpg|thumb|250px|''The pale orange dot'', an artist's impression of the [[early Earth]] which is believed to have appeared orange through its [[haze|hazy]], [[methane]] rich, [[prebiotic atmosphere|prebiotic second atmosphere]]. Earth's atmosphere at this stage was somewhat comparable to today's [[atmosphere of Titan]].<ref>{{cite journal |last1=Trainer |first1=Melissa G. |last2=Pavlov |first2=Alexander A. |last3=DeWitt |first3=H. Langley |last4=Jimenez |first4=Jose L. |last5=McKay |first5=Christopher P. |last6=Toon |first6=Owen B. |last7=Tolbert |first7=Margaret A. |date=2006-11-28 |title=Organic haze on Titan and the early Earth |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |issn=0027-8424 |doi=10.1073/pnas.0608561103 |doi-access=free |pmid=17101962 |pmc=1838702 |volume=103 |issue=48 |pages=18035–18042}}</ref>]]

The Archean atmosphere is thought to have almost completely lacked [[Dioxygen in biological reactions|free oxygen]]; oxygen levels were less than 0.001% of their present atmospheric level,<ref name=AnoxicArchaeanAtmosphere>{{cite journal |last1=Pavlov |first1=A. A. |last2=Kasting |first2=J. F. |date=5 Jul 2004 |title=Mass-Independent Fractionation of Sulfur Isotopes in Archean Sediments: Strong Evidence for an Anoxic Archean Atmosphere |journal=[[Astrobiology (journal)|Astrobiology]] |doi=10.1089/153110702753621321 |pmid=12449853 |volume=2 |issue=1 |pages=27–41 |bibcode=2002AsBio...2...27P |url=https://www.liebertpub.com/doi/abs/10.1089/153110702753621321 |access-date=12 November 2022|url-access=subscription }}</ref><ref>{{cite journal |last1=Zhang |first1=Shuichang |last2=Wang |first2=Xiaomei |last3=Wang |first3=Huajian |last4=Bjerrum |first4=Christian J. |last5=Hammarlund |first5=Emma U. |last6=Costa |first6=M. Mafalda |last7=Connelly |first7=James N. |last8=Zhang |first8=Baomin |last9=Su |first9=Jin |last10=Canfield |first10=Donald Eugene |date=4 January 2016 |title=Sufficient oxygen for animal respiration 1,400 million years ago |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |doi=10.1073/pnas.1523449113 |doi-access=free |pmid=26729865 |pmc=4763753 |bibcode=2016PNAS..113.1731Z |volume=113 |issue=7 |pages=1731–1736}}</ref> with some analyses suggesting they were as low as 0.00001% of modern levels.<ref name=AtmosphericOxygenTheory>{{cite journal |last1=Laakso |first1=T. A. |last2=Schrag |first2=D. P. |date=5 April 2017 |title=A theory of atmospheric oxygen |journal=[[Geobiology (journal)|Geobiology]] |doi=10.1111/gbi.12230 |pmid=28378894 |s2cid=22594748 |volume=15 |issue=3 |pages=366–384 |bibcode=2017Gbio...15..366L }}</ref> However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980–2,960 Ma,<ref name=Oxygen3Ga>{{cite journal |last1=Crowe |first1=Sean A. |last2=Døssing |first2=Lasse N. |last3=Beukes |first3=Nicolas J. |last4=Bau |first4=Michael |last5=Kruger |first5=Stephanus J. |last6=Frei |first6=Robert |last7=Canfield |first7=Donald Eugene |date=25 September 2013 |title=Atmospheric oxygenation three billion years ago |journal=[[Nature (journal)|Nature]] |doi=10.1038/nature12426 |pmid=24067713 |s2cid=4464710 |volume=501 |issue=7468 |pages=535–538 |bibcode=2013Natur.501..535C |url=https://www.nature.com/articles/nature12426#citeas |access-date=12 November 2022|url-access=subscription }}</ref> 2,700 Ma,<ref name=LargeEtAl2022>{{cite journal |last1=Large |first1=Ross R. |last2=Hazen |first2=Robert M. |last3=Morrison |first3=Shaunna M. |last4=Gregory |first4=Dan D. |last5=Steadman |first5=Jeffrey A. |last6=Mukherjee |first6=Indrani |date=May 2022 |title=Evidence that the GOE was a prolonged event with a peak around 1900 Ma |journal=Geosystems and Geoenvironment |doi=10.1016/j.geogeo.2022.100036 |s2cid=246755121 |volume=1 |issue=2 |article-number=100036 |bibcode=2022GsGe....100036L |doi-access=free }}</ref> and 2,501 Ma.