Proterozoic

The Proterozoic ( /ˌproʊtərəˈzoʊɪk, prɔː-, -trə-/[1][2]) is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two forms of ultimately Greek origin: protero- meaning "former, earlier", and -zoic, a suffix related to zoe "life". The Proterozoic Eon extended from 2500 mya to 541 mya (million years ago), and is the most recent part of the Precambrian "supereon". The Proterozoic is the longest eon of the Earth's geologic time scale and it is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.[3]

The well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; several glaciations, which produced the hypothesized Snowball Earth during the Cryogenian Period in the late Neoproterozoic Era; and the Ediacaran Period (635 to 541 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms and provides us with the first obvious fossil evidence of life on earth.

Proterozoic Eon
2500–541 million years ago
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The Proterozoic record

The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. 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 those rocks are less metamorphosed than are Archean ones, and many are unaltered.[4]:315 Studies of these rocks have shown that the eon continued the massive continental accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity (orogeny).[4]:315–18, 329–32

There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during the Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations.[4]:320–1, 325

The accumulation of oxygen

One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere. Though oxygen is believed to have been released by photosynthesis as far back as Archean Eon, it could not build up to any significant degree until mineral sinks of 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]:323 The Banded iron formations, which provide most of the world's iron ore, are one mark of that mineral sink process. Their accumulation ceased after 1.9 billion years ago, after the iron in the oceans had all been oxidized.[4]:324

Red beds, which are colored by hematite, indicate an increase in atmospheric oxygen 2 billion years ago. Such massive iron oxide formations are not found in older rocks.[4]:324 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.[4]:325

Subduction processes

The Proterozoic Eon was a very tectonically active period in the Earth’s history. The late Archean Eon to Early Proterozoic Eon corresponds to a period of increasing crustal recycling, suggesting subduction. Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6 Ga.[5] The appearance of eclogites, which metamorphic rocks created by high pressure (>1 GPa), are explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor the formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon.[6] As a result of remelting of basaltic oceanic crust due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes. The long-term tectonic stability of those cratons is why we find continental crust ranging up to a few billion years in age.[7] It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic.[5] Studies by Condie 2000 and Rino et al. 2004 suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period.[5]

Tectonic history (supercontinents)

Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. Paleomagnetic and geochronological dating mechanisms have allowed the deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as orogenic belts or ophiolite complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian, the Earth went through several supercontinent breakup and rebuilding cycles (Wilson cycle).[5] In the late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called Laurentia. An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the Grenville orogeny located in Eastern North America. Rodinia formed after the breakup of the supercontinent Columbia and prior to the assemblage of the supercontinent Gondwana (~500 Ma).[8] The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the Pan-African orogeny.[9]

Columbia was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are a few plausible models that explain tectonics of the early Earth pre-Columbia, but the current most plausible theory is that prior to Columbia, there were only a few independent craton formations scattered around the Earth (not necessarily a supercontinent formation like Rodinia or Columbia).[5]

Life

Stromatolites Cochabamba
South America
ZebRivStromatolites 2010.3
Western Namibia

The first advanced single-celled, eukaryotes and multi-cellular life, Francevillian Group Fossils, roughly coincides with the start of the accumulation of free oxygen.[10] This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria.[4]:325 It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved.[4]: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 1200 million years ago.[4]: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 including 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 541 Ma.

See also

References

  1. ^ "Proterozoic – definition of Proterozoic in English from the Oxford dictionary". OxfordDictionaries.com. Retrieved 2016-01-20.
  2. ^ "Proterozoic". Merriam-Webster Dictionary.
  3. ^ Speer, Brian. "The Proterozoic Eon". University of California Museum of Paleontology.
  4. ^ a b c d e f g h i j Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. ISBN 978-0-7167-2882-5.
  5. ^ a b c d e Kearey, P., Klepeis, K., Vine, F., Precambrian Tectonics and the Supercontinent Cycle, Global Tectonics, Third Edition, pp. 361–377, 2008.
  6. ^ Bird, P. (2003). "An updated digital model of plate boundaries". Geochemistry, Geophysics, Geosystems. 4 (3): 1027. Bibcode:2003GGG.....4.1027B. doi:10.1029/2001GC000252.
  7. ^ Mengel, F., Proterozoic History, Earth System: History and Variablility, volume 2, 1998.
  8. ^ Condie, K. C.; O'Neill, C. (2011). "The Archean-Proterozoic boundary: 500 my of tectonic transition in Earth history". American Journal of Science. 310 (9): 775–790. Bibcode:2010AmJS..310..775C. doi:10.2475/09.2010.01.
  9. ^ Huntly, C., The Mozambique Belt, Eastern Africa: Tectonic Evolution of the Mozambique Ocean and Gondwana Amalgamation. The Geological Society of America. 2002.
  10. ^ El Albani, A.; Bengtson, S.; Canfield, D. E.; Bekker, A.; Macchiarelli, R.; Mazurier, A.; Hammarlund, E. U.; Boulvais, P.; Dupuy, J.-J.; Fontaine, C.; Fürsich, F. T.; Gauthier-Lafaye, F.; Janvier, P.; Javaux, E.; Ossa, F. O.; Pierson-Wickmann, A.-C.; Riboulleau, A.; Sardini, P.; Vachard, D.; Whitehouse, M.; Meunier, A. (2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago". Nature. 466 (7302): 100–104. Bibcode:2010Natur.466..100A. doi:10.1038/nature09166. PMID 20596019.

