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Archean Era


Abstract

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Keywords: xxx

Introduction

follows the Hadean

approx 3,500 Ma to 2,500 Ma

followed by the Proterozoic

 

 
 

Related Topics


Further Reading

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Related Pages

Other Web Sites

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Stratigraphy

Type Section/Sections

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The Lower (Hadean–Archean) Boundary

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approx 3,500 Ma ~ formation of persistent crust

The oldest rocks known are those of the Isua Supracrustal Group of southwestern Greenland, surviving from around 3700 Ma but, unfortunately, preserving no fossils. The Isua rocks are strongly metamorphosed; although some sequences have been demonstrated to be of sedimentary origin, and may have once contained fossils, the heat and pressure to which they have been subjected will have erased any traces.

The Upper (Archean–Proterozoic) Boundary

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approx 2,500 Ma

Current Chronology of the Vendian

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Paleogeography

Major Tectonic Events

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Land and Sea

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Climate

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Detrital uraninite and paleosol profiles indicate that the late Archean ceiling for oxygen concentrations remained about 1-2% PAL.

 

SCIENCE, Volume 298, Issue 5602, December 20 2002

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Reducing Schemes
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Geological evidence suggests that Earth’s early atmosphere had little free oxygen, but direct evidence of oxygen content during Earth’s history has been unavailable. Two reports take advantage of the better record of sulfur isotope values for much of the Precambrian to infer the abundance of oxygen and the sulfur chemistry of the early Earth (see the Perspective by Wiechert). Isotopic fractionation is normally mass dependent, but exceptions are known, including the mass-independent photochemical fractionation of sulfur that occurs today in the upper atmosphere where ozone is absent. For sulfur, evidence for mass-independent effects that reflect a paucity of atmospheric oxygen has been found in Archean crustal rocks. Farquhar et al. (p. 2369) now report evidence of mass-independent isotope effects in sulfide inclusions in ancient diamonds, which are derived from deep in Earth’s mantle. These data provide direct evidence that these diamonds were sampling material that reflected processes in Earth’s atmosphere and had been subducted into the mantle during the Archean. Archean sulfide rocks are much less depleted than are modern sulfides in 34S, an isotope that reflects bacterial processing. In culture experiments, Habicht et al. (p. 2372) show that the isotope data limit Archean ocean sulfate concentrations to 200 micromolar or less, considerably lower than previously thought. These data also imply that the atmosphere had little free oxygen but an abundance of the greenhouse-gas methane produced through the action of more prevalent methanogens.

 

 

Bjerrum & Canfield 2004 (p. 7) challenges the long-standing opinion that the fraction of total carbon which is buried as organic carbon (the f ratio) through the Precambrian was generally similar to today. Significant removal of isotopically depleted dissolved inorganic carbon from deep ocean waters "can have a large influence on the f ratio, and it is likely that the f ratio was less than half the present ratio during nearly all of the Archean, and could have been as low as zero for some of the time. Low f ratios mean less organic carbon burial, and imply relatively lower rates of oxygen release to the surface environment.... Low rates of organic carbon burial could have acted alone, or more likely together, with high fluxes of mantle-derived reduced gases ... to explain the low concentrations of O2 in the Archean atmosphere...."

Paleontology

General Characteristics

"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.)

... Late Archean ...

Another ancient cyanobacteria collection is reported from the Nauga Formation, Prieska, South Africa, between 2588 ± 6 and 2549 ± 7 Ma (Kazmierczak & Altermann 2002).

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.

"Bacteria microfossils dating back 3.3 to 3.4 billion years have also been discovered in rocks from the Barberton greenstone belt, South Africa. 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 Archaean 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 1,000 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 Archaean, when there was no protective ozone layer" (Brack 2002).

Almost a billion years later, between about 2,900 and 2,600 Ma, stromatolites appear more or less abruptly, forming widespread and thick carbonate platforms such as the South African Campbellrand platform. From this platform comes another ancient cyanobacterial collection, reported from the Nauga Formation, Prieska, South Africa, and dated between 2,588 ± 6 and 2,549 ± 7 Ma (Kazmierczak & Altermann 2002). From then on, stromatolites provide a virtually unbroken fossil record into the Phanerozoic, including such famous occurrences as the 2,100 Ma Gunflint Formation from Canada and the 850 Ma Bitter Springs Formation in Australia.

For further reading, see Schopf 1999.

Lagerstätten

Lagerstätten (sing. lagerstätte) are fossil localities which are highly remarkable for for either their diversity or quality of preservation; sometimes both.

Both these criteria are relative and can only be appreciated in some sort of context. In fact, any form of fossil preservation is remarkable in rocks as ancient as these. Thus, although the term is hardly ever applied, there is a case to be made for calling the following Archean fossil beds ‘lagerstätten.’

 

supplement from Cradle

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.

