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the time from 3,800 million years to 2,500 million years ago; earths crust formed; unicellular organisms are earliest forms of life (同)Archean eon, Archean aeon, Archeozoic, Archaeozoic, Archeozoic eon, Archaeozoic aeon
of or relating to the earliest known rocks formed during the Precambrian Eon (同)archaean

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/03/04 12:22:22」(JST)

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"Archaean" redirects here. For the division of living organisms, see Archaea.

Archean Eon 4000 - 2500 million years ago
The geological eras view • discuss • edit -4500 — – -4000 — – -3500 — – -3000 — – -2500 — – -2000 — – -1500 — – -1000 — – -500 — – 0 — Hadean Archean Protero -zoic Phanero -zoic Eo Paleo Meso Neo Paleo Meso Neo Paleo Meso Ceno Scale: Millions of years

The Archean (pron.: /ɑrˈkiːən/, also spelled Archaean; formerly Archaeozoic /ɑrkiəˈzoʊɨk/), also spelled Archeozoic or Archæozoic) is a geologic eon before the Proterozoic Eon, before 2.5 Ga (billion years), or 2,500 million years ago. The Archean era is generally agreed to have started at 3.8 billion years ago, but this boundary is informal.

Classification issues

Instead of being based on stratigraphy as all other geological ages are, the beginning of the Archean eon is defined chronometrically. The lower boundary (starting point) of 3.8 billion years has not been officially recognized by the International Commission on Stratigraphy.

The Archean customarily starts at 3.8 Ga—at the end of the Hadean Eon. In older literature, the Hadean is included as part of the Archean. The name comes from the ancient Greek Αρχή (Arkhē), meaning "beginning, origin".

Earth

The Archean is one of the four principal eons of Earth history. When the Archean began, the Earth's heat flow was nearly three times higher than it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma). The extra heat was the result of a mix of remnant heat from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements.

Most surviving Archean rocks are metamorphic or igneous. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids.

The Earth of the early Archean may have supported a tectonic regime unlike that of the present. Some scientists argue that, because the Earth was much hotter, tectonic activity was more vigorous than it is today, resulting in a much faster rate of recycling of crustal material. This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Others argue that the oceanic lithosphere was too buoyant to subduct, and that the rarity of Archean rocks is a function of erosion by subsequent tectonic events. The question of whether plate tectonic activity existed in the Archean is an active area of modern research.^[1]

There are two schools of thought concerning the amount of continental crust that was present in the Archean. One school maintains that no large continents existed until late in the Archean: small protocontinents were the norm, prevented from coalescing into larger units by the high rate of geologic activity.^{[citation needed]} The other school follows the teaching of Richard Armstrong, who argued that the continents grew to their present volume in the first 500 million years of Earth history and have maintained a near-constant ever since: throughout most of Earth history, recycling of continental material crust back to the mantle in subduction or collision zones balances crustal growth.^{[citation needed]}

Opinion is also divided about the mechanism of continental crustal growth. Those scientists who doubt that plate tectonics operated in the Archean argue that the felsic protocontinents formed at hotspots rather than subduction zones. Through a process called "sagduction", which refers to partial melting in downward-directed diapirs, a variety of mafic magmas produce intermediate and felsic rocks.^{[citation needed]} Others accept that granite formation in island arcs and convergent margins was part of the plate tectonic process, which has operated since at least the start of the Archean.^{[citation needed]}

An explanation for the general lack of Hadean rocks (older than 3800 Ma) is the efficiency of the processes that either cycled these rocks back into the mantle or effaced any isotopic record of their antiquity. All rocks in the continental crust are subject to metamorphism, partial melting and tectonic erosion during multiple orogenic events and the chance of survival at the surface decreases with increasing age. In addition, a period of intense meteorite bombardment in the period 4.0-3.8 Ga pulverized all rocks at the Earth's surface during the period. The similar age of the oldest surviving rocks and the "late heavy bombardment" is thought to be not accidental.^{[citation needed]}

Palaeoenvironment

The Archean atmosphere is thought to have nearly lacked free oxygen. Astronomers think that the sun had about 70–75% of the present luminosity, yet temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, which is puzzling (the faint young sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The equable temperatures may reflect the presence of larger amounts of greenhouse gases than later in the Earth's history.^[2]^[3] Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.^[4]

By the end of the Archaean c. 2500 Mya, plate tectonic activity may have been similar to that of the modern Earth. 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. Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.