External links

Anorthosite

Anorthosite ( ) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Anorthosites are of enormous geologic interest, because it is still not fully understood how they form. Most models involve separating plagioclase crystals based on their density. Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there.Anorthosite on Earth can be divided into five types:

Archean-age anorthosites

Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth

Layers within Layered Intrusions (e.g., Bushveld and Stillwater intrusions)

Mid-ocean ridge and transform fault anorthosites

Anorthosite xenoliths in other rocks (often granites, kimberlites, or basalts)Of these, the first two are the most common. These two types have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins.Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

Atlantica

Atlantica (Greek: Ατλαντικα; Atlantika) is an ancient continent that formed during the Proterozoic about 2,000 million years ago (two billion years ago, Ga) from various 2 Ga cratons located in what is now West Africa and eastern South America.

The name, introduced by Rogers 1996, was chosen because the continent opened up to form the South Atlantic Ocean.

Beaverhead crater

The Beaverhead crater is an impact structure spanning the U.S. states of Idaho and Montana. Estimated at 60 kilometers (37 mi) in diameter, it is one of the largest impact craters on Earth.

With an estimated age of 600 million years (Neoproterozoic), the impact's original shatter cones along the crater's perimeter provide some of the structure's only remaining visible evidence.

It is named for the Beaverhead region of southwestern Montana in which it was first discovered.

Calymmian

The Calymmian Period (from Greek κάλυμμα (kálymma), meaning "cover") is the first geologic period in the Mesoproterozoic Era and lasted from 1600 Mya to 1400 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.

The period is characterised by expansion of existing platform covers, or by new platforms on recently cratonized basements.

The supercontinent Columbia broke up during the Calymmian some 1500 Mya.

Cryogenian

The Cryogenian ( , from Greek κρύος (krýos), meaning "cold" and γένεσις (génesis), meaning "birth") is a geologic period that lasted from 720 to 635 million years ago. It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran.

The Sturtian and Marinoan glaciations occurred during the Cryogenian period, which are the greatest ice ages known to have occurred on Earth. These events are the subject of much scientific controversy. The main debate contests whether these glaciations covered the entire planet (the so-called "Snowball Earth") or a band of open sea survived near the equator (termed "slushball Earth").

Ediacaran

The Ediacaran Period ( ), spans 94 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 541 Mya. It marks the end of the Proterozoic Eon, and the beginning of the Phanerozoic Eon. It is named after the Ediacara Hills of South Australia.

The Ediacaran Period's status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS), making it the first new geological period declared in 120 years.

Although the period takes its name from the Ediacara Hills where geologist Reg Sprigg first discovered fossils of the eponymous Ediacara biota in 1946, the type section is located in the bed of the Enorama Creek within Brachina Gorge in the Flinders Ranges of South Australia, at 31°19′53.8″S 138°38′0.1″E.

Era (geology)

A geologic era is a subdivision of geologic time that divides an eon into smaller units of time. The Phanerozoic Eon is divided into three such time frames: the Paleozoic, Mesozoic, and Cenozoic (meaning "old life", "middle life" and "recent life") that represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.

The Hadean, Archean and Proterozoic eons were as a whole formerly called the Precambrian. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, however, the Archean and Proterozoic eons have been subdivided into eras of their own.

Geologic eras are further subdivided into geologic periods, although the Archean eras have yet to be subdivided in this way.

Geology of Antarctica

The geology of Antarctica covers the geological development of the continent through the Proterozoic Eon, Paleozoic, Mesozoic and Cenozoic eras.

More than 170 million years ago, Antarctica was part of the supercontinent Gondwana. Over time Gondwana broke apart and Antarctica as we know it today was formed around 35 million years ago.

Geology of Egypt

The geology of Egypt includes rocks from Archaean - early Proterozoic times onwards. These oldest rocks are found as inliers in Egypt’s Western Desert. In contrast, the rocks of the Eastern Desert are largely late Proterozoic in age. Throughout the country this older basement is overlain by Palaeozoic sedimentary rocks. Cretaceous rocks occur commonly whilst sediments indicative of repeated marine transgression and regression are characteristic of the Cenozoic Era.

Grypania

Grypania is an early, tube-shaped fossil from the Proterozoic eon. The organism, with a size over one centimeter and consistent form, could have been a giant bacterium, a bacterial colony, or a eukaryotic alga. The oldest probable Grypania fossils date to about 1870 million years ago (redated from the previous 2100 million) and the youngest extended into the Ediacaran period. This implies that the time range of this taxon extended for 1200 million years.

