Difference between revisions of "Precambrian" - New World Encyclopedia
Rick Swarts (talk | contribs) |
Rick Swarts (talk | contribs) |
||
| Line 1: | Line 1: | ||
{{Contracted}} | {{Contracted}} | ||
{{Precambrian Infobox}} | {{Precambrian Infobox}} | ||
| − | The '''Precambrian''' (or '''Pre-Cambrian''') is an informal name for the [[Geologic time scale#Terminology|eons]] of the [[geologic timescale]] that came before the current ''[[Phanerozoic]]'' eon. | + | The '''Precambrian''' (or '''Pre-Cambrian''') is an informal name for the [[Geologic time scale#Terminology|eons]] of the [[geologic timescale]] that came before the current ''[[Phanerozoic]]'' eon. The Precambrian spans from the formation of [[Earth]] around 4500 Ma (million years ago) to the appearance of abundant macroscopic hard-shelled [[fossil]]s, which marked the beginning of the [[Cambrian]], the first period of the Phanerozoic eon, some 542 Ma. |
| − | The | + | The Precambrian is divided, from earliest to most recent, into the [[Hadean]], [[Archaean]], and [[Proterozic]] eons. During the Precambrian, there is fossil evidence of [[prokaryote]] organisms, followed by [[eukaryote]]s in the Proterozic period (2.7 to 1.7 billion years ago.) |
| − | The Precambrian | + | The Cambrian, which follows the Proterozoic era, is the age of the first major flowering of multicellular eukaryotes, when a majority of the skeleton-bearing phyla of animals appear in the fossil record (Mayr 2001). Among the phyla that were first evident in the Cambrian were the [[brachiopod]]s, [[mollusk]]s, [[arthropod]]s ([[trilobite]]s), and [[echinoderm]]s. The entire preceding history of the Earth is referred to as the Precambrian, while the fossil-rich time from the Cambrian to the present is referred to as the Phaenerozoic eon, which is divided into the [[Paleozoic]], [[Mesozoic]], and [[Cenozoic]] eras (Mayr 2001). Mayr (2001) concludes that the apparent explosion of new phyla in the early Cambrian was possibly due to the skeletonization of a variety of soft-bodies organisms that already existed in the Precambrian. |
{{Eon Footer}} | {{Eon Footer}} | ||
| Line 11: | Line 11: | ||
==Overview== | ==Overview== | ||
| − | Remarkably little is known about the Precambrian, despite it making up roughly seven-eighths of the | + | Remarkably little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history. What little is known has largely been discovered in the past four or five decades. The Precambrian [[fossil]] record is poor, and what fossils are present(such as those of stromatolites) are of limited use for biostratigraphic work (Monroe and Wicander 1997). Many Precambrian rocks are heavily metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath [[Phanerozoic]] strata (Monroe and Wicander 1997, Gore 2006). |
| − | It is thought that the Earth itself coalesced from material in orbit around the sun roughly | + | It is thought that the Earth itself coalesced from material in orbit around the sun roughly 4.5 billion years ago (4500 Ma) and may have been struck by a very large Mars-sized planetesimal shortly after it formed, splitting off material that came together to form the Moon. A stable crust was apparently in place by 4400 Ma, since zircon crystals from Western Australia have been dated at 4404 Ma. |
| − | The term ''Precambrian'' is somewhat dated, but is still in common use among [[geologist]]s and [[paleontologists]]. It was briefly also called the '''Cryptozoic''' | + | The term ''Precambrian'' is somewhat dated, but is still in common use among [[geology|geologist]]s and [[paleontology|paleontologists]]. It was briefly also called the '''Cryptozoic''' eon. It seems likely that it will eventually be replaced by the preferred terms ''[[Proterozoic]]'', ''[[Archaean]]'', and ''[[Hadean]]'', and become a deprecated term. (See [[geologic time scale]].) |
==Life before the Cambrian== | ==Life before the Cambrian== | ||
| − | It is not known when life originated, but carbon in | + | It is not known when life originated, but [[carbon]] in 3.8 billion-year-old rocks (3800 Ma) from islands off western [[Greenland]] may be of organic origin. Well-preserved [[bacteria]] older than 3460 million years have been found in Western [[Australia]]. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian. |
