Difference between revisions of "Proterozoic" - New World Encyclopedia

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{{Proterozoic Infobox}}
 
{{Proterozoic Infobox}}
 
The '''Proterozoic''' is the [[geology|geologic]] [[Geologic time scale#Terminology|eon]] extending from about 2500 million years ago (mya), at the end of the [[Archean]] eon, to about 542.0 ± 1.0 mya, at the start of the current ''[[Phanerozoic]]'' eon.
 
The '''Proterozoic''' is the [[geology|geologic]] [[Geologic time scale#Terminology|eon]] extending from about 2500 million years ago (mya), at the end of the [[Archean]] eon, to about 542.0 ± 1.0 mya, at the start of the current ''[[Phanerozoic]]'' eon.

Revision as of 23:13, 20 January 2007

Proterozoic eon
The Proterozoic eon comprises the Paleo-, Meso-, and Neo- proterozoic eras, and is preceded by the Archaean eon.
2500 - 542 millions of years ago

(expand)

The Proterozoic is the geologic eon extending from about 2500 million years ago (mya), at the end of the Archean eon, to about 542.0 ± 1.0 mya, at the start of the current Phanerozoic eon.

The Proterozoic is the most recent division of the Precambrian, an informal name for the billions of years of the geologic timescale that came before the current Cambrian, the first period of the Phanerozoic. The Cambrian marked the appearance of abundant macroscopic hard-shelled fossils; the Proterozic represents a period of time before the first abundant complex life on Earth.

The Proterozoic marks the first appearance in the fossil record of eukaryotes, tracing to about 1.7 billion years ago. There is speculation, however, that it is likely that the first eukaryotes actually originated near the end of the previous Archean, perhaps 2.7 billion years ago (Mayr 2001). During the Proterozic, eukaryotes greatly diversified. There were also abundant soft-bodied, multicellular organisms during the late Proterozoic, but being soft-bodied fossilization was rare. Another process set in motion during the Proterozoic was a transition to an oxygenated atmosphere. The Proterozoic thus provided the foundation for the explosion of life forms during the Cambrian explosion, when most phyla of animals appeared.

Geologic Time (ca. 4500 million years ago - present)
Hadean Archean Proterozoic Phanerozoic
Precambrian (ca. 4500 - 542 million years ago)  

Overview

The Proterozoic consists of 3 geologic eras, from oldest to youngest:

  • Paleoproterozoic
  • Mesoproterozoic
  • Neoproterozoic

The Neoproterozoic is the era from 1000 mya to 542 mya. It covers one of the more interesting time in the geological record, during which the Earth was hit by the most severe glaciations known (during which ice sheets reached the equator), and in the later parts of the era, the Ediacaran period, the earliest evidence of multicelled life is found, including the earliest animals.

During the Mesoproterozoic, there was a transition to an oxygenated atmosphere.

The geologic record of the Proterozoic is much better than that for the preceding Archean. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas; furthermore, many of these rocks are less metamorphosed than Archean-age ones, and plenty are unaltered (Stanley 1999). Study of these rocks show that the eon featured massive, rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and modern orogenic (mountain building) activity (Stanley 1999).

The first known glaciations occurred during the Proterozoic; one began shortly after the beginning of the eon, while there were at least four during the Neoproterozoic, climaxing with the "Snowball Earth" of the Varangian glaciation (Stanley 1999).

Proterozoic eon (2500 - 542 million years ago)
Paleoproterozoic era Mesoproterozoic era Neoproterozoic era
Siderian Rhyacian Orosirian Statherian Calymmian Ectasian Stenian Tonian Cryogenian Ediacaran
----------X--------------------------Three probable snowball earth episodes.------------------------------------X—X----

The Build-up of Oxygen

One of the most important events of the Proterozoic was the gathering up of oxygen in the Earth's atmosphere. Though oxygen was undoubtedly released by photosynthesis well back in Archean times, it could not build up to any significant degree until chemical sinks—unoxidized sulfur and iron—had been filled; until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level (Stanley 1999). Banded iron formations, which provide most of the world's iron ore, were also a prominent chemical sink; most accumulation ceased after 1.9 billion years ago, either due to an increase in oxygen or a more thorough mixing of the oceanic water column (Stanley 1999).

Red beds, which are colored by hematite, indicate an increase in atmospheric oxygen after 2 billion years ago; they are not found in older rocks (Stanley 1999). The oxygen build-up was probably due to two factors: a filling of the chemical sinks, and an increase in carbon burial, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere (Stanley 1999).

Proterozoic Life

One of the major developments in life is the transition from prokaryotes to eukaryotes. Mayr (2001) states that the origin of eukaryotes is "perhaps the most important and dramatic event in the history of life." Prokaryotes originated in the Archean, with fossils known from about 3.5 billion years ago (3500 mya). The first fossils of eukaryote organisms is known from the subsequent Proterozoic, tracing to about 1.7 billion years ago (Mayr 2001). Eukaryotes, however, may have actually originated during the Archean and simply not left any fossils (Stanley 1999).

Mayr (2001) speculates that eukaryotes may have originated 1 billion years prior to the first findings of eukaryote fossils. He notes that lipids, by-products of eukaryotic metabolism, have been found in rocks that are 2700 million years old, tracing to the Archean. There is a possibility, however, that these molecules percolated down from recent strata into these older strata; although most geologists deny this possibility (Mayr 2001).

Certainly, during the Proterozic, eukaryotes greatly diversified. Likewise, there were abundant soft-bodied, multicellular organisms during the late Proterozoic.

The oldest shelled fossils are those of the Ediacaran period, which ranges from approximately 635 to 542 million years before the present. The Ediacaran biota is occasionally referred to as the Vendian biota. This usage echoes the former name Vendian, by which the Ediacaran period was known in Russia and some other parts of the world before the official naming of the period in 2004. The Ediacaran fossils are the oldest definite multicellular fossils. According to Mayr (2001), Ediacaran fossils from the late Proterozoic contain "many strange types as well as others clearly related to the Cambrian types."

The first advanced single-celled and multi-cellular life roughly coincides with the oxygen accumulation; this may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria (Stanley 1999). It was also during the Proterozoic that the first symbiotic relationship between mitochondria (for animals and protists) and chloroplasts (for plants) and their hosts evolved (Stanley 1999).

The blossoming of eukaryotes, such as acritarchs, did not preclude the expansion of cyanobacteria; in fact, stromatolites (structures thought to have been formed by microorganisms, particularly cyanobacteria) reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago (Stanley 1999).

Classically, the boundary between the Proterozoic and the Paleozoic was set at the base of the Cambrian period, when the first fossils of animals known as trilobites and archeocyathids appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks (the Ediacaran fossils), but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 542 mya. Most animal phyla appeared in the late Proterozoic and early Cambrian, about 565-530 mya. Evolutionist Mayr (2001) notes that the appeared "seemingly full-fledged" and that "no fossils intermediate between them have been found and no living intermediates are in existence." Particularly during the Cambrian explosion there was a geologically sudden and simultaneous appearance of most phyla of animals (Mayr 2001).

References
ISBN links support NWE through referral fees

  • Hagadorni, J. W., and B. Waggoner. 2000. Ediacaran fossils from the Southwestern Great Basin, United States. Journal of Paleontology 74(2): 349-359.
  • Mayr, E. 2001. What Evolution Is. New York, NY: Basic Books. ISBN 0465044263.
  • Stanley, S. M. 1999. Earth System History. New York: W.H. Freeman and Company. ISBN 0-7167-2882-6.


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