<ref name=WhiffOfOxygen>{{cite journal |last1=Anbar |first1=Ariel D. |last2=Duan |first2=Yun |last3=Lyons |first3=Timothy W. |last4=Arnold |first4=Gail N. |last5=Kendall |first5=Brian |last6=Creaser |first6=Robert A. |last7=Kaufman |first7=Alan J. |last8=Gordon |first8=Gwyneth W. |last9=Scott |first9=Clinton |last10=Garvin |first10=Jessica |last11=Buick |first11=Roger |date=28 September 2007 |title=A Whiff of Oxygen Before the Great Oxidation Event? |journal=[[Science (journal)|Science]] |doi=10.1126/science.1140325 |pmid=17901330 |s2cid=25260892 |volume=317 |issue=5846 |pages=1903–1906 |bibcode=2007Sci...317.1903A |url=https://www.science.org/doi/10.1126/science.1140325 |access-date=12 November 2022|url-access=subscription }}</ref><ref name=ArchaeanOxidativeWeathering>{{cite journal |last1=Reinhard |first1=Christopher T. |last2=Raiswell |first2=Robert |last3=Scott |first3=Clinton |last4=Anbar |first4=Ariel D. |last5=Lyons |first5=Timothy W. |date=30 October 2009 |title=A Late Archean Sulfidic Sea Stimulated by Early Oxidative Weathering of the Continents |journal=[[Science (journal)|Science]] |volume=326 |issue=5953 |pages=713–716 |doi=10.1126/science.1176711 |pmid=19900929 |bibcode=2009Sci...326..713R |s2cid=25369788 |url=https://www.science.org/doi/10.1126/science.1176711 |access-date=12 November 2022|url-access=subscription }}</ref> The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of the [[Great Oxygenation Event]],<ref name=LargeEtAl2022/><ref>{{cite journal |last1=Warke |first1=Matthew R. |last2=Di Rocco |first2=Tommaso |last3=Zerkle |first3=Aubrey L. |last4=Lepland |first4=Aivo |last5=Prave |first5=Anthony R. |last6=Martin |first6=Adam P. |last7=Ueno |first7=Yuichiro |last8=Condon |first8=Daniel J. |last9=Claire |first9=Mark W. |date=2020-06-16 |title=The Great Oxidation Event preceded a Paleoproterozoic "snowball Earth" |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |issn=0027-8424 |doi=10.1073/pnas.2003090117 |doi-access=free |pmc=7306805 |pmid=32482849 |volume=117 |issue=24 |pages=13314–13320|bibcode=2020PNAS..11713314W }}</ref> which most scholars consider to have begun in the [[Palaeoproterozoic]]  ({{circa|2,400 Ma}}).<ref>{{cite journal |last1=Luo |first1=Genming |last2=Ono |first2=Shuhei |last3=Beukes |first3=Nicolas J. |last4=Wang |first4=David T. |last5=Xie |first5=Shucheng |last6=Summons |first6=Roger E. |date=2016-05-06 |title=Rapid oxygenation of Earth's atmosphere 2.33 billion years ago |journal=[[Science Advances]] |issn=2375-2548 |doi=10.1126/sciadv.1600134 |pmc=4928975 |pmid=27386544 |volume=2 |issue=5 |article-number=e1600134|bibcode=2016SciA....2E0134L }}</ref><ref>{{cite journal |last1=Poulton |first1=Simon W. |last2=Bekker |first2=Andrey |last3=Cumming |first3=Vivien M. |last4=Zerkle |first4=Aubrey L. |last5=Canfield |first5=Donald E. |last6=Johnston |first6=David T. |date=April 2021 |title=A 200-million-year delay in permanent atmospheric oxygenation |journal=Nature |issn=1476-4687 |doi=10.1038/s41586-021-03393-7 |pmid=33782617 |s2cid=232419035 |volume=592 |issue=7853 |pages=232–236 |bibcode=2021Natur.592..232P |url=https://www.nature.com/articles/s41586-021-03393-7 |access-date=7 January 2023|hdl=10023/24041 |hdl-access=free |url-access=subscription }}</ref><ref name=GumsleyEtAl2017PNAS>{{cite journal |last1=Gumsley |first1=Ashley P. |last2=Chamberlain |first2=Kevin R. |last3=Bleeker |first3=Wouter |last4=Söderlund |first4=Ulf |last5=De Kock |first5=Michiel O. |last6=Larsson |first6=Emilie R. |last7=Bekker |first7=Andrey |date=6 February 2017 |title=Timing and tempo of the Great Oxidation Event |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |issn=0027-8424 |doi=10.1073/pnas.1608824114 |doi-access=free |pmc=5338422 |pmid=28167763 |volume=114 |issue=8 |pages=1811–1816|bibcode=2017PNAS..114.1811G }}</ref> Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean.