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 supereon comprising the eons of geologic time preceding the current eon, which is called the Phanerozoic ("revealed life") eon. The Precambrian is a time period of roughly 4 billion years beginning with the formation of Earth around 4500 million years ago (mya) and continuing until the abrupt appearance in the fossil record some 542 mya of abundant macroscopic hard-shelled animals, an event that marks the beginning of both the Phanerozoic eon and its earliest component era, the Cambrian.

During the vast depths of Precambrian time, the newly-formed planet congealed, cooled, differentiated into solid, liquid, and gaseous parts, and became the site for the origin of microscopic life forms that proliferated and became major participants shaping the planet. Photosynthetic bacteria in particular released so much oxygen that it caused the waters' heavy load of dissolved iron to precipitate out as iron oxides (which form today's rich veins of iron ore). As the bacteria continued producing oxygen while levels of iron in the water declined, the oxygen accumulated in the atmosphere, achieving the level needed to support the forms of multicellular life that were developing in the waters—on the foundation of an earlier development from non-nucleated cells (prokaryotes) to nucleated cells (eukaryotes).

The Precambrian is divided, from earliest to most recent, into the Hadean, Archaean (or Archean), and Proterozoic eons. Some scientists recognize only two subdivisions, the Archaean and the Proterozoic eons, beginning the Precambrian from the formation of the earth's crust and the oldest earth rocks 3800-4000 mya. In rock formations dating from the Archaean eon, the first fossil evidence of prokaryotic cells appears about 3500 mya and the first fossil evidence of eukaryotic cells appears about 2700 mya.

The Cambrian era, following immediately after the Proterozoic eon, is the age when a majority of the skeleton-bearing phyla of animals appear in the fossil record (Mayr 2001). Mayr concludes that the apparent explosion of new phyla in the early Cambrian was possibly due to the skeletonization of a variety of soft-bodied 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 been discovered largely 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 4500 mya and may have been struck by a 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 mya, since zircon crystals from Western Australia have been dated at 4404 mya.

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 rocks dated at 3800 mya 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 mya. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 600 Mya and 542 Mya (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 mya, starting in the late 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," giving evidence of a 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.

Continental land masses protruding about the surface of the waters covering Earth have assembled, broken apart, and re-assembled several times during the eons of geologic time, but details of plate motions in the Precambrian are only hazily known. It is generally believed that most of Earth's landmasses collected into a single supercontinent, Rodinia, around 1000 mya, and then broke apart around 600 mya. A number of glacial periods have been identified going as far back as the Huronian epoch, roughly 2200 mya. The best studied is the Sturtian-Varangian glaciation, around 600 mya, 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 bacteria developed the biochemical machinery for performing photosynthesis, they began to produce oxygen in large quantities, causing an ecological crisis sometimes called the Oxygen Catastrophe. In an early phase of the high volume production of oxygen, 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

Diverse terminologies and frameworks for characterizing the early years of the Earth's existence have been developed in the past century. With improvements in and greater use of radiometric dating methods, however, scientists are assigning plausible real dates to specific formations and features, and the field is becoming more settled.

• Proterozoic. In modern use, the Proterozoic eon is most often the period extending back from the beginning of the earliest Cambrian boundary to 2500 mya. The earliest Cambrian boundary boundary has been placed at various times by various authors, but that boundary has now become 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, extending back from the rough Cambrian boundary to as far as 900 mya, although modern use tends to represent a shorter interval from 542-600 mya. The Neoproterozoic corresponds to "Precambrian Z" rocks of older North American geology.
    • Ediacaran (part of the Neoproterozoic of the Proterozoic). In March 2004, the International Union of Geological Sciences officially defined the term Ediacaran period as beginning at the time of deposition of a particular stratigraphic boundary, about 620 mya and ending at the beginning of the Cambrian, 542 mya. In the Ediacaran period, the Ediacaran fauna appeared. The Ediacaran period is longer than the time period since the disappearance of the dinosaurs some 65 mya; it is a time period sufficient for significant change and development of diverse body types as are seen in the Cambrian explosion.
    • 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 eon, extending roughly from 1600-2500 mya. 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 mya. This term was intended originally to cover the time before any preserved rocks were deposited, although a very few rock beds seem to be dated slightly earlier than 3800 mya. Some zircon crystals from about 4400 mya 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 the 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

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• 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.

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