The oldest fossils known to date derive from the Apex Chert, a formation of the Pilbara Supergroup occuring in northwestern Western Australia, and dated at 3,465 Ma (± 5 Ma, see Schopf 1999, pp. 88-89). However, the fossils occur in fragments of rock within the chert; thus they are even older, though by how much, is unknown. The organisms themselves are filamentous, composed of distinct, organic walled cells occurring as a uniserial string, and are interpreted as cyanobacteria.

Questioning the evidence for Earth’s oldest fossils
MARTIN D. BRASIER*, OWEN R. GREEN*, ANDREW P. JEPHCOAT*, ANNETTE K. KLEPPE*, MARTIN J. VAN KRANENDONK?, JOHN F. LINDSAY?, ANDREW STEELE & NATHALIE V. GRASSINEAU

Nature 416, 76 - 81 (2002)

Correspondence and requests for materials should be addressed to M.D.B. (e-mail: martinb@earth.ox.ac.uk).

Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from 3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer-Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.

EARLY ARCHEAN (3500-3000 Ma)
Lithostratigraphic Unit Age (Ma) Location References
Warrawoona Group 3500-3400 Australia Schopf & Packer (1987); Schopf (1992, 1993)
Onverwacht & Fig Tree Group 3500-3400 South Africa Knoll & Barghoorn (1977); Walsh & Lowe (1985); Walsh (1992)

 

LATE ARCHEAN (3000-2500 Ma)

Lithostratigraphic Unit Age (Ma) Location References
Fortescue Group 2800 Australia Schopf & Walter (1983)
Transvaal Supergroup 2500 South Africa Lanier (1986); Klein et al. (1987)

Major Evolutionary Events

"There are only three places on Earth with sedimentary rocks older than 3300 million years: the greenstone belts of Isua in southwest Greenland, the Barberton area east of South Africa, and the Pilbara area of northwest Australia. The oldest sediments on Earth have been found in Greenland. The isotopic signatures of the organic carbon in these sediments provide indirect evidence that life may be 3.85 billion years old" (Brack 2002).

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Major Taxa

"The early Archean fossil record at best contains a handful of morphotypes, none of which is taxonomically diagnostic or physiologically informative. Indeed the low apparent diversity of early Archean fossils cannot itself be taken at face value. Studies of Proterozoic formations show that with increasing diagenetic/metamorphic alteration, the apparent diversity of microfossil assemblages decreases (Knoll et al.1988). Thus, when subjected to lower greenschist facies metamorphism, an assemblage with an original diversity comparable to, say, the Gunflint Formation might well yield a morphological record much like that actually seen in early Archean cherts."

"Stromatolites shed some light on the nature of early Archean life, but again there are uncertainties and differing interpretations of reported structures."

"[T]he microbial mat origin of the stratiform mats remains well supported. Complex communities of microorganisms including phototactic mat builders, certainly colonized early Archean coastal environments. It is reasonable to suggest that these communities included photoautotrophs, but this is not beyond question (Walter 1983)."

"The issue of cyanobacterial antiquity is directly related to the question of Earth’s atmospheric history. Most students agree that prior to the evolution of cyanobacterial photosynthesis there could have been only trace amounts of oxygen in the atmosphere, perhaps 10-10 atm 02, (Holland 1984). Higher oxygen concentrations must have been generated photosynthetically. The presence of hematite iron formations has been used as prima facie evidence for cyanobacterial oxygen production, but it is possible that these sedimentary deposits were generated by the photooxidation of ferrous iron dissolved in anoxic early Archean oceans."

"… Despite low global pO2, relatively high concentrations of oxygen could have accumulated locally in association with high cyanobacterial productivity; however, oxygen oases would have been transient in time and space. Therefore, obligately aerobic organisms could not have evolved until pO2 reached stable and global levels of 1-2% PAL [present atmospheric level] (Chapman & Schopf 1983)."

The late Archean record (3000-2500 Ma)

"Schopf & Walther (1983) reported rare trichomes from the ca. 2800 Ma Fortescue Group, Western Australia. The fossils resemble oscillatorian cyanobacteria, but they are not taxonomically diagnostic; similar morphologies occur among both sulfur-oxidizing and sulfate-reducing bacteria. More diverse microfossils have been reported from the ca 2500 Ma Transvaal Supergroup, South Africa. Silicified microstromatolites and associated intraclasts from platform environments contain 1-5 m m diameter coccoids and thin filamentous sheaths interpreted as primary producers as well as tiny rods interpreted as heterotrophic bacteria (Lanier 1986). Chert nodules in deeper basinal limestones contain carbonate-lined filamentous sheaths up to 27 m m in cross-sectional diameter (Klein et al. 1987)."

"Stromatolites become increasingly abundant in younger Archean successions, a pattern as likely to reflect craton growth as evolutionary change. By the end of the eon, extensive carbonate platforms supported widespread mat-building communities that almost certainly included cyanobacteria."

(After Knoll 1996.)

Extinctions

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Conclusion

"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).

References

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.


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