Nena (supercontinent)

Nena, an acronym for Northern Europe–North America, is the Early Proterozoic amalgamation of Baltica and Laurentia into a single "cratonic landmass", a name first proposed in 1990. Since then several similar Proterozoic supercontinents have been proposed, including Nuna and Arctica, that include other Archaean cratons, such as Siberia and East Antarctica.In the original concept Nena formed c. 1,900 million years ago in the Penkean, Makkovikan, Ketilidian, and Svecofennian orogenies. However, because Nena excludes several known Archaean cratons, including those in India and Australia, it is strictly speaking not a supercontinent. Nena, or Nuna, can, nevertheless be though of as the core of Columbia, another supercontinent concept with several proposed configurations.Nena as a continent has been associated with the Sudbury Basin Impact.

Neoproterozoic

The Neoproterozoic Era is the unit of geologic time from 1,000 to 541 million years ago.It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran Periods. It is preceded by the Mesoproterozoic era and succeeded by the Paleozoic era.

The most severe glaciation known in the geologic record occurred during the Cryogenian, when ice sheets reached the equator and formed a possible "Snowball Earth".

The earliest fossils of multicellular life are found in the Ediacaran, including the Ediacarans, which were the earliest animals.

According to Rino and co-workers, the sum of the continental crust formed in the Pan-African orogeny and the Grenville orogeny makes the Neoproterozoic the period of Earth's history that has produced most continental crust.

Paleoproterozoic

Paleoproterozoic Era ( ;), spanning the time period from 2,500 to 1,600 million years ago (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's geological history. It was during this era that the continents first stabilized.

Paleontological evidence suggests that the Earth's rotational rate during this era resulted in 20-hour days ~1.8 billion years ago, implying a total of ~450 days per year.

Precambrian

The Precambrian (or Pre-Cambrian, sometimes abbreviated pЄ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.

The Precambrian (colored green in the timeline figure) is an informal unit of geologic time, subdivided into three eons (Hadean, Archean, Proterozoic) of the geologic time scale. It spans from the formation of Earth about 4.6 billion years ago (Ga) to the beginning of the Cambrian Period, about 541 million years ago (Ma), when hard-shelled creatures first appeared in abundance.

Shoemaker crater

Shoemaker (formerly known as Teague Ring) is an impact structure, the deeply eroded remnant of a former impact crater, situated in arid central Western Australia, about 100 km (62 mi) north-northeast of Wiluna. It is named in honour of planetary geologist Eugene Shoemaker.

Siderian

The Siderian Period ( ; Greek: σίδηρος, translit. sídēros, meaning "iron") is the first geologic period in the Paleoproterozoic Era and lasted from 2500 Ma to 2300 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.

The laying down of the banded iron formations (BIFs) peaked early in this period. BIFs were formed as anaerobic cyanobacteria produced waste oxygen that combined with iron, forming magnetite (Fe3O4, an iron oxide). This process removed iron from the Earth's oceans, presumably turning greenish seas clear. Eventually, with no remaining iron in the oceans to serve as an oxygen sink, the process allowed the buildup of an oxygen-rich atmosphere. This second, follow-on event is known as the oxygen catastrophe, which, some geologists believe triggered the Huronian glaciation.Since the time period from 2420 Ma to 2250 Ma is well-defined by the lower edge of iron-deposition layers, an alternative period named the Oxygenian, based on stratigraphy instead of chronometry, was suggested in 2012 by Gradstein et al. in a geological timescale review but, as of February 2017, this has not yet been officially adopted by the IUGS.

Statherian

The Statherian Period ( ; Greek: σταθερός (statherós), meaning "stable, firm") is the final geologic period in the Paleoproterozoic Era and lasted from 1800 Mya to 1600 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.

The period was characterized on most continents by either new platforms or final cratonization of fold belts.

By the beginning of the Statherian, the supercontinent Columbia had assembled.

Stenian

The Stenian Period (from Greek στενός (stenós), meaning "narrow") is the final geologic period in the Mesoproterozoic Era and lasted from 1200 Mya to 1000 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically. The name derives from narrow polymetamorphic belts formed over this period.

Preceded by the Ectasian period and followed by the Neoproterozoic Era.

The supercontinent Rodinia assembled during the Stenian. It would last into the Tonian period.

This period includes the formation of the Keweenawan Rift at about 1100 Mya.

Tonian

The Tonian (from Greek τόνος (tónos), meaning "stretch") is the first geologic period of the Neoproterozoic Era. It lasted from 1000 Mya to 720 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry. The Tonian is preceded by the Stenian Period of the Mesoproterozoic era and followed by the Cryogenian.

Rifting leading to the breakup of supercontinent Rodinia, which had formed in the mid-Stenian, occurred during this period, starting from 900 to 850 Mya.

Cenozoic era
(present–66.0 Mya)
Mesozoic era
(66.0–251.902 Mya)
Paleozoic era
(251.902–541.0 Mya)
Proterozoic eon(541.0 Mya–2.5 Gya)
Archean eon (2.5–4 Gya)
Hadean eon (4–4.6 Gya)

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