Archean

Source: Wikipedia, the free encyclopedia.

Not to be confused with Archaea.
Archean
4000 – 2500 Ma
Archean.png
Artist's impression of an Archean landscape.
Chronology
Etymology
Name formalityFormal
Alternate spelling(s)Archaean, Archæan
Synonym(s)Eozoic
Eon
Stratigraphic unitEonothem
Time span formalityFormal
Lower boundary definitionDefined Chronometrically
Lower GSSA ratified1991[citation needed]
Upper boundary definitionDefined Chronometrically
Upper GSSA ratified1991[1]

The Archean Eon (

geologic eons of Earth's history and by definition representing the time from 4 to 2.5 billion years ago. The Archean was preceded by the Hadean Eon and followed by the Proterozoic
.

The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under an ocean deeper than today's ocean. Except for some trace minerals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The earliest known life started in the Archean. Life was simple throughout the Archean, mostly represented by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen.

Etymology and changes in classification

The word Archean comes from the Greek word arkhē (αρχή), meaning 'beginning, origin'.[2] It was first used in 1872, when it meant 'of the earliest geological age'.[a] 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 on

chronometrically. The eon's lower boundary or starting point of 4 billion years ago is officially recognized by the International Commission on Stratigraphy.[4]

Geology

When the Archean began, the Earth's

radioactive elements. As a result, the Earth's mantle was significantly hotter than today.[5]

The evolution of Earth's radiogenic heat
flow 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 in

Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite.[6] Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.[7] Greenstone belts 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 arcs, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts, being both types of metamorphosed rock, represent sutures between the protocontinents.[8]
: 302–303 

Plate tectonics likely started vigorously in the Hadean, but slowed down in the Archean.[9][10] The slowing of plate tectonics was probably due to an increase in the viscosity of the mantle due to outgassing of its water.[9] Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely.[11] Only at the end of the Archean did the continents likely emerge from the ocean.[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 called Ur as of 3,100 Ma.[13] A differing conflicting hypothesis is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as 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.[9]

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, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from basalt into eclogite as they sank.[15][16] There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed.

Asteroid impacts were frequent in the early Archean.

spherule layers 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 the Chicxulub impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.[18]

Environment

The pale orange dot, an artist's impression of the early Earth which is believed to have appeared orange through its hazy, methane rich, prebiotic second atmosphere. Earth's atmosphere at this stage was somewhat comparable to today's atmosphere of Titan.[19]

The Archean atmosphere is thought to have nearly lacked

redox gradient, that 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, though δ18O values decreased to ones 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 the present luminosity,[36] yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (the faint young Sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history.[37][38][39] Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.[40]

Early life

Main article: Earliest known life forms
For details on how life got started, see 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 origin found in 3.7 billion–year-old metasedimentary rocks discovered in Western Greenland.[41]

Lithified stromatolites 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

microbial mats formed in shallow water by cyanobacteria. The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia.[42][43] Stromatolites are found throughout the Archean[44] and become common late in the Archean.[8]: 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-old

sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34.[46]

Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation.[47][48]

Earth was very hostile to life before 4.2–4.3 Ga 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.[49]

Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called

intracellular replicators such as viruses
.

Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago.[51][52]

See also

Footnotes

  1. ^ The name Archean was coined by American geologist James Dwight Dana (1813–1895).[3] The Pre-Cambrian eon had been believed to be without life (azoic); however, because fossils had been found in deposits that had been judged to belong to the Azoic age, "... I propose to use for the Azoic era and its rocks the general term Archæn (or Arche'an), from the Greek άρχαιος, pertaining to the beginning."[3]: 253 

References

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External links

Retrieved from "https://en.wikipedia.org/w/index.php?title=Archean&oldid=1146142423"