Proterozoic


Proterozoic

The Proterozoic (pronEng|ˌproʊtərəˈzoʊɪk) 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), 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.

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. [cite book| last=Stanley| first=Steven M.| title=Earth System History| location=New York| publisher=W.H. Freeman and Company| year=1999| id=ISBN 0-7167-2882-6 | pages= 315] Study of these rocks show that the eon featured massive, rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and wholly-modern orogenic activity. [Stanley, 315-18, 329-32]

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, 320-1, 325]

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, 323] 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, 324]

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. 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, 325]

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. [ [http://www.lpi.usra.edu/meetings/lpsc97/pdf/1247.PDF Lunar and Planetary Science XXVIII] ]

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. 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. [Stanley 321-2]

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. [Stanley, 321-3]

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.

ee also

*Timetable of the Precambrian

References

External links

* [http://www.palaeos.com/Proterozoic/Proterozoic.htm Proterozoic] - "Palaeos"


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  • proterozoic — PROTEROZÓIC, Ă, proterozoici, ce, subst., adj. (geol.) Algonchian. [pr.: zo ic] – Din fr. protérozoïque, engl. proterozoic. Trimis de ana zecheru, 13.09.2007. Sursa: DEX 98  PROTEROZÓIC adj., s. v …   Dicționar Român

  • Proterozoic — [prät΄ə rə zō′ik, prōt΄ə rō zō′ik] adj. [< Gr proteros, former, compar. of pro, forward (see PRO 1) + ZO + IC] [sometimes p ] designating or of the geologic eon characterized by the development of sedimentary rock, shallow seas, and the first… …   English World dictionary

  • Proterozoic — adjective Etymology: Greek proteros former, earlier (from pro before) + International Scientific Vocabulary zoic more at for Date: 1899 of, relating to, or being the eon of geologic time or the corresponding segment of rocks that includes the… …   New Collegiate Dictionary

  • Proterozoic — /prot euhr euh zoh ik, proh teuhr /, Geol. adj. 1. noting or pertaining to the latter half of the Precambrian Era, from about 2.5 billion to 570 million years ago, characterized by the appearance of bacteria and marine algae; Algonkian. n. 2. the …   Universalium

  • Proterozoic — 1. adjective Of, or relating to the geologic eon from about 2,500 to 570 million years ago; comprises the Paleoproterozoic, Mesoproterozoic and Neoproterozoic eras; marked by the build up of oxygen in the atmosphere and the emergence of primitive …   Wiktionary

  • Proterozoic — Prot•er•o•zo•ic [[t]ˌprɒt ər əˈzoʊ ɪk, ˌproʊ tər [/t]] adj. 1) gel noting or pertaining to the latter half of the Precambrian Era, from about 2.5 billion to 570 million years ago, when bacteria and marine algae were the principal forms of life 2) …   From formal English to slang

  • proterozoic — proterozojus statusas T sritis ekologija ir aplinkotyra apibrėžtis Antroji Žemės geologinės istorijos antroji era, prasidėjusi prieš 2,6 mlrd. m., po archėjaus, pasibaigusi prieš 570 mln. m. prieš paleozojų; per tą laikotarpį susidariusių uolienų …   Ekologijos terminų aiškinamasis žodynas

  • proterozoic —   an eon of geologic time lasting from 2500m to 570m years ago …   Geography glossary

  • proterozoic — pro|te|ro|zoic Mot Agut Adjectiu variable …   Diccionari Català-Català

  • proterozóic — adj. m. (sil. zo ic), pl. proterozóici; f. sg. proterozóicã, pl. proterozóice …   Romanian orthography


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