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Archean

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Not to be confused withArchaea.
Archean
4031 ± 3 – 2500Ma
Artist's impression of an Archean landscape
Chronology
Etymology
Name formalityFormal
Alternate spelling(s)Archaean, Archæan
Synonym(s)Eozoic
J.W. Dawson,1865
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitEon
Stratigraphic unitEonothem
Time span formalityFormal
Lower boundary definitionTen oldest U-Pb zircon ages
Lower boundary GSSAAlong the Acasta River,Northwest Territories,Canada
65°10′26″N115°33′14″W/ 65.1738°N 115.5538°W/65.1738; -115.5538
Lower GSSA ratified2023[1]
Upper boundary definitionDefined Chronometrically
Upper GSSA ratified1991[2]

TheArcheanEon (IPA:/ɑːrˈkiːən/ar-KEE-ən,also spelledArchaeanorArchæan), in older sources sometimes called theArchaeozoic,is the second of the fourgeologic eonsofEarth'shistory,preceded by theHadean Eonand followed by theProterozoic.The Archean represents the time period from4,031 to 2,500Ma(millions of years ago). TheLate Heavy Bombardmentis hypothesized to overlap with the beginning of the Archean. TheHuronian glaciationoccurred at the end of the eon.

The Earth during the Archean was mostly awater world:there wascontinental crust,but much of it was under anoceandeeper than today's oceans. Except for some rarerelict crystals,today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. TheEarth's atmospherewas also vastly different incompositionfrom today's: theprebiotic atmospherewas areducing atmosphererich inmethaneand lacking freeoxygen.

Theearliest known life,mostly represented by shallow-watermicrobial matscalledstromatolites,started in the Archean and remained simpleprokaryotes(archaeaandbacteria) throughout the eon. The earliestphotosyntheticprocesses, especially those by earlycyanobacteria,appeared in the mid/late Archean and led toa permanent chemical changein the ocean and the atmosphere after the Archean.

Etymology and changes in classification[edit]

The wordArcheanis derived from the Greek wordarkhē(αρχή), meaning 'beginning, origin'.[3]ThePre-Cambrianhad 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 million years ago until 2,500 million years ago.

Instead of being based onstratigraphy,the beginning and end of the Archean Eon are definedchronometrically.The eon's lower boundary or starting point of 4,031±3 million years ago is officially recognized by theInternational Commission on Stratigraphy,[1]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[edit]

When the Archean began, the Earth'sheat flowwas 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 (2,500Ma). The extra heat was partly remnant heat fromplanetary accretion,from the formation of themetallic core,and partly arose from the decay ofradioactiveelements. As a result, the Earth's mantle was significantly hotter than today.[4]

The evolution of Earth'sradiogenic heatflow over time

Although a few mineral grains are known to be Hadean, the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found inGreenland,Siberia,theCanadian Shield,Montana,Wyoming(exposed parts of theWyoming Craton),Minnesota(Minnesota River Valley), theBaltic Shield,theRhodope Massif,Scotland,India,Brazil,westernAustralia,and southernAfrica.[citation needed]Graniticrocks predominate throughout the crystalline remnants of the surviving Archean crust. These include great melt sheets and voluminousplutonicmasses ofgranite,diorite,layered intrusions,anorthositesandmonzonitesknown assanukitoids.Archean rocks are often heavily metamorphized deep-water sediments, such asgraywackes,mudstones,volcanic sediments, andbanded iron formations.Volcanicactivity was considerably higher than today, with numerous lava eruptions, including unusual types such askomatiite.[5]Carbonaterocks are rare, indicating that the oceans were more acidic, due to dissolvedcarbon dioxide,than during the Proterozoic.[6]Greenstone beltsare typical Archean formations, consisting of alternating units of metamorphosedmaficigneous and sedimentary rocks, includingArchean felsic volcanic rocks.The metamorphosed igneous rocks were derived from volcanicisland arcs,while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in aforearcbasin. Greenstone belts, which include both types of metamorphosed rock, representsuturesbetween the protocontinents.[7]: 302–303 

Plate tectonicslikely started vigorously in theHadean,but slowed down in the Archean.[8][9]The slowing of plate tectonics was probably due to an increase in the viscosity of themantledue to outgassing of its water.[8]Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely.[10]Only at the end of the Archean did the continents likely emerge from the ocean.[11]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.[12]

