Proterozoic
| Proterozoic eon |
|---|
| The Proterozoic eon comprises the Paleo-, Meso-, and Neo- proterozoic eras, and is preceded by the Archaean eon. |
| 2500 - 542 millions of years ago |
 (expand) |
The Proterozoic is the geologic eon extending from about 2500 million years ago (mya), at the end of the Archean eon, to about 542.0 ± 1.0 mya, at the start of the current Phanerozoic eon.
The Proterozoic is the most recent part of the Precambrian, an informal name for the billions of years of the geologic timescale that came before the current Cambrian, the first period of the Phanerozoic. The Cambrian marked the appearance of abundant macroscopic hard-shelled fossils. The Proterozic, on the other hand, represents a period of time before the first abundant complex life on Earth.
The origin of life traces to the prior Archean, with prokaryote fossils known from 3.5 billion years ago (3500 mya). Quite possibly, eukaryote organisms appeared near the end of the Archean, perhaps 2.7 billion years ago, although the earliest eukaryote fossils trace to about 1.7 billion years ago, during the Proterozic (Mayr 2001). During the Proterozic, eukaryotes greatly diversified, and it is held that there also were abundant soft-bodied, multicellular organisms during the late Proterozoic, but being soft-bodied they failed to yield fossils. Another processes set in motion during the Precambrian were a transition to an oxygenated atmosphere. These provide the foundation for the explosion of life forms during the Cambrian explosion, when most phyla of animals appeared.
| Geologic Time (ca. 4500 million years ago - present) | |||
|---|---|---|---|
| Hadean | Archean | Proterozoic | Phanerozoic |
| Precambrian (ca. 4500 - 542 million years ago) | |||
Overview
The Proterozoic consists of 3 geologic eras, from oldest to youngest:
- Paleoproterozoic
- Mesoproterozoic
- Neoproterozoic
The Neoproterozoic is the era from 1000 mya to 542 mya. It covers one of the more interesting time in the geological record, during which the Earth was hit by the most severe glaciations known (during which ice sheets reached the equator), and in the later parts of the era (the Ediacaran period) the earliest evidence of multicelled life is found, including the earliest animals.
During the Mesoproterozoic, there was a transition to an oxygenated atmosphere.
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 (Stanley 1999). Study of these rocks show that the eon featured massive, rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and modern orogenic (mountain building) activity (Stanley 1999).
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 Varangian glaciation (Stanley 1999).
| Proterozoic eon (2500 - 542 million years ago) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Paleoproterozoic era | Mesoproterozoic era | Neoproterozoic era | |||||||
| Siderian | Rhyacian | Orosirian | Statherian | Calymmian | Ectasian | Stenian | Tonian | Cryogenian | Ediacaran |
| ----------X--------------------------Three probable snowball earth episodes.------------------------------------X—X---- | |||||||||
The Build-up 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.[1] 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.[2]
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.[2] The oxygen build-up 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.[3]
Proterozoic Life
The first advanced single-celled and multi-cellular life roughly coincides with the oxygen accumulation; this may have been due to an increase in the oxidised nitrates that eukaryotes use, as opposed to cyanobacteria.[3] It was also during the Proterozoic that the first symbiotic relationship between mitochondria (for animals and protists) and chloroplasts (for plants) and their hosts evolved.[4]
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.[5]
Classically, the boundary between the Proterozoic and the Paleozoic was set at the base of the Cambrian period when the first fossils of animals known as 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
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