| − | Excepting a few contested reports of much older forms from Texas and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. | + | Excepting a few contested reports of much older forms from Texas and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 600 Ma and 542 Ma (the beginning of the [[Cambrian]]). These are referred to as ''Ediacaran'' or ''Vendian'' biota. Hard-shelled creatures appeared toward the end of that timespan. |
| − | A very diverse collection of forms appeared around 544 Ma, starting in the latest Precambrian with a poorly understood '' | + | A very diverse collection of forms appeared around 544 Ma, starting in the latest Precambrian with a poorly understood ''small shelly fauna'' and ending in the very early Cambrian with a very diverse, and quite modern ''Burgess shale fauna'', the rapid radiation of forms called the ''[[Cambrian#Cambrian explosion|Cambrian explosion]]'' of life. |
==Planetary environment and the oxygen catastrophe== | ==Planetary environment and the oxygen catastrophe== | ||
| − | Details of | + | Details of plate motions and such are only hazily known in the Precambrian. It is generally believed that most of the Earth's landmasses collected into a single supercontinent around 1000 Ma. The supercontinent, known as [[Rodinia]], broke up around 600 Ma. A number of [[glacier|glacial periods]] have been identified going as far back as the Huronian epoch, roughly 2200 Ma. The best studied is the Sturtian-Varangian glaciation, around 600 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth". |
| − | The [[Earth's atmosphere|atmosphere]] of the early Earth is poorly known, but it is thought to have been | + | The [[Earth's atmosphere|atmosphere]] of the early Earth is poorly known, but it is thought to have been high in reducing gases, containing very little free [[oxygen]]. The young planet likely had a reddish tint, and its seas were thought to be olive green. Many materials with insoluble oxides appear to have been present in the oceans for hundreds of millions of years after the Earth's formation. |
| − | When evolving life forms developed [[photosynthesis]], oxygen began to be produced in large quantities, causing an ecological crisis sometimes called the | + | When evolving [[life]] forms developed [[photosynthesis]], oxygen began to be produced in large quantities, causing an [[ecology|ecological]] crisis sometimes called the Oxygen Catastrophe. The oxygen was immediately tied up in chemical reactions, primarily with [[iron]], until the supply of oxidizable surfaces ran out. After that, the modern high-oxygen atmosphere developed. Older rocks contain massive banded iron formations that were apparently laid down as iron and oxygen first combined. |
| − | ==Subdivisions== | + | ==Subdivisions of the Precambrian== |
A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to settle out and come into greater use as [[radiometric dating]] allows plausible real dates to be assigned to specific formations and features. The terms '''[[Archean]]''' (older than about 2500 Ma), '''[[Proterozoic]]''' (2500-600 Ma), and '''[[Neoproterozoic]]''' (600-542 Ma) appear to have general currency. Some additional terms are included in the geological time line. See [[Timetable of the Precambrian]]. | A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to settle out and come into greater use as [[radiometric dating]] allows plausible real dates to be assigned to specific formations and features. The terms '''[[Archean]]''' (older than about 2500 Ma), '''[[Proterozoic]]''' (2500-600 Ma), and '''[[Neoproterozoic]]''' (600-542 Ma) appear to have general currency. Some additional terms are included in the geological time line. See [[Timetable of the Precambrian]]. | ||
| Line 63: | Line 63: | ||
{{FootnotesSmall|resize={{{1|100%}}}}} | {{FootnotesSmall|resize={{{1|100%}}}}} | ||
* Valley, John W., William H. Peck, Elizabeth M. King (1999) ''Zircons Are Forever'', The Outcrop for 1999, University of Wisconsin-Madison [http://www.geology.wisc.edu/~valley/zircons/zircon_home.html Wgeology.wisc.edu] – ''Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago'' Accessed Jan. 10, 2006 | * Valley, John W., William H. Peck, Elizabeth M. King (1999) ''Zircons Are Forever'', The Outcrop for 1999, University of Wisconsin-Madison [http://www.geology.wisc.edu/~valley/zircons/zircon_home.html Wgeology.wisc.edu] – ''Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago'' Accessed Jan. 10, 2006 | ||
| + | |||
| + | [http://gpc.edu/~pgore/geology/geo102/precamb.htm Pamela J.W. Gore, "The Precambrian". Retrieved on 12/6/06.]</ref> 2006? | ||
