Precambrian

Precambrian
The Precambrian comprises the Hadean, Archaean, and Proterozoic eons.
Ca. 4500 - 542 millions of years ago

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.

During the Precambrian, processes were set in motion that prepared the foundation for the subsequent time period with multicellular animals. These processes included the origin of life, the development of an oxygen atmosphere, and the development of eukaryotes.

The Precambrian is divided, from earliest to most recent, into the Hadean, Archaean (or Archean), and Proterozoic eons. Some only recognize two subdivisions, the Archaean and the Proterozoic, beginning the Precambrian from the formation of the earth's crust and the oldest earth rocks 3.8-4.0 billion years ago. 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.

Overview

Remarkably little is known about the Precambrian, despite the fact that it makes 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.

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 multicellular 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," with the rapid radiation of forms called the Cambrian explosion of life.

Planetary environment and the oxygen catastrophe

File:Rodinia1.jpg

A map showing Rodinia, one of the oldest known supercontinents. Geologic evidence suggests that Rodinia formed and broke apart in the Neoproterozoic.

Details of plate motions 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. However, this process is becoming settled as greater use of radiometric dating has allowed plausible real dates to be assigned to specific formations and features.

• Proterozoic. In modern use, the Proterozoic eon is most often the period from the beginning of the lower Cambrian boundary through 2500 Ma. The lower boundary has been placed at various times by various authors, but has now been settled at 542 Ma. As originally used, the term Proterozoic was a synonym for Precambrian, and hence included everything prior to the Cambrian boundary.
  • Neoproterozoic. The Neoproterozoic era is considered to be 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 from 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 Ediacaran to describe this geologic period, which is part of the Neoproterozoic of the Proterozoic. 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. This is a proposed subdivision of the Neoproterozoic.
    • Tonian. The Tonian is another proposed subdivision of the Neoproterozoic.
  • Mesoproterozoic. The Mesoproterozoic era is the the middle division of the Proterozoic, and extends from roughly 900 Ma to 1600 Ma. This time period corresponds to "Precambrian Y" rocks of older North American geology.
  • Paleoproterozoic. The Paleoproterozic era is the oldest subdivision of the Proterozoic, extending roughly from 1600-2500 Ma. It corresponds to "Precambrian X" rocks of older North American geology.
• Archaean. The Archaen eon extends roughly from 2500-3800 Ma.
• Hadean. The Hadean eon is the division corresponding to the time prior to 3800 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although a very few 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 Global Boundary Stratotype Sections and Points (GSSPs) (internationally agreed upon stratigraphic sections, which serves as the reference section for a particular boundary on the geologic time scale). The Precambrian could be divided into five "natural" eons, characterized as follows (Bleeker 2004):

1. Accretion and differentiation: A period of planetary formation until giant Moon-forming impact event.
2. Hadean: The Late Heavy Bombardment period.
3. 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.
4. Transition: A period of continued iron banded formation until the first continental red beds.
5. Proterozoic: A period of modern plate tectonics until the first animals.

References

ISBN links support NWE through referral fees

• Bleeker, W. 2004. Toward a "natural" Precambrian time scale. In F. M. Gradstein, J. G. Ogg, and A. G. Smith (eds.), A Geologic Time Scale 2004. Cambridge University Press. ISBN 0-521-78673-8
• Gore, P. J. W. 1999. The Precambrian. Retrieved December 6, 2006.
• Mayr, E. 2001. What Evolution Is. New York, NY: Basic Books. ISBN 0465044263
• Monroe, J., and R. Wicander. 1997. The Changing Earth, 2nd ed. Belmont: Wadsworth Publishing Company.
• Wilde S. A., J. W. Valley, W. H. Peck, and C. M. Graham. 2001. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409: 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 51(1): 31. Retrieved January 10, 2006.
• Valley, J. W., W. H. Peck, and E. M. King. 1999. Zircons are forever. University of Wisconsin-Madison Geology Alumni Newsletter. Retrieved January 10, 2006.

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:

• Precambrian  ^history
• Rodinia  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Precambrian"

Note: Some restrictions may apply to use of individual images which are separately licensed.