The Proterozoic is a geological eon representing the time just before the
proliferation of complex life on Earth. The name Proterozoic comes from Greek
and means "earlier life". The Proterozoic Eon extended from
7016788940000000000♠2500 Ma to 7016171042192000000♠542.0±1.0 Ma, and is
the most recent part of the Precambrian. It is subdivided into three geologic
eras: the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.
The well-identified events of this eon were the transition to an oxygenated
atmosphere during the Paleoproterozoic; several glaciations, including the
hypothesized Snowball Earth during the Cryogenian period in the late
Neoproterozoic; and the Ediacaran Period 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. Study of these rocks shows that the eon continued the massive
continental accretion that had begun late in the Archean, as well as featured
the first definitive supercontinent cycles and wholly modern orogenic
activity. Evidence for 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, 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 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. Banded iron formations, which provide most of the
world's iron ore, were also a prominent chemical sink; their 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.
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
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.
Subduction processes The Proterozoic was a very tectonically
active period in the Earth’s history. The late Archean to Early Proterozoic
corresponds to a period of increasing crustal recycling, which may be due to
subduction. Evidence for this increased subduction activity comes from the
abundance of old granites originating mostly after 2.6 Ga. The appearance of
eclogites, which are high pressure metamorphic rocks, are explained using a
model incorporating subduction. The lack of Archean eclogites in today’s rock
record suggests that conditions at the time did not favor the formation of high
grade metamorphism and therefore did not achieve the same levels of subduction as
in the Proterozoic. As a result of remelting of basaltic oceanic crust due
to subduction, the cores of the first continents grew large enough to
withstand crustal recycling processes. The long-term tectonic stability of
these cratons is the reason we find continental crust ranging up to a few
billion years in age. It is believed that 43% of modern continental crust
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 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 been decreasing in magnitude after every
period. Tectonic history
Evidence of collision and rifting between continents raises questions as
to what exactly the movements of the Archean cratons composing Proterozoic
continents were Paleomagnetic and geochronological dating mechanisms
enable the advancement in deciphering Precambrian tectonics. It is known that
tectonic processes in the Proterozoic resemble greatly what we see today. This
is due to closely resembling features in the rock record, such as orogenic belts
or ophiolite complexes. Most geologists would conclude that the Earth was active
at the time. It is also commonly accepted that in the Precambrian the
Earth went through several Wilson cycles. In the late Proterozoic, the
dominant supercontinent was Rodinia. It consisted of a series of continents all
attached to a central craton called Laurentia. An example of an orogeny
associated with the construction of Rodinia is the Grenville orogeny located
in Eastern North America. Rodinia formed after the breakup of Columbia and prior
to the assemblage of the supercontinent Gondwana. 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 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.
Life The first advanced single-celled,
eukaryotes and multi-cellular life, Francevillian Group Fossils, 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 and chloroplasts 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.
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 542 Ma.
See also Timetable of the Precambrian
References External links
Palaeos.com: Proterozoic eon