Paleoarchean
Vaalbara Continent.jpg
The Vaalbara continent, a continent dating from 2.7 to 3.6 billion years ago
Chronology
Proposed redefinition(s)4031–3490 Ma
Gradstein et al., 2012
Proposed subdivisionsAcastan Period, 4031–3810 Ma

Gradstein et al., 2012
Isuan Period, 3810–3490 Ma

Gradstein et al., 2012
Etymology
Name formalityFormal
Alternate spelling(s)Palaeoarchaean
Synonym(s)Early Archean
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitEra
Stratigraphic unitErathem
Time span formalityFormal
Lower boundary definitionDefined Chronometrically
Lower boundary GSSPN/A
GSSP ratifiedN/A
Upper boundary definitionDefined Chronometrically
Upper boundary GSSPN/A
GSSP ratifiedN/A

The Paleoarchean (/ˌpli.ɑːrˈkən/), also spelled Palaeoarchaean (formerly known as early Archean), is a geologic era within the Archaean Eon. The name derives from Greek "Palaios" ancient. It spans the period of time 3,600 to 3,200 million years ago. The era is defined chronometrically and is not referenced to a specific level of a rock section on Earth. The earliest confirmed evidence of life comes from this era, and Vaalbara, one of Earth's earliest supercontinents, may have formed during this era.[2][3]

Early life

A stromatolite formed by Paleoarchean microbial mats, preserved as a fossil, from Pilbara craton, Western Australia.
A stromatolite formed by Paleoarchean microbial mats, preserved as a fossil, from Pilbara craton, Western Australia.

The geological record from the Paleoarchean era is very limited. Due to deformation and metamorphism, most rocks from the Paleoarchean era cannot provide any useful information. There are only two locations in the world containing rock formations that are intact enough to preserve evidence of early life: the Kaapvaal Craton in Southern Africa and the Pilbara Craton in Western Australia.[4]

The Dresser Formation, located in the Pilbara Craton, contains samples of sedimentary rock from the Paleoarchean Era.[4] Inside the rocks, there are microbial mats containing the oldest ascertained life form, fossilized bacteria estimated to be 3.48 billion years old.[2] The Strelley Pool Chert, also located in the Pilbara Craton, contains stromatolites that may have been created by bacteria 3.4 billion years ago. However, it is possible that these stromatolies are abiogenic and were actually formed through evaporitic precipitation then deposited on the sea floor.[5]

The Barberton Greenstone Belt, located in the Kaapvaal Craton, also contains evidence of life. It was created around 3.26 Ga when a large asteroid, about 37 to 58 kilometres (23–36 mi) wide, collided with the Earth.[6] The Buck Reef chert and the Josefsdal chert, two rock formations in the Barberton Greenstone Belt, both contain microbial mats with fossilized bacteria from the Paleoarchean era.[4] The Kromberg Formation, near the top of the Onverwacht Group which itself is a part of the Barberton Greenstone Belt, dates back to approximately 3.416–3.334 Ga and contains evidence of microbial life reproducing via multiple fission and binary fission.[7]

Continental development

Artist's impression of what Vaalbara may have looked like.
Artist's impression of what Vaalbara may have looked like.
A map of the Barberton Greenstone Belt in southern Africa.

Similarities between the Barberton Greenstone Belt in the Kaapvaal Craton and the eastern part of the Pilbara Craton indicate that the two formations were once joined as part of the supercontinent Vaalbara, one of Earth's earliest supercontinents.[3] Both cratons formed at the beginning of the Paleoarchean era.[8] While some paleomagnetic data suggests that they were connected during the Paleoarchean era, it is possible that Vaalbara did not form until the Mesoarchean or Neoarchean eras.[3]

It is also unclear whether there was any exposed land during the Paleoarchean era. Although several Paleoarchean formations such as the Dresser Formation, the Josefsdal Chert, and the Mendon Formation show some evidence of being above the surface, over 90 percent of Archean continental crust has been destroyed, making the existence of exposed land practically impossible to confirm or deny. It is likely that during the Paleoarchean era, there was a large amount of continental crust, but it was still underwater and would not emerge until later in the Archean era. Hotspot islands may have been the only exposed land at the time.[9]

