|Palæos:||The Archean Eon|
|Archean||The Eoarchean Era|
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|Introduction to the Archean
The Isua Greenstone Belt
As far as official geology and stratigraphy goes, the Eoarchian is still shrouded in mystery. The term Eoarchean Era is used by the International Commission on Stratigraphy in 2000 for everything prior to the 3.6 Bya that arbitrarily marks the start of the Paleoarchean (both replacing Early or Lower Archean). Precambrian chronostratigraphic boundaries (Global Standard Stratigraphic Age (GSSA) were added in 2004, but neither the lower boundary of the era, nor that of the preceding Hadean Eon, was recognised. More recently, in 2009, the Hadean is restored informally, and the boundary placed arbitrarily at 4 gigayears ago, as well as adding a few chronostratigraphic boundaries for the later Archean eras. The 2009 Time Scale by the Geological Society of America, also restores the Hadean, and puts the boundary between the Hadean and Eoarchean as 3850 mya, which better fits the end of the Late Heavy Bombardment (more or less). We have followed that chronology here. - MAK110911
The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland. The belt is aged between 3.7 and 3.8 Ga, making it among the oldest rock in the world. The belt contains variably metamorphosed mafic volcanic and sedimentary rocks. The occurrence of Boninitic geochemical signatures offers evidence that plate tectonic processes may be responsible for the creation of the belt. Pillowed basalts indicate that liquid water existed on the surface at this time.- Wikipedia (includes (as of edit of 15 June 2011 ) comprehensive list of references)
The most ancient sedimentary rocks – thoseolder than about 3,300 Ma – occur at only a few places on Earth: the Isua supracrustal belt in southwest Greenland, the Barberton area in eastern South Africa, and the Pilbara area of northwest Australia.
The greenstones of the Isua Supracrustal Group date from around 3,700 Ma and possibly more than 3,800 Ma. Unfortunately, although some sequences are thought to be of sedimentary origin, they are strongly metamorphosed (to amphibolite facies) and no fossils have been recovered from them.
However, carbon isotope signatures recovered from these rocks provide indirect evidence that life may have existed in Isua times. "This isotopic evidