Proterozoic
Proterozoic | |
---|---|
Upper GSSP ratified | 1992 |
The Proterozoic (
The Proterozoic is subdivided into three
Well-identified events of this eon were the
The Proterozoic record
The geologic record of the Proterozoic Eon is more complete than that for the preceding
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
The accumulation of oxygen
One of the most important events of the Proterozoic was the
The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the
Subduction processes
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
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.[15]
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.[16] 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.[14] Studies by Condie (2000)[17] and Rino et al. (2004)[18] 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.[14]
Supercontinent tectonic history
-
Rodinia, about 900 Mya
-
Pannotia, 545 Mya (disputed[clarification needed]), centered on South Pole
-
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.
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).[19] 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.[20]
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).[14]
Life
million years ago) |
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
The emergence of advanced single-celled
By the late Palaeoproterozoic, eukaryotic organisms had become moderately biodiverse.
The earliest
The Viridiplantae evolved sometime in the Palaeoproterozoic or Mesoproterozoic, according to molecular data.[25]
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.[26]
Classically, the boundary between the Proterozoic and the
See also
References
- ^ Smithsonian National Museum. flickr (photo album).
- .
- OxfordDictionaries.com.
- ^ "Proterozoic". Merriam-Webster.com Dictionary. Merriam-Webster.
- ^ "Proterozoic". Dictionary.com Unabridged (Online). n.d.
- International Stratigraphic Commission. February 2022. Retrieved 22 April 2022.
- ^ Speer, Brian. "The Proterozoic eon". University of California Museum of Paleontology (ucmp.berkeley.edu). Berkeley, CA.
- ^ "Proterozoic, adj. and n.". OED Online. Oxford University Press. June 2021. Archived from the original on 25 June 2021. Retrieved 25 June 2021.
- ^ ISBN 978-0-7167-2882-5.
- .
- .
- S2CID 24761414.
- .
- ^ a b c d e Kearey, P.; Klepeis, K.; Vine, F. (2008). Precambrian Tectonics and the Supercontinent Cycle. Global Tectonics (Third ed.). pp. 361–377.
- .
- ^ Mengel, F. (1998). Proterozoic History. Earth System: History and variablility. Vol. 2.
- .
- S2CID 140166194.
- S2CID 128469935.
- The Geological Society of America.
- .
- S2CID 257761993.
- .
- S2CID 25586788.
- PMID 37696791.
- ISSN 0036-8075.
- PMID 18952316.
External links
- "Proterozoic eon". Palaeos.com. Archived from the original on 2 July 2007.
- "Proterozoic". chronostratigraphy scale.