| + | |||
| + | |||
| + | |||
| + | .<ref>James Monroe and Reed Wicander, ''The Changing Earth'', 2nd ed, (Belmont: Wadsworth Publishing Company, 1997), p. 492.</ref> | ||
| + | |||
* Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (2001) ''Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.'' Nature, v. 409, pp. 175-178. | * Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (2001) ''Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.'' Nature, v. 409, pp. 175-178. | ||
* Wyche, S., D. R. Nelson and A. Riganti (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 Volume 51 [http://www.blackwell-synergy.com/doi/abs/10.1046/j.1400-0952.2003.01042.x?cookieSet=1 Zircon ages from W. Australia - Absract] Accessed Jan. 10, 2006 | * Wyche, S., D. R. Nelson and A. Riganti (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 Volume 51 [http://www.blackwell-synergy.com/doi/abs/10.1046/j.1400-0952.2003.01042.x?cookieSet=1 Zircon ages from W. Australia - Absract] Accessed Jan. 10, 2006 | ||
Revision as of 14:54, 1 January 2007
| Precambrian |
|---|
| The Precambrian comprises the Hadean, Archaean, and Proterozoic eons. |
| Ca. 4500 - 542 millions of years ago |
 (expand) |
The Precambrian (or Pre-Cambrian) is an informal name for the eons of the geologic timescale that came before the current Phanerozoic eon. The Precambrian spans from the formation of Earth around 4500 Ma (million years ago) to the appearance of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, the first period of the Phanerozoic eon, some 542 Ma.
The Precambrian is divided, from earliest to most recent, into the Hadean, Archaean, and Proterozic eons. During the Precambrian, there is fossil evidence of prokaryote organisms, followed by eukaryotes in the Proterozic period (2.7 to 1.7 billion years ago.)
The Cambrian, which follows the Proterozoic era, is the age of the first major flowering of multicellular eukaryotes, when a majority of the skeleton-bearing phyla of animals appear in the fossil record (Mayr 2001). Among the phyla that were first evident in the Cambrian were the brachiopods, mollusks, arthropods (trilobites), and echinoderms. The entire preceding history of the Earth is referred to as the Precambrian, while the fossil-rich time from the Cambrian to the present is referred to as the Phaenerozoic eon, which is divided into the Paleozoic, Mesozoic, and Cenozoic eras (Mayr 2001). Mayr (2001) concludes that the apparent explosion of new phyla in the early Cambrian was possibly due to the skeletonization of a variety of soft-bodies organisms that already existed in the Precambrian.
| Geologic Time (ca. 4500 million years ago - present) | |||
|---|---|---|---|
| Hadean | Archean | Proterozoic | Phanerozoic |
| Precambrian (ca. 4500 - 542 million years ago) | |||
Overview
Remarkably little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history. What little is known has largely been discovered in the past four or five decades. The Precambrian fossil record is poor, and what fossils are present(such as those of stromatolites) are of limited use for biostratigraphic work (Monroe and Wicander 1997). Many Precambrian rocks are heavily metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata (Monroe and Wicander 1997, Gore 2006).
It is thought that the Earth itself coalesced from material in orbit around the sun roughly 4.5 billion years ago (4500 Ma) and may have been struck by a very large Mars-sized planetesimal shortly after it formed, splitting off material that came together to form the Moon. A stable crust was apparently in place by 4400 Ma, since zircon crystals from Western Australia have been dated at 4404 Ma.
The term Precambrian is somewhat dated, but is still in common use among geologists and paleontologists. It was briefly also called the Cryptozoic eon. It seems likely that it will eventually be replaced by the preferred terms Proterozoic, Archaean, and Hadean, and become a deprecated term. (See geologic time scale.)
Life before the Cambrian
It is not known when life originated, but carbon in 3.8 billion-year-old rocks (3800 Ma) from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian.
Excepting a few contested reports of much older forms from Texas and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 600 Ma and 542 Ma (the beginning of the Cambrian). These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan.
