Lower Proterozoic Stromatolites from Bolivia, South America

The Proterozoic ( /ˌproʊtərɵˈzoʊ.ɨk/) 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 2,500 Ma to 542.0±1.0 Ma (million years ago), and is the most recent part of the informally named ‘Precambrian’ time. 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 Mesoproterozoic; several glaciations, including the hypothesized Snowball Earth during the Cryogenian period in the late Neoproterozoic; and 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.^[1] 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.^[2]

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 Sturtian and Marinoan glaciations.^[3]

  The buildup 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.^[4] 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.^[5]

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.^[5] 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.^[6]

  Paleogeography and tectonics

Throughout the history of the Earth, there have been times when the continental mass came together to form a supercontinent, followed by the break-up of the supercontinent and new continents moving apart again. This repetition of tectonic events is called a Wilson cycle. It is at least clear that, about 1,000–830 Ma, most continental mass was united in the supercontinent Rodinia.^[7] Rodinia was not the first supercontinent; it formed at about 1.0 Ga by accretion and collision of fragments produced by breakup of the older supercontinent, called Nuna or Columbia, which was assembled by global-scale 2.0–1.8 Ga collisional events.^[8][9] This means plate tectonic processes similar to today's must have been active during the Proterozoic.

After the break-up of Rodinia about 800 Ma, it is possible the continents joined again around 550 Ma. The hypothetical supercontinent is sometimes referred to as Pannotia or Vendia. The evidence for it is a phase of continental collision known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South-America, Antarctica and Australia. It is extremely likely, however, that the aggregation of continental masses was not completed, since a continent called Laurentia (roughly equivalent to current-day North America) had already started breaking off around 610 Ma. It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole.^[10]

  Life

  Stromatolites (Proterozoic) Zebra River Canyon, 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.^[11] This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria.^[6] 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.^[12]

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.^[13]

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

Wikimedia Commons has media related to: Proterozoic

• Proterozoic - Palaeos