Proterozoic
Template:Short description Template:Use dmy dates Template:Infobox geologic timespan The Proterozoic (Template:IPAc-en Template:Respell)<ref>Template:Cite dictionary</ref><ref>Template:MerriamWebsterDictionary</ref><ref>Template:Dictionary.com</ref> is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8Template:NbspMya,<ref name="StratChart 2022">Template:Cite report</ref> and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozoic, and is the most recent part of the Precambrian "supereon".
The Proterozoic is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic and Neoproterozoic.<ref>Template:Cite web</ref> It covers the time from the appearance of free oxygen in Earth's atmosphere to just before the proliferation of complex life on the Earth during the Cambrian Explosion. The name Proterozoic combines two words of Greek origin: Template:Lang meaning "former, earlier", and Template:Lang, meaning "of life".<ref>Template:Cite dictionary</ref>
Well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; the evolution of eukaryotes via symbiogenesis; several global glaciations, which produced the 300 million years-long Huronian glaciation (during the Siderian and Rhyacian periods of the Paleoproterozoic) and the hypothesized Snowball Earth (during the Cryogenian period in the late Neoproterozoic); and the Ediacaran period (635–538.8 Ma), which was characterized by the evolution of abundant soft-bodied multicellular organisms such as sponges, algae, cnidarians, bilaterians and the sessile Ediacaran biota (some of which had evolved sexual reproduction) and provides the first obvious fossil evidence of life on Earth.
The Proterozoic record
[edit]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 Archean rocks, and many are unaltered.<ref name=Stanley>Template:Cite book</ref>Template:Rp 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 Template:Clarify span mountain building activity (orogeny).<ref name=Stanley/>Template:Rp
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.<ref name=Stanley/>Template:Rp
The accumulation of oxygen
[edit]Template:Main 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 the Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur and iron had been exhausted. Until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level.<ref name=Stanley/>Template:Rp 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.<ref name=Stanley/>Template:Rp
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.<ref name=Stanley/>Template:Rp The oxygen buildup was probably due to two factors: Exhaustion of the chemical sinks, and an increase in carbon sequestration, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere.<ref name=Stanley/>Template:Rp
The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the Great Oxygenation Event, or alternately the Oxygen Catastrophe – to reflect the mass extinction of almost all life on Earth, which at the time was virtually all obligate anaerobic. A second, later surge in oxygen concentrations is called the Neoproterozoic Oxygenation Event,<ref name=Shields-Zhou2011>Template:Cite journal</ref> occurred during the Middle and Late Neoproterozoic<ref name="Och2012">Template:Cite journal</ref> and drove the rapid evolution of multicellular life towards the end of the era.<ref>Template:Cite journal</ref><ref name=EdiacaranOxygenationIronIsotopes>Template:Cite journal</ref>
Subduction processes
[edit]The Proterozoic Eon was a very tectonically active period in the Earth's history. Oxygen changed the chemistry allowing for extensive geological changes. Volcanism was also extensive resulting in more geologic changes.
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.<ref name=Kearey>Template:Cite book</ref>
The occurrence of eclogite (a type of metamorphic rock created by high pressure, > 1 GPa), is 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.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite book</ref> 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.<ref name=Kearey/> Studies by Condie (2000)<ref>Template:Cite journal</ref> and Rino et al. (2004)<ref>Template:Cite journal</ref> 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.<ref name=Kearey/>
Supercontinent tectonic history
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Rodinia, about 900 Mya
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Pannotia, 545 Mya (disputedTemplate:Clarify), centered on South Pole
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Gondwana 420 Mya, centered on South Pole
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).<ref name=Kearey/>
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).<ref>Template:Cite journal</ref> 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.<ref>Template:Cite book</ref>
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 prior to the formation of Columbia, but the current most plausible hypothesis is that prior to Columbia, there were only a few independent cratons scattered around the Earth (not necessarily a supercontinent, like Rodinia or Columbia).<ref name=Kearey/>
Life
[edit]Template:Life timeline Template:Also The Proterozoic can be roughly divided into seven biostratigraphic zones which correspond to informal time periods. The first was the Labradorian, lasting from 2.0–1.65 Ga. It was followed by the Anabarian, which lasted from 1.65–1.2 Ga and was itself followed by the Turukhanian from 1.2–1.03 Ga. The Turukhanian was succeeded by the Uchuromayan, lasting from 1.03–0.85 Ga, which was in turn succeeded by the Yuzhnouralian, lasting from 0.85–0.63 Ga. The final two zones were the Amadeusian, spanning the first half of the Ediacaran from 0.63–0.55 Ga, and the Belomorian, spanning from 0.55–0.542 Ga.<ref>Template:Cite journal</ref>
The emergence of advanced single-celled eukaryotes began after the Oxygen Catastrophe.<ref>Template:Cite journal</ref> This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria.<ref name=Stanley/>Template:Rp 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.<ref name=Stanley/>Template:Rp
By the late Palaeoproterozoic, eukaryotic organisms had become moderately biodiverse.<ref>Template:Cite journal</ref> 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.<ref name=Stanley/>Template:Rp
The earliest fossils possessing features typical of fungi date to the Paleoproterozoic Era, some 2.4 billion years ago; these multicellular benthic organisms had filamentous structures capable of anastomosis.<ref>Template:Cite journal</ref>
The Viridiplantae evolved sometime in the Palaeoproterozoic or Mesoproterozoic, according to molecular data.<ref>Template:Cite journal</ref>
Eukaryote fossils from before the Cryogenian are sparse, and there seems to be low and relatively constant rates of species appearance, change, and extinction. This contrasts with the Ediacaran and early Cambrian periods, in which the quantity and variety of speciations, changes, and extinctions exploded.<ref name="Tang, Zheng, Zhang, Fan et all 2024">Template:Cite journal</ref>
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, as well as the animal-like Caveasphaera, appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, particularly in ones from the Ediacaran, proving that multicellular life had already become widespread tens of millions of years before the Cambrian Explosion in what is known as the Avalon Explosion.<ref name="OnTheEveOfAnimalRadiation">Template:Cite journal</ref> Nonetheless, the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 538.8 Ma. Template:Clear