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| Artist's depiction
of life on the ocean floor as it may have
appeared in the late Proterozoic. |
Proterozoic
The Proterozoic ( /ˌproʊtərəˈzoʊɪk, prɒt-, -əroʊ-, -trə-,
-troʊ-/) 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.
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. |
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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 there are
Archean ones, and many are unaltered. 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).
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.
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 exhausted. Until
roughly 2.3 billion years ago, oxygen was probably only
1% to 2% of its current level. 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.
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. The oxygen buildup was probably due to two
factors: exhaustion of the chemical sinks, and an
increase in carbon burial, which sequestered organic
compounds that would have otherwise been oxidized by the
atmosphere. |
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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. 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. 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. 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.
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. |
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).
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). 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.
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). |
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Life
The first advanced single-celled, eukaryotes and
multi-cellular life, preserved as the Francevillian
biota, 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 (found in nearly all eukaryotes)
and chloroplasts (found in plants and some protists
only) and their hosts evolved.
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.
The earliest fossils possessing features typical of
fungi date to the Paleoproterozoic era, some 2,400
million years ago; these multicellular benthic organisms
had filamentous structures capable of anastomosis.
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, but the upper boundary of
the Proterozoic has remained fixed at the base of the
Cambrian, which is currently placed at 541 Ma. |
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|
 Kiddle: Proterozoic
Wikipedia: Proterozoic |
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