Due to a much hotter mantle and an elevated oceanic geothermal gradient compared to the present day, plate tectonics in its modern form did not exist during the Paleoarchean. Instead, a model of "flake tectonics" has been proposed for this era of geologic time. According to this model, instead of normal subduction of oceanic plates, extensively silicified upper oceanic crust delaminated from lower oceanic crust and was deposited in a manner similar to ophiolites from the later Proterozoic and Phanerozoic eons.[10]

References

  1. ^ Caredona, Tanai (6 March 2018). "Early Archean origin of heterodimeric Photosystem I". Heliyon. 4 (3): e00548. doi:10.1016/j.heliyon.2018.e00548. PMC 5857716. PMID 29560463. Retrieved 25 February 2021.
  2. ^ a b Lepot, Kevin (2020). "Signatures of early microbial life from the Archean (4 to 2.5 Ga) eon". Earth-Science Reviews. 209: 103296. Bibcode:2020ESRv..20903296L. doi:10.1016/j.earscirev.2020.103296. ISSN 0012-8252. S2CID 225413847.
  3. ^ a b c Bradley, Kyle; Weiss, Benjamin P.; Buick, Roger (2015). "Records of geomagnetism, climate, and tectonics across a Paleoarchean erosion surface". Earth and Planetary Science Letters. 419: 1–13. Bibcode:2015E&PSL.419....1B. doi:10.1016/j.epsl.2015.03.008. ISSN 0012-821X.
  4. ^ a b c Homann, Martin (2019). "Earliest life on Earth: Evidence from the Barberton Greenstone Belt, South Africa". Earth-Science Reviews. 196: 102888. Bibcode:2019ESRv..19602888H. doi:10.1016/j.earscirev.2019.102888. ISSN 0012-8252. S2CID 198424907.
  5. ^ van Kranendonk, Martin J. (2007). "Chapter 7.2 A review of the evidence for putative Paleoarchean life in the Pilbara craton, Western Australia". In van Kranendonk, Martin J.; Smithies, R. Hugh; Bennett, Vickie C. (eds.). Developments in Precambrian Geology. Earth's Oldest Rocks. Vol. 15. Elsevier. pp. 855–877. doi:10.1016/s0166-2635(07)15072-6. ISBN 9780444528100. Retrieved 2021-11-26.
  6. ^ "Scientists reconstruct ancient impact that dwarfs dinosaur-extinction blast" (Press release). American Geophysical Union. 9 April 2014.
  7. ^ Kaźmierczak, Józef; Kremer, Barbara (1 September 2019). "Pattern of cell division in ∼3.4 Ga-old microbes from South Africa". Precambrian Research. 331: 1–9. doi:10.1016/j.precamres.2019.105357. Retrieved 17 December 2022.
  8. ^ van Kranendonk, Martin J.; Smithies, R. Hugh; Griffin, William L.; Huston, David L.; Hickman, Arthur H.; Champion, David C.; Anhaeusser, Carl R.; Pirajno, Franco (2015). "Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons". Geological Society, London, Special Publications. 389 (1): 83–111. Bibcode:2015GSLSP.389...83V. doi:10.1144/SP389.12. ISSN 0305-8719. S2CID 130084679.
  9. ^ Korenaga, Jun (2021). "Was there land on the early Earth?". Life. 11 (11): 1142. doi:10.3390/life11111142. PMC 8623345. PMID 34833018.
  10. ^ Grosch, Eugene G.; Viola, Giulio; Ndlela, Sibusisiwe (15 August 2020). "Geological record of Paleoarchean oceanic flake tectonics preserved in the c. 3.3 Ga Kromberg volcanic type-section, Barberton greenstone belt, South Africa". Precambrian Research. 346. doi:10.1016/j.precamres.2020.105815. Retrieved 17 December 2022.