<ref>{{cite journal |last1=Eickmann |first1=Benjamin |last2=Hofmann |first2=Axel |last3=Wille |first3=Martin |last4=Bui |first4=Thi Hao |last5=Wing |first5=Boswell A. |last6=Schoenberg |first6=Ronny |date=15 January 2018 |title=Isotopic evidence for oxygenated Mesoarchaean shallow oceans |journal=[[Nature Geoscience]] |doi=10.1038/s41561-017-0036-x |s2cid=135023426 |volume=11 |issue=2 |pages=133–138 |bibcode=2018NatGe..11..133E |url=https://www.nature.com/articles/s41561-017-0036-x?error=cookies_not_supported&code=54e99c94-9890-439d-a813-c26d056ce863 |access-date=25 December 2022|url-access=subscription }}</ref> The ocean was broadly [[reducing agent|reducing]] and lacked any persistent [[redoxcline]], a water layer between oxygenated and anoxic layers with a strong [[reduction–oxidation|redox]] gradient, which would become a feature in later, more oxic oceans.<ref>{{cite journal |last1=Zhou |first1=Hang |last2=Zhou |first2=Wenxiao |last3=Wei |first3=Yunxu |last4=Chi Fru |first4=Ernest |last5=Huang |first5=Bo |last6=Fu |first6=Dong |last7=Li |first7=Haiquan |last8=Tan |first8=Mantang |date=December 2022 |title=Mesoarchean banded iron-formation from the northern Yangtze Craton, South China and its geological and paleoenvironmental implications |journal=[[Precambrian Research]] |doi=10.1016/j.precamres.2022.106905 |volume=383 |article-number=106905 |bibcode=2022PreR..38306905Z |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926822003497 |access-date=17 December 2022|url-access=subscription }}</ref> Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present.<ref>{{cite journal |last1=Fischer |first1=W. W. |last2=Schroeder |first2=S. |last3=Lacassie |first3=J. P. |last4=Beukes |first4=N. J. |last5=Goldberg |first5=T. |last6=Strauss |first6=H. |last7=Horstmann |first7=U. E. |last8=Schrag |first8=D. P. |last9=Knoll |first9=A. H. |date=March 2009 |title=Isotopic constraints on the Late Archean carbon cycle from the Transvaal Supergroup along the western margin of the Kaapvaal Craton, South Africa |journal=[[Precambrian Research]] |doi=10.1016/j.precamres.2008.10.010 |volume=169 |issue=1–4 |pages=15–27 |bibcode=2009PreR..169...15F |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926808002490 |access-date=24 November 2022|url-access=subscription }}</ref> Due to extremely low oxygen levels, sulphate was rare in the Archean ocean, and sulphides were produced primarily through reduction of organically sourced sulphite or through mineralisation of compounds containing reduced sulphur.<ref>{{cite journal |last1=Fakhraee |first1=Mojtaba |last2=Katsev |first2=Sergei |date=7 October 2019 |title=Organic sulfur was integral to the Archean sulfur cycle |journal=[[Nature Communications]] |doi=10.1038/s41467-019-12396-y |pmid=31591394 |pmc=6779745 |volume=10 |issue=1 |page=4556|bibcode=2019NatCo..10.4556F }}</ref> The Archean ocean was enriched in heavier oxygen isotopes relative to the modern ocean, though [[δ18O]] values decreased to levels comparable to those of modern oceans over the course of the later part of the eon as a result of increased continental weathering.<ref>{{cite journal |last1=Johnson |first1=Benjamin W. |last2=Wing |first2=Boswell A. |date=2 March 2020 |title=Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean |journal=[[Nature Geoscience]] |doi=10.1038/s41561-020-0538-9 |s2cid=211730235 |volume=13 |issue=3 |pages=243–248 |bibcode=2020NatGe..13..243J |url=https://www.nature.com/articles/s41561-020-0538-9?error=cookies_not_supported&code=bee768fe-63d9-40f0-9508-cddc3686d03a |access-date=7 January 2023|url-access=subscription }}</ref>