Geology

Although a few mineral grains are known that are Hadean, the oldest rock formations exposed on the surface of the Earth are Archean or slightly older. Archean rocks are known from Greenland, the Canadian Shield, the Baltic Shield, Scotland, India, Brazil, western Australia, and southern Africa. Although the first continents formed during this eon, rock of this age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, evidence suggests that continental crust equivalent to only 5-40% of the present amount formed during the Archean.^[5]

In contrast to Proterozoic rocks, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.^[6] Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks. The meta-igneous rocks were derived from volcanic island arcs, while the metasediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts represent sutures between protocontinents.^[7]

Life

Fossils of cyanobacterial mats (stromatolites, which were instrumental in creating the free oxygen in the atmosphere^[8] ) are found throughout the Archean, becoming especially common late in the eon, while a few probable bacterial fossils are known from chert beds.^[9] In addition to the domain Bacteria (once known as Eubacteria), microfossils of the domain Archaea have also been identified.

Life was probably present throughout the Archean, but may have been limited to simple non-nucleated single-celled organisms, called Prokaryota (formerly known as Monera). There are no known eukaryotic fossils, though they might have evolved during the Archean without leaving any fossils.^[10] No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.

References

^ Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. pp. 297–301. ISBN 0-7167-2882-6.
^ Walker, James C. G. (June 1985). "Carbon dioxide on the early earth". Origins of Life and Evolution of the Biosphere 16 (2): 117−127. Bibcode 1985OLEB...16..117W. doi:10.1007/BF01809466. http://deepblue.lib.umich.edu/bitstream/2027.42/43349/1/11084_2005_Article_BF01809466.pdf. Retrieved 2010-01-30.
^ Pavlov, Alexander A.; Kasting, James F.; Brown, Lisa L.; Rages, Kathy A.; Freedman, Richard (May 2000). "Greenhouse warming by CH₄ in the atmosphere of early Earth". Journal of Geophysical Research 105 (E5): 11981−11990. Bibcode 2000JGR...10511981P. doi:10.1029/1999JE001134.
^ Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; Bjerrum, Christian J. (April 1, 2010). "No climate paradox under the faint early Sun". Nature 464 (7289): 744–747. Bibcode 2010Natur.464..744R. doi:10.1038/nature08955. PMID 20360739.
^ Stanley, pp. 301-2
^ Cooper, John D.; Miller, Richard H.; Patterson, Jacqueline (1986). A Trip Through Time: Principles of Historical Geology. Columbus: Merrill Publishing Company. p. 180. ISBN 0675201403.
^ Stanley, pp. 302-3
^ "Early life: Oxygen enters the atmosphere". BBC. http://www.bbc.co.uk/science/earth/earth_timeline/first_life. Retrieved September 20, 2012.
^ Stanley, 307
^ Stanley, pp. 306, 323

External links

Wikimedia Commons has media related to: Archean

GeoWhen Database
When Did Plate Tectonics Begin?

Archean Eon
Eoarchean	Paleoarchean	Mesoarchean	Neoarchean

English Journal

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Nitrogen is a major nutrient for all life on Earth and could plausibly play a similar role in extraterrestrial biospheres. The major reservoir of nitrogen at Earth's surface is atmospheric N2, but recent studies have proposed that the size of this reservoir may have fluctuated significantly over the
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Missing Archean sulfur returned from the mantle.

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Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume.

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The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle
PMID 27791057

Life: the first two billion years.

Knoll AH1, Bergmann KD2, Strauss JV3.
Philosophical transactions of the Royal Society of London. Series B, Biological sciences.Philos Trans R Soc Lond B Biol Sci.2016 Nov 5;371(1707). pii: 20150493. doi: 10.1098/rstb.2015.0493.
Microfossils, stromatolites, preserved lipids and biologically informative isotopic ratios provide a substantial record of bacterial diversity and biogeochemical cycles in Proterozoic (2500-541 Ma) oceans that can be interpreted, at least broadly, in terms of present-day organisms and metabolic proc
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