Proterozoic

Information about Proterozoic

Proterozoic eon • • [ e]
Geologic timescale of the Proterozoic
(millions of years ago)
()

The Proterozoic (IPA: /ˌprəʊt(ə)rəˈzəʊɪk/) is a geological eon representing a period before the first abundant complex life on Earth. The Proterozoic Eon extended from 2500 Ma to 542.0 ± 1.0 Ma (million years ago). The Proterozoic is the most recent part of the old, informally named ‘Precambrian’ time.

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

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.

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 show 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 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.^[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.^[5] 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.^[6]

Paleogeography

Proterozoic life

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.^[6] 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.^[7]

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.^[8]

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.

References

1. ^ Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company, 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, 325
7. ^ Stanley 321-2
8. ^ Stanley, 321-3

External links

Proterozoic - Palaeos

Precambrian
(Hadean)	Archean	Proterozoic	Phanerozoic

Proterozoic eon
Paleoproterozoic era				Mesoproterozoic era			Neoproterozoic era
Siderian	Rhyacian	Orosirian	Statherian	Calymmian	Ectasian	Stenian	Tonian	Cryogenian	Ediacaran

The geological time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of Earth.
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IPA for English The
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eon (sometimes spelled aeon) is a period of time arbitrarily designated by humans. Geologists refer to an eon as the largest subdivision of time on the geologic time scale.
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EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001. Their greatest hit, their debut single "time after time", peaked at #13 in the Oricon singles chart.
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Annum is a Latin noun meaning year. It is the accusative singular of the second declension masculine noun annus (nominative), anni (genitive) [1] .

As a unit of time, it is defined as exactly 365.
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Precambrian (Pre-Cambrian) is an informal name for the supereon comprising the eons of the geologic timescale that came before the current Phanerozoic eon. It spans from the formation of Earth around 4500 Ma (million years ago) to the evolution of abundant
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A geologic era is a subdivision of geologic time that divides an Eon into smaller buckets. The Phanerozoic Eon is divided into three such timeframes: the Paleozoic, Mesozoic, and Cenozoic represent the major stages in the macroscopic fossil record.
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The Paleoproterozoic (IPA: /ˌpeɪlɪəʊˌprəʊt(ə)rəˈzəʊɪk/, also spelled Palaeoproterozoic
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The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma (million years ago).

The major events of this era are the formation of the Rodinia supercontinent, the breakup of the Columbia supercontinent, and the evolution of sexual reproduction.
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The Neoproterozoic Era is the unit of geologic time from 1,000 to 542 +/- 0.3 million years ago.^[1] The terminal Era of the formal Proterozoic Eon (or the informal "Precambrian"), it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods.
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2, −1
(neutral oxide)
Electronegativity 3.44 (Pauling scale)
Ionization energies
(more) 1st: 1313.9 kJmol⁻¹
2nd: 3388.3 kJmol⁻¹
3rd: 5300.5 kJmol⁻¹

Atomic radius 60 pm
Atomic radius (calc.
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glacier is a large, slow moving river of ice, formed from compacted layers of snow, that slowly deforms and flows in response to gravity. Glacier ice is the largest reservoir of fresh water on Earth, and second only to oceans as the largest reservoir of total water.
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Snowball Earth hypothesis as it currently stands^[2] proposes that the Earth was entirely covered by ice in part of the Cryogenian period of the Proterozoic eon, and perhaps at other times in the history of Earth.
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Please [improve the article] or discuss this issue on the talk page. This article has been tagged since May 2007.
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Cryogenian

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Archean (IPA: /ɑːˈkiːən/, also spelled Archaean, formerly called the Archaeozoic (IPA:
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stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces.
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An epeiric sea (also known as an epicontinental sea) is a large but shallow body of salt water that lies over a part of a continent.

Epeiric seas are usually associated with the marine transgressions of the early Cenozoic and other eras.
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Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat (greater than 150 degrees Celsius) and extreme pressure causing profound
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continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criteria, but seven areas are commonly regarded as continents – they are (from largest in size to smallest): Asia, Africa, North America, South America,
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The supercontinent cycle describes the quasi-periodic aggregration and dispersal of Earth's continental crust. There are varying opinions as to whether Earth's budget of continental crust is increasing, decreasing, or remaining about constant, but it is agreed that this inventory
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Orogeny (Greek for "mountain generating") is the process of mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity,
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Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed.

This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the
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6
(strongly acidic oxide)
Electronegativity 2.58 (Pauling scale)
Ionization energies
(more) 1st: 999.6 kJmol⁻¹
2nd: 2252 kJmol⁻¹
3rd: 3357 kJmol⁻¹

Atomic radius 100 pm
Atomic radius (calc.
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3, 4, 6
(amphoteric oxide)
Electronegativity 1.83 (Pauling scale)
Ionization energies
(more) 1st: 762.5 kJmol⁻¹
2nd: 1561.9 kJmol⁻¹
3rd: 2957 kJmol⁻¹

Atomic radius 140 pm
Atomic radius (calc.
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Banded iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite, alternating with bands
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