Astronomers think that the Sun had about 75–80&nbsp;percent of its present luminosity,<ref name=DauphasKasting2011>{{cite journal |last1=Dauphas |first1=Nicolas |last2=Kasting |first2=James Fraser |date=1 June 2011 |title=Low pCO2 in the pore water, not in the Archean air |journal=Nature |doi=10.1038/nature09960 |pmid=21637211 |s2cid=205224575 |volume=474 |issue=7349 |pages=E2-3; discussion E4-5 |bibcode=2011Natur.474E...1D |doi-access=free }}</ref> yet temperatures on Earth appear to have been near modern levels only 500&nbsp;million years after Earth's formation (the [[faint young Sun paradox]]). The presence of liquid water is evidenced by certain highly deformed [[gneiss]]es produced by metamorphism of [[sediment]]ary [[protolith]]s. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history.<ref name=Walker1982>{{cite journal |last1=Walker |first1=James C. G. |date=November 1982 |title=Climatic factors on the Archean earth |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |doi=10.1016/0031-0182(82)90082-7 |hdl=2027.42/23810 |volume=40 |issue=1–3 |pages=1–11 |bibcode=1982PPP....40....1W |url=https://www.sciencedirect.com/science/article/abs/pii/0031018282900827 |access-date=12 November 2022|hdl-access=free }}</ref><ref name=Walker1985>{{cite journal |last=Walker |first=James C.G. |date=June 1985 |title=Carbon dioxide on the early earth |journal=[[Origins of Life and Evolution of Biospheres]] |doi=10.1007/BF01809466 |bibcode=1985OrLi...16..117W |hdl=2027.42/43349 |hdl-access=free |pmid=11542014 |s2cid=206804461 |volume=16 |issue=2 |pages=117–127 |url=http://deepblue.lib.umich.edu/bitstream/2027.42/43349/1/11084_2005_Article_BF01809466.pdf |access-date=30 January 2010 }}</ref><ref name=Pavlov2000>{{cite journal |vauthors=Pavlov AA, Kasting JF, Brown LL, Rages KA, Freedman R |date=May 2000 |title=Greenhouse warming by CH<sub>4</sub> in the atmosphere of early Earth |journal=[[Journal of Geophysical Research]] |doi=10.1029/1999JE001134 |doi-access=free |bibcode=2000JGR...10511981P |pmid=11543544 |volume=105 |issue=E5 |pages=11981–11990}}</ref> Extensive abiotic denitrification took place on the Archean Earth, pumping the greenhouse gas [[nitrous oxide]] into the atmosphere.<ref>{{cite journal |last1=Buessecker |first1=Steffen |last2=Imanaka |first2=Hiroshi |last3=Ely |first3=Tucker |last4=Hu |first4=Renyu |last5=Romaniello |first5=Stephen J. |last6=Cadillo-Quiroz |first6=Hinsby |date=5 December 2022 |title=Mineral-catalysed formation of marine NO and N2O on the anoxic early Earth |url=https://www.nature.com/articles/s41561-022-01089-9 |journal=[[Nature Geoscience]] |volume=15 |issue=1 |pages=1056–1063 |doi=10.1038/s41561-022-01089-9 |bibcode=2022NatGe..15.1056B |s2cid=254276951 |access-date=28 April 2023|url-access=subscription }}</ref> Alternatively, Earth's [[albedo]] may have been lower at the time, due to less land area and cloud cover.<ref name=Rosing>{{cite journal |vauthors=Rosing MT, Bird DK, Sleep NH, Bjerrum CJ |date=April 2010 |title=No climate paradox under the faint early Sun |journal=[[Nature (journal)|Nature]] |pmid=20360739 |doi=10.1038/nature08955 |bibcode=2010Natur.464..744R |s2cid=205220182 |volume=464 |issue=7289 |pages=744–747}}</ref>

