|The Archean Eon|
|Archean||The Paleoarchean Era|
(Timeline - earlier)
(Timeline - later)
(Timeline - earlier)
(Life - earlier)
(Life - later)
(Timeline - later)
The Paleoarchean (also spelled Palaeoarchaean) is the second geologic era of the Archean, immediately following the Eoarchean. It dates from 3600 to 3200 million years ago, and is arbitrarily defined chronometrically rather than (as with Phanerozoic strata) referenced with specific rock sections. Although the oldest unambiguous microfossils and stromatolites date from this era, it is generally agreed that life appeared earlier. Wikipedia
Vaalbara is theorized to be Earth's first supercontinent, beginning its formation about 3,600 million years ago, completing its formation by about 3,100 million years ago and breaking up by 2,500 million years ago. The name Vaalbara is derived from the South African Kaapvaal craton and the West Australian Pilbara craton. These cratons were combined during the time of the Vaalbara supercontinent. Identical radiometric ages of 3,470 ± 2 million years ago have been obtained for the ejecta from the oldest impact events in each craton. Remarkably similar structural sequences between these two cratons have been noted for the period between 3,500 to 2,700 million years ago. Wikipedia
Pending revision of this page, the following material is lifted verbatim from Peripatus - Archaean Eon:
"The early Archean [3500-3000 Ma] paleontological record is meagre. Virtually all critical data come from two successions, the Warrawoona Group of Western Australia and the Onverwacht and Fig Tree Groups of South Africa. There may even be redundancy in these two successions, in that some geologists believe that they are tectonically separated portions of a single depositional basin.
"[B]oth successions contain carbonaceous microstructures. These structures are uncommon, and their interpretation as microfossils has been challenged repeatedly (Schopf & Walter 1983; Buick 1991).
"During the 1970s, further research on Onverwacht and Fig Tree cherts produced a second round of paleontological reports (Muir & Grant 1976; Knoll & Barghoorn 1977). The case for the biogenicity of at least some of these structures is stronger. For example, the structures reported by Knoll & Barghoorn (1977) have a well-defined unimodal size frequency distribution with a mean of 2.5 m m; about 25% of the individuals in a large sample population are clearly paired or have a distinct hour-glass morphology comparable to those of cells undergoing binary fission; the cells have a distinct wall layer and collapsed internal contents, much like that seen in younger microfossils; and individual microstructures may be flattened parallel to bedding, indicating their emplacement prior to sediment compaction. Are they fossils? Quite possibly, but given their simple morphology , unequivocal acceptance of biogenicity is impossible.
"Perhaps a nearly thirty year tradition of rejecting previously reported material while presenting new "unequivocal" evidence is at an end. Schopf (1992, 1993) has discovered poorly preserved but convincingly biological filaments in cross-bedded Warrawoona chert grainstones. Having visited the outcrop in question, I regard the early Archean age of these fossils as beyond question."
(After Knoll 1996.)
The oldest plausible fossils reported to date derive from the Apex Chert, a formation of the Pilbara Supergroup occurring in north-western Western Australia (Schopf 1994, p. 6735; Schopf 1999, pp. 88-89). The rocks are dated at 3,465 ± 5 Ma. However, the putative fossils occur in fragments of rock within the chert; thus they are even older, though by how much is unknown. The structures are filamentous, apparently composed of distinct, organic walled cells occurring as a uniserial string, and were originally interpreted as cyanobacteria. Sceptics (notably Brasier et al. 2002) have questioned the biological attribution of these forms but, although the cyanobacterial affinity has been conceded as improbable, the debate continues.
Apex Chert (Pilbara Supergroup): Eleven species of filamentous fossil microbes comprising the oldest diverse microbial assemblage now known in the geological record were discovered in cherts from the Pilbara greenstone belt, northwest Australia. This prokaryotic assemblage establishes that cyanobacterium-like microorganisms were extant and both morphologically and taxonomically diverse at least as early as ~3.465 billion years ago. Barberton: (= Fig Tree?) Bacteria microfossils dating back 3.3 to 3.4 billion years have also been discovered in rocks from the Barberton greenstone belt, South Africa.
Strelley Group: Long, fine filaments probably representing thermophilic microorganisms living in the vicinity of a hydrothermal vent have been found in a massive sulfide deposit from the Early Archean Strelley Group (about 3.235 billion years old) of the Pilbara greenstone belt, northwest Australia. Although the temperature of the hydrothermal fluids was about 300°C, the microorganisms more likely developed at temperatures below 110°C and at water depths of about 1000 m. Under such environmental conditions, the microorganisms would have been anaerobic chemotrophs metabolizing in a reducing environment and obtaining their energy and nutrients from the hydrothermal fluids. This deep environment would have provided the microbiota with protection from the harmful UV radiation prevalent at the surface of the Earth during the Archean, when there was no protective ozone layer. (Brack 2002).
"The early Archean record tells us that life was present at least 3500 Ma ago. Microbial ecosystems were driven by autotrophy, most likely photoautotrophy, and oxygenic cyanobacteria may already have appeared. Heterotrophs included prokaryotes and, possibly, primitive amitochondrial eukaryotes capable of feeding by phagocytosis. Depending on the amount of 02 available, the biota could also have included aerobic prokaryotes and mitochondrion bearing eukaryotic heterotrophs (but perhaps not eukaryotic algae; see below, and Knoll & Holland 1995). Although impossible to test empirically, the possibility that early communities included organisms unlike anything represented in the modern biota cannot be excluded. Clearly, early Archean ecosystems remain poorly understood" (Knoll 1996, p. 55).
Bjerrum, Christian J.; Canfield, Donald E. 2004: New insights into the burial history of organic carbon on the early Earth. Geochemistry Geophysics Geosystems 5, Q08001, doi:10.1029/2004GC000713.
Brack, André 2002 (in press): Origin of Life. In Encyclopedia of Life Sciences. Nature Publishing Group, Macmillan.
Kazmierczak, Józef; Altermann, Wladyslaw 2002: Neoarchean Biomineralization by Benthic Cyanobacteria. Science 298: 2351.
Knoll, A.H. 1996: Chapter 4. Archean and Proterozoic Paleontology. In Jansonius, J.; McGregor, D.C. (eds.) 1996: Paleontology: Principles and Applications. American Association of Stratigraphic Palynologists Foundation, v. 1, pp. 51-80.
Links GeoWhen Database - Paleoarchean; Peripatus - Archaean Eon, Wikipedia
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