A very diverse collection of forms appeared around 544 Ma, starting in the latest Precambrian with a poorly understood small shelly fauna and ending in the very early Cambrian with a very diverse, and quite modern Burgess shale fauna, the rapid radiation of forms called the Cambrian explosion of life.
Planetary environment and the oxygen catastrophe
Details of plate motions and such are only hazily known in the Precambrian. It is generally believed that most of the Earth's landmasses collected into a single supercontinent around 1000 Ma. The supercontinent, known as Rodinia, broke up around 600 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly 2200 Ma. The best studied is the Sturtian-Varangian glaciation, around 600 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 poorly known, but it is thought to have been high in reducing gases, containing very little free oxygen. The young planet likely had a reddish tint, and its seas were thought to be olive green. Many materials with insoluble oxides appear to have been present in the oceans for hundreds of millions of years after the Earth's formation.
When evolving life forms developed photosynthesis, oxygen began to be produced in large quantities, causing an ecological crisis sometimes called the Oxygen Catastrophe. The oxygen was immediately tied up in chemical reactions, primarily with iron, until the supply of oxidizable surfaces ran out. After that, the modern high-oxygen atmosphere developed. Older rocks contain massive banded iron formations that were apparently laid down as iron and oxygen first combined.
Subdivisions of the Precambrian
A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to settle out and come into greater use as radiometric dating allows plausible real dates to be assigned to specific formations and features. The terms Archean (older than about 2500 Ma), Proterozoic (2500-600 Ma), and Neoproterozoic (600-542 Ma) appear to have general currency. Some additional terms are included in the geological time line. See Timetable of the Precambrian.
- Proterozoic : Modern use is most often the period from the beginning of the lower Cambrian boundary, through 2500 Ma. The boundary has been placed at various times by various authors, but has now been settled at 542 Ma. As originally used, it was a synonym for Precambrian and hence included everything prior to the Cambrian boundary.
- Neoproterozoic : the earliest subdivision of the Proterozoic roughly from the Cambrian boundary back to as far as 900 Ma, although modern use tends to represent a shorter interval : 542-600 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
- Ediacaran : In March 2004, the International Union of Geological Sciences officially defined the term to describe this geologic period. The period begins at the time of deposition of a particular stratigraphic boundary, about 620 Ma. The period ends at the beginning of the Cambrian, 542 Ma. In this period the Ediacaran fauna appeared.
- Cryogenian a proposed subdivision of the Neoproterozoic.
- Tonian a proposed subdivision of the Neoproterozoic.
- Mesoproterozoic : the middle division of the Proterozoic. Roughly from 900-1600 Ma. Corresponds to Precambrian Y rocks of older North American geology.
- Paleoproterozoic : The oldest subdivision of the Proterozoic. Roughly from 1600-2500 Ma. Corresponds to Precambrian X rocks of older North American geology.
- Neoproterozoic : the earliest subdivision of the Proterozoic roughly from the Cambrian boundary back to as far as 900 Ma, although modern use tends to represent a shorter interval : 542-600 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
- Archaean : Roughly from 2500-3800 Ma.
- Hadean : Prior to 3800 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although a very few old rock beds seem to be slightly older than 3800 Ma. 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.
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.[1]
- Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
- Hadean: the Late Heavy Bombardment period.
- Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due 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.
ReferencesISBN links support NWE through referral fees
- ↑ Bleeker, W. [2004]. "Toward a "natural" Precambrian time scale", in Felix M. Gradstein, James G. Ogg, and Alan G. Smith: A Geologic Time Scale 2004. Cambridge University Press. ISBN 0-521-78673-8.
- Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison Wgeology.wisc.edu – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
Pamela J.W. Gore, "The Precambrian". Retrieved on 12/6/06.</ref> 2006?
.[1]
- Wilde S.A., Valley J.W., Peck W.H. and Graham C.M. (2001) Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature, v. 409, pp. 175-178.
- Wyche, S., D. R. Nelson and A. Riganti (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 Volume 51 Zircon ages from W. Australia - Absract Accessed Jan. 10, 2006
Add Mayr, 2001.
External links
- PALEOMAP Project: Late Precambrian Supercontinent and Ice House World
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
- ↑ James Monroe and Reed Wicander, The Changing Earth, 2nd ed, (Belmont: Wadsworth Publishing Company, 1997), p. 492.