==Early life==
{{Main|Earliest known life forms}}

{{For|details on how life got started|Abiogenesis}}
The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon  or early in the Archean Eon.

The earliest evidence for life on Earth is [[graphite]] of [[Biogenic substance|biogenic]] origin found in 3.7&nbsp;billion–year-old [[metasedimentary rock]]s discovered in [[Kitaa|Western Greenland]].<ref name=NG-20131208>{{cite journal |vauthors=Ohtomo Y, Kakegawa T, Ishida A, Nagase T, Rosing MT |date=8 December 2013 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=Nature Geoscience |doi=10.1038/ngeo2025 |bibcode=2014NatGe...7...25O |volume=7 |issue=1 |pages=25–28}}</ref>

[[File:Lake Thetis-Stromatolites-LaRuth.jpg|thumb|right|[[Lithification|Lithified]] [[stromatolite]]s on the shores of [[Lake Thetis]], [[Western Australia]]. Archean stromatolites are the first direct fossil traces of life on Earth.]]

The earliest identifiable fossils consist of [[stromatolite]]s, which are [[microbial mats]] formed in shallow water by [[cyanobacteria]]. The earliest stromatolites are found in 3.48&nbsp;billion-year-old [[sandstone]] discovered in [[Western Australia]].<ref name=AP-20131113>{{cite news |last=Borenstein |first=Seth |agency=[[AP News]] |date=13 November 2013 |title=Oldest fossil found: Meet your microbial mom |url=http://apnews.excite.com/article/20131113/DAA1VSC01.html |access-date=15 November 2013}}</ref><ref name=AST-20131108>{{cite journal |vauthors=[[Nora Noffke|Noffke N]], Christian D, Wacey D, Hazen RM |date=December 2013 |title=Microbially induced sedimentary structures recording an ancient ecosystem in the ca.&nbsp;3.48&nbsp;billion-year-old Dresser Formation, Pilbara, Western Australia |journal=Astrobiology |pmid=24205812 |pmc=3870916 |doi=10.1089/ast.2013.1030 |bibcode=2013AsBio..13.1103N |volume=13 |issue=12 |pages=1103–1124}}</ref> Stromatolites are found throughout the Archean<ref name=PiP>{{cite journal |last=Garwood |first=Russell J. |year=2012 |title=Patterns In Palaeontology: The first 3&nbsp;billion years of evolution |journal=Palaeontology Online |volume=2 |issue=11 |pages=1–14 |url=http://www.palaeontologyonline.com/articles/2012/patterns-in-palaeontology-the-first-3-billion-years-of-evolution/ |access-date=June 25, 2015 }}</ref> and become common late in the Archean.<ref name=Stanley1999/>{{rp|page=307}} Cyanobacteria were instrumental in creating free oxygen in the atmosphere.{{citation needed|date=March 2023|reason=There was a citation here but it turned out to a page that was using Wikipedia's Great Oxidation Event article as source; so not a good source.}}

Further evidence for early life is found in 3.47&nbsp;billion-year-old [[baryte]], in the [[Warrawoona Group]] of Western Australia. This mineral shows sulfur [[fractionation]] of as much as 21.1%,<ref>{{cite journal |vauthors=Shen Y, Buick R, Canfield DE |date=March 2001 |title=Isotopic evidence for microbial sulphate reduction in the early Archaean era |journal=Nature |pmid=11242044 |doi=10.1038/35065071 |bibcode=2001Natur.410...77S |s2cid=25375808 |volume=410 |issue=6824 |pages=77–81}}</ref> which is evidence of [[sulfate-reducing bacteria]] that metabolize [[Isotopes of sulfur|sulfur-32]] more readily than sulfur-34.<ref>{{cite journal |vauthors=Seal RR |year=2006 |title=Sulfur isotope geochemistry of sulfide minerals |journal=Reviews in Mineralogy and Geochemistry |doi=10.2138/rmg.2006.61.12 |bibcode=2006RvMG...61..633S |volume=61 |issue=1 |pages=633–677 |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1354&context=usgsstaffpub|url-access=subscription }}</ref>

Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in [[zircon]]s dated to 4.1&nbsp;billion years ago, but this evidence is preliminary and needs validation.<ref name=AP-20151019>{{cite news |last=Borenstein |first=Seth |agency=[[Associated Press]] |date=19 October 2015 |title=Hints of life on what was thought to be desolate early Earth |website=[[Excite (web portal)|Excite]] |publisher=[[Mindspark Interactive Network]] |location=Yonkers, NY |url=http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html |access-date=2015-10-20 }}</ref><ref name=PNAS-20151014-pdf>{{cite journal |vauthors=Bell EA, Boehnke P, Harrison TM, [[Wendy Mao|Mao WL]] |date=November 2015 |title=Potentially biogenic carbon preserved in a 4.1&nbsp;billion-year-old zircon |edition=Early, published online before print |journal=Proceedings of the National Academy of Sciences of the United States of America |pmid=26483481 |pmc=4664351 |doi=10.1073/pnas.1517557112 |doi-access=free |bibcode=2015PNAS..11214518B |volume=112 |issue=47 |pages=14518–14521}}</ref>

Earth was very hostile to life before 4,300 to 4,200 Ma, and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before the Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon.<ref>{{cite journal |last=Nisbet |first=Euan |year=1980 |title=Archaean stromatolites and the search for the earliest life |journal=Nature |bibcode=1980Natur.284..395N |doi=10.1038/284395a0 |s2cid=4262249 |volume=284 |issue=5755 |pages=395–396}}</ref>

Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called [[prokaryote]]s. In addition to the domain [[Bacteria]], microfossils of the domain [[Archaea]] have also been identified. There are no known [[eukaryote|eukaryotic]] fossils from the earliest Archean, though they might have evolved during the Archean without leaving any.<ref name=Stanley1999/>{{rp|pages=306,323}} Fossil [[sterane]]s, indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter.<ref>{{cite journal |vauthors=French KL, Hallmann C, Hope JM, Schoon PL, Zumberge JA, Hoshino Y, Peters CA, George SC, Love GD, Brocks JJ, Buick R, Summons RE |date=May 2015 |title=Reappraisal of hydrocarbon biomarkers in Archean rocks |journal=Proceedings of the National Academy of Sciences of the United States of America |pmid=25918387 |pmc=4434754 |doi=10.1073/pnas.1419563112 |doi-access=free |bibcode=2015PNAS..112.5915F |volume=112 |issue=19 |pages=5915–5920}}</ref> No fossil evidence has been discovered for [[ultramicroscopic]] [[intracellular]] replicators such as [[virus]]es.

Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22&nbsp;Ga.<ref>{{cite journal |last1=Homann |first1=Martin |last2=Sansjofre |first2=Pierre |last3=Van Zuilen |first3=Mark |last4=Heubeck |first4=Christoph |last5=Gong |first5=Jian |last6=Killingsworth |first6=Bryan |last7=Foster |first7=Ian S. |last8=Airo |first8=Alessandro |last9=Van Kranendonk |first9=Martin J. |last10=Ader |first10=Magali |last11=Lalonde |first11=Stefan V. |date=23 July 2018 |title=Microbial life and biogeochemical cycling on land 3,220 million years ago |journal=[[Nature Geoscience]] |doi=10.1038/s41561-018-0190-9 |s2cid=134935568 |volume=11 |issue=9 |pages=665–671 |bibcode=2018NatGe..11..665H |url=https://www.nature.com/articles/s41561-018-0190-9?error=cookies_not_supported&code=de9a5c60-153f-4b56-a4d4-e0ae6ad9222d |access-date=14 January 2023}}</ref><ref>{{cite web |last=Woo |first=Marcus |date=30 July 2018 |title=Oldest Evidence for life on land unearthed in South Africa |website=Live Science |url=https://www.livescience.com/63199-oldest-life-on-land.html}}</ref>

''[[Thuchomyces]]'', a Mesoarchean-Neoarchean fossil from South Africa, may be the first evidence of macroscopic land life, however it is likely a microbial mat due to a lack of eukaryotic features.<ref>{{cite journal |last1=Lücking |first1=Robert |last2=Nelsen |first2=Matthew P. |title=Ediacarans, Protolichens, and Lichen-Derived Penicillium |journal=Transformative Paleobotany |date=2018 |pages=551–590 |doi=10.1016/B978-0-12-813012-4.00023-1|isbn=978-0-12-813012-4 }}</ref>

==See also==
{{div col|colwidth=25em}}
* {{annotated link|Abiogenesis}}
* {{annotated link|Cosmic Calendar}}
* {{annotated link|Earliest known life forms}}
* {{annotated link|Geologic time scale}}
* {{annotated link|Geological history of oxygen}}
* {{annotated link|History of Earth}}
* {{annotated link|Precambrian}}
* {{annotated link|Timeline of natural history}}
* {{Annotated link|Archean felsic volcanic rocks}}
{{div col end}}

==References==
{{reflist|25em}}

==External links==
*{{cite web |title=Archean |department=GeoWhen Database |website=stratigraphy.org |url=http://www.stratigraphy.org/bak/geowhen/stages/Archean.html |access-date=17 September 2010 |archive-url=https://web.archive.org/web/20100822004931/http://www.stratigraphy.org/bak/geowhen/stages/Archean.html |archive-date=22 August 2010}}
*{{cite web |title=When did plate tectonics begin? |website=utdallas.edu |publisher=University of Texas – Dallas |url=http://www.utdallas.edu/~rjstern/PlateTectonicsStart/}}
*{{cite web |title=Archean (chronostratigraphy scale) |website=ghkclass.com |url=https://ghkclass.com/ghkC.html?archean}}

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[[Category:Archean| ]]
[[Category:Precambrian geochronology]]