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 calledUras of 3,100 Ma.[13]Another hypothesis, which conflicts with the first, is that rocks from western Australia and southern Africa were assembled in a continent calledVaalbaraas far back as 3,600 Ma.[14]Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled.[8]

By theNeoarchean,plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence ofslab detachmentresulting from a hotter mantle,rheologicallyweaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing frombasaltintoeclogiteas they sank.[15][16]There are well-preservedsedimentary basins,and evidence ofvolcanic arcs,intracontinentalrifts,continent-continent collisions and widespread globe-spanningorogenic eventssuggesting the assembly and destruction of one and perhaps severalsupercontinents.Evidence from banded iron formations,chertbeds, chemical sediments andpillow basaltsdemonstrates that liquid water was prevalent and deep oceanic basins already existed.

Asteroid impacts were frequent in the early Archean.[17]Evidence fromspherulelayers suggests that impacts continued into the later Archean, at an average rate of about one impactor with a diameter greater than 10 kilometers (6 mi) every 15 million years. This is about the size of theChicxulubimpactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.[18]

Environment[edit]

The pale orange dot,an artist's impression of theearly Earthwhich is believed to have appeared orange through itshazy,methanerich,prebiotic second atmosphere.Earth's atmosphere at this stage was somewhat comparable to today'satmosphere of Titan.[19]

The Archean atmosphere is thought to have almost completely lackedfree oxygen;oxygen levels were less than 0.001% of their present atmospheric level,[20][21]with some analyses suggesting they were as low as 0.00001% of modern levels.[22]However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980–2,960 Ma,[23]2,700 Ma,[24]and 2,501 Ma.[25][26]The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of theGreat Oxygenation Event,[24][27]which most scholars consider to have begun in thePalaeoproterozoic.[28][29][30]Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean.[31]The ocean was broadlyreducingand lacked any persistentredoxcline,a water layer between oxygenated and anoxic layers with a strongredoxgradient, which would become a feature in later, more oxic oceans.[32]Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present.[33]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.[34]The Archean ocean was enriched in heavier oxygen isotopes relative to the modern ocean, thoughδ18Ovalues 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.[35]

Astronomers think that the Sun had about 75–80 percent of its present luminosity,[36]yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (thefaint young Sun paradox). The presence of liquid water is evidenced by certain highly deformedgneissesproduced by metamorphism ofsedimentaryprotoliths.The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history.[37][38][39]Extensive abiotic denitrification took place on the Archean Earth, pumping the greenhouse gas nitrous oxide into the atmosphere.[40]Alternatively, Earth'salbedomay have been lower at the time, due to less land area and cloud cover.[41]

Early life[edit]

Main article:Earliest known life forms
For details on how life got started, seeAbiogenesis.

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 isgraphiteofbiogenicorigin found in 3.7 billion–year-oldmetasedimentary rocksdiscovered inWestern Greenland.[42]

Lithifiedstromatoliteson the shores ofLake Thetis,Western Australia.Archean stromatolites are the first direct fossil traces of life on Earth.

The earliest identifiable fossils consist ofstromatolites,which aremicrobial matsformed in shallow water bycyanobacteria.The earliest stromatolites are found in 3.48 billion-year-oldsandstonediscovered inWestern Australia.[43][44]Stromatolites are found throughout the Archean[45]and become common late in the Archean.[7]: 307 Cyanobacteria were instrumental in creating free oxygen in the atmosphere.[citation needed]

Further evidence for early life is found in 3.47 billion-year-oldbaryte,in theWarrawoona Groupof Western Australia. This mineral shows sulfurfractionationof as much as 21.1%,[46]which is evidence ofsulfate-reducing bacteriathat metabolizesulfur-32more readily than sulfur-34.[47]

Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected inzirconsdated to 4.1 billion years ago, but this evidence is preliminary and needs validation.[48][49]

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.[50]

Life in the Archean was limited to simple single-celled organisms (lacking nuclei), calledprokaryotes.In addition to the domainBacteria,microfossils of the domainArchaeahave also been identified. There are no knowneukaryoticfossils from the earliest Archean, though they might have evolved during the Archean without leaving any.[7]: 306, 323 Fossilsteranes,indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter.[51]No fossil evidence has been discovered forultramicroscopicintracellularreplicators such asviruses.

Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago.[52][53]

See also[edit]

References[edit]

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External links[edit]

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