Proterozoic

Of or relating to the later of the two divisions of Precambrian time, from approximately 2.5 billion to 570 million years ago, marked by the buildup of oxygen and the appearance of the first multicellular eukaryotic life forms.

The Proterozoic Eon.

[Greek proteros, earlier, former + –ZOIC.]

A major division of geologic time spanning from 2500 to 543 million years before present (Ma). The beginning of Proterozoic time is an arbitrary boundary that roughly coincides with the transition from a tectonic style dominated by extensive recycling of the Earth's continental crust to a style characterized by preservation of the crust as stable continental platforms. The end of the Proterozoic coincides with the Precambrian-Cambrian boundary, which is formally defined on the basis of the first appearance of diverse coelomate invertebrate animals. Proterozoic Earth history testifies to several remarkable biogeochemical events, including the formation and dispersal of the first supercontinent, the maturation of life and evolution of animals, the rise of atmospheric oxygen, and the decline of oceanic carbonate saturation. Tremendous iron and lead-zinc mineral deposits occur in Proterozoic rocks, as do the first preserved accumulations of oil and gas. See also Cambrian; Precambrian.

Many of the Earth's Archean cratons are blanketed by little-deformed sequences of Proterozoic sedimentary rocks, which indicate that vigorous recycling of the Earth's crust, characteristic of Archean time, had slowed markedly by the beginning of Proterozoic time. This decrease in crustal recycling is attributed to the development of thick continental roots, which stabilized the cratons, and the decrease in heat that was escaping from the Earth's interior, believed to drive thermal convection in the Earth's mantle and recycling of the crust. Most of the Earth's Archean cratons appear to have participated in the formation of a supercontinent in Mesoproterozoic time, about 1200 Ma. This supercontinent, called Rodinia, seems to have assembled with the North American craton (Laurentia) at its center. Rodinia persisted until the latest part of the Neoproterozoic, about 600 Ma. See also Archean; Continents, evolution of; Earth, heat flow in; Earth crust; Earth interior; Plate tectonics.

Giant iron oxide deposits were formed by precipitation from seawater about 2000 Ma, whereupon oxygen was free to accumulate in the atmosphere and shallow ocean. During most of Paleoproterozoic time the oceans and atmosphere were reducing and ferrous iron was abundant in seawater.

The partial pressure of carbon dioxide on the early Earth was very high. During Proterozoic time, much of the mass of carbon shifted from the ocean and atmosphere to the solid Earth. Enormous volumes of limestone [CaCO₃] and dolostone [CaMg(CO₃)₂] were deposited and testify to this shift. See also Dolomite rock; Limestone; Sedimentary rocks.

Glaciers covered significant parts of the Earth during two widely separated times in Proterozoic history. The first episode occurred about 2200 Ma, and glacial deposits of that age cover various parts of North America and Scandinavia. The second episode consisted of at least two different pulses spanning from 750 to 600 Ma during Neoproterozoic time. Glaciers formed at that time were of almost global extent, and were thought to have extended from the poles to the Equator, according to the snowball Earth hypothesis. See also Glacial epoch.

A number of significant events in the evolution of life occurred during Proterozoic time. The record of biological activity is rich, consisting of actual body fossils, in addition to organism traces and impressions, and complex chemical biomarkers. Eukaryotic microbes appear to have evolved by about 1900 Ma, when they became major players in ecosystems present at that time. By the beginning of Neoproterozoic time, about 1000 Ma, multicellular eukaryotic algae are present in numerous sedimentary basins around the world. See also Eukaryotae; Ribonucleic acid (RNA).

The evolution of animals did not take place until the close of Neoproterozoic time. Why these organisms evolved at this particular time in Earth history remains unanswered. General opinion proposes that it was likely the result of the confluence of a number of environmental factors, such as the rise in oxygen. Whatever the cause of their origin, these existed until at least 543 Ma, when another major evolutionary adaptive radiation began which marks the onset of Cambrian time and the end of the Proterozoic Eon. See also Animal evolution; Extinction (biology); Geologic time scale.

The noun has one meaning:

Meaning #1: from 2,500 to 544 million years ago; bacteria and fungi; primitive multicellular organisms
  Synonym: Preoterozoic era

Proterozoic eon 2500 - 542 million years ago

The geological eras view • discuss • edit -4500 — – -4000 — – -3500 — – -3000 — – -2500 — – -2000 — – -1500 — – -1000 — – -500 — – 0 — Hadean Archean Protero -zoic Phanero -zoic Eo Paleo Meso Neo Paleo Meso Neo Paleo Meso Ceno     Scale: Millions of years

Lower Proterozoic Stromatolites from Bolivia, South America

The Proterozoic (pronounced /ˌproʊt^ərəˈzoʊɪk/) is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life." The Proterozoic Eon extended from 2500 Ma to 542.0 ± 1.0 Ma (million years ago), and is the most recent part of the old, informally named ‘Precambrian’ time.

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

• Paleoproterozoic
• Mesoproterozoic
• Neoproterozoic

The well-identified events were:

• The transition to an oxygenated atmosphere during the Mesoproterozoic.
• Several glaciations, including the hypothesized Snowball Earth during the Cryogenian period in the late Neoproterozoic.
• The Ediacaran Period (635 to 542 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms.

Contents 1 The Proterozoic record 2 The buildup of oxygen 3 Paleogeography 4 Proterozoic life 5 See also 6 References 7 External links

The Proterozoic record

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.^[1] Study of these rocks shows that the eon featured massive, rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and wholly-modern orogenic activity.^[2]

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.^[3]

The buildup 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.^[4] 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.^[5]

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.^[6] The oxygen buildup 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.^[7]

Paleogeography

The Mackenzie dike swarm in Canada's Canadian Shield is the largest known dike swarm on Earth, and was a source for significant massive flood basalt eruptions throughout the Proterozoic period. The source for the Mackenzie dike swarm is thought to have been a mantle plume center called the Mackenzie hotspot.^[8]

This section requires expansion.

Proterozoic life

The first advanced single-celled and multi-cellular life roughly coincides with the start of the accumulation of free oxygen; this may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria.^[7] It was also during the Proterozoic that the first symbiotic relationships between mitochondria (for nearly all eukaryotes) and chloroplasts (for plants and some protists only) and their hosts evolved.^[9]

The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago.^[10]

Classically, the boundary between the Proterozoic and the Phanerozoic eons 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, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 542 Ma.

See also

• Timetable of the Precambrian

References

1. ^ Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. pp. 315. ISBN 0-7167-2882-6. 
2. ^ Stanley, 315-18, 329-32
3. ^ Stanley, 320-1, 325
4. ^ Stanley, 323
5. ^ Stanley, 324
6. ^ Stanley, 324
7. ^ ^{a b} Stanley, 325
8. ^ Lunar and Planetary Science XXVIII
9. ^ Stanley 321-2
10. ^ Stanley, 321-3

External links

Wikimedia Commons has media related to: Proterozoic

• Proterozoic - Palaeos

Preceded by Archean eon	2.5 Ga - Proterozoic eon - 542 Ma										Followed by Phanerozoic eon
	2.6 Ga - Paleoproterozoic era - 1.6 Ga				1.6 Ga - Mesoproterozoic era - 1.0 Ga			1.0 Ga - Neoproterozoic era - 542 Ma
	Siderian	Rhyacian	Orosirian	Statherian	Calymmian	Ectasian	Stenian	Tonian	Cryogenian	Ediacaran

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