Precambrian
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The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of , where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.The Precambrian is an informal unit of geologic time, OverviewRelatively little is known about the Precambrian, despite it making up roughly seven-eighths of the stromatolites) are of limited biostratigraphic use.[3] This is because many Precambrian rocks have been heavily metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.[3][4][5]
It is thought that Theia shortly after it formed, splitting off material that formed the Moon (see Giant-impact hypothesis). A stable crust was apparently in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma.[6][7]
The term "Precambrian" is used by paleontologists for general discussions not requiring a more specific eon name. However, both the United States Geological Survey[8] and the International Commission on Stratigraphy regard the term as informal.[9] Because the span of time falling under the Precambrian consists of three eons (the Hadean, the Archean, and the Proterozoic), it is sometimes described as a supereon,[10][11] but this is also an informal term, not defined by the ICS in its chronostratigraphic guide.[12]
Eozoic (from eo- “earliest”) was a synonym for pre-Cambrian,[13][14] or more specifically Archean.[15] Life formsA specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks (Archean Eon) from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia.[16] Probable fossils 100 million years older have been found in the same area. However, there is evidence that life could have evolved over 4.280 billion years ago.[17][18][19][20] There is a fairly solid record of bacterial life throughout the remainder (Proterozoic Eon) of the Precambrian. Complex multicellular organisms may have appeared as early as 2100 Ma.[21] However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers."[22] Other possible early complex multicellular organisms include a possible 2450 Ma red alga from the Kola Peninsula,[23] 1650 Ma carbonaceous biosignatures in north China,[24] the 1600 Ma Rafatazmia,[25] and a possible 1047 Ma Bangiomorpha red alga from the Canadian Arctic.[26] The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period.[27][28] A very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic Eon. By the middle of the following Cambrian Period, a very diverse fauna is recorded in the Burgess Shale, including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life.[29][30] While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic mats that covered terrestrial areas.[31] Tracks from an animal with leg-like appendages have been found in what was mud 551 million years ago.[32][33] Planetary environment and the oxygen catastrophe Evidence of the details of Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth ".
The atmosphere of the early Earth is not well understood. Most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and was lacking in free oxygen. There is, however, evidence that an oxygen-rich atmosphere existed since the early Archean.[34]
At present, it is still believed that oxygen catastrophe. At first, oxygen would have quickly combined with other elements in Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, and the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides.
Subdivisions
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A terminology has evolved covering the early years of the Earth's existence, as
- Proterozoic: this eon refers to the time from the lower Cambrian boundary, 538.8 Ma, back through 2500 Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary. The Proterozoic Eon is divided into three eras: the Neoproterozoic, Mesoproterozoic and Paleoproterozoic.
- geologic era of the Proterozoic Eon, from the CambrianPeriod lower boundary (538.8 Ma) back to 1000 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American stratigraphy.
- geologic period within the Neoproterozoic Era. The "2012 Geologic Time Scale" dates it from 538.8 to 635 Ma. In this period the Ediacaran biotaappeared.
- Cryogenian: The middle period in the Neoproterozoic Era: 635-720 Ma.
- Tonian: the earliest period of the Neoproterozoic Era: 720-1000 Ma.
- Mesoproterozoic: the middle era of the Proterozoic Eon, 1000-1600 Ma. Corresponds to "Precambrian Y" rocks of older North American stratigraphy.
- Paleoproterozoic: oldest era of the Proterozoic Eon, 1600-2500 Ma. Corresponds to "Precambrian X" rocks of older North American stratigraphy.
- Archean Eon: 2500-4000 Ma.
- Hadean Eon: 4000–4567.3 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although some zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the moon and meteorites.[36][37]
It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows:[38]
- Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
- Hadean: dominated by heavy bombardment from about 4.51 Ga (possibly including a Cool Early Earth period) to the end of the Late Heavy Bombardmentperiod.
- Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formationsdue to increasing atmospheric oxygen content.
- Transition: a period of continued iron banded formation until the first continental red beds.
- Proterozoic: a period of modern plate tectonics until the first animals.
Precambrian supercontinents
The movement of Earth's
See also
- Phanerozoic – Fourth and current eon of the geological timescale
References
- ^ Cohen, Kim. "New edition of the Chart - 2022-10". International Commission on Stratigraphy. International Commission on Stratigraphy. Retrieved 16 January 2023.
- ISBN 978-0-44-459390-0.
- ^ ISBN 9781285981383.
- ISBN 978-0470387740. Outlined in Gore, Pamela J.W. (25 October 2005). "The Earliest Earth: 2,100,000,000 years of the Archean Eon".
- ^ Davis, C.M. (1964). "The Precambrian Era". Readings in the Geography of Michigan. Michigan State University.
- ^ "Zircons are Forever". Department of Geoscience. 2005. Archived from the original on 18 May 2019. Retrieved 28 April 2007.
- ISBN 9780444528100.
- ^ U.S. Geological Survey Geologic Names Committee (2010), "Divisions of geologic time – major chronostratigraphic and geochronologic units", U.S. Geological Survey Fact Sheet 2010–3059, United States Geological Survey, p. 2, retrieved 20 June 2018
- International Chronostratigraphic Chart. Retrieved 10 May 2018.
- S2CID 85652369.
- ISBN 9780470659632.
- ^ "Stratigraphic Guide". International Commission on Stratigraphy. Table 3. Retrieved 9 December 2020.
{{cite web}}: CS1 maint: location (link) - J.W. Dawson, of Montreal, in 1865. He did not fully define the limits of its application at that time; but it seems to have been generally understood by geologists to embrace all the obscurely fossiliferous rocks older than the Cambrian.
- J. W. Dawsonprefers the term "Eozoic" [to Archean], and would have it include all the Pre-Cambrian strata.
- ISBN 978-3-642-68684-9.
a possibility of dividing the Precambrian history into two eons: the Eozoic, embracing the Archean Era only, and the Protozoic, comprising all the remaining Precambrian Eras.
- ISBN 978-1-55581-158-7.
- PMID 28252057.
- ^ Zimmer, Carl (1 March 2017). "Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest". The New York Times. Retrieved 2 March 2017.
- ^ Ghosh, Pallab (1 March 2017). "Earliest evidence of life on Earth 'found'". BBC News. Retrieved 2 March 2017.
- ^ Dunham, Will (1 March 2017). "Canadian bacteria-like fossils called oldest evidence of life". Reuters. Retrieved 1 March 2017.
- S2CID 4331375.
- S2CID 4396466.
- S2CID 15774804.
- .
- PMID 28291791.
- doi:10.1130/G39829.1.
- PMID 25114212.
- S2CID 134885946.
- OL 17256629M.
- ISBN 9780618619160.
- ISBN 978-0470016176.
- Independent.co.uk. 7 June 2018. The Independent
- PMID 29881773.
- .
- ^ "Geological Society of America's "2009 GSA Geologic Time Scale."".
- .
- .
- ISBN 978-0-521-78673-7. also available at Stratigraphy.org: Precambrian subcommission
- .
- .
- doi:10.1016/j.precamres.2007.04.021. Archived from the original(PDF) on 4 March 2016. Retrieved 6 February 2016.
Further reading
- Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison Wgeology.wisc.edu Archived 2012-03-16 at the Wayback Machine – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
- Wilde, S. A.; Valley, J. W.; Peck, W. H.; Graham, C. M. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago". Nature. 409 (6817): 175–178. S2CID 4319774.
- Wyche, S.; Nelson, D. R.; Riganti, A. (2004). "4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton". Australian Journal of Earth Sciences. 51 (1): 31–45. .
External links
- Late Precambrian Supercontinent and Ice House World from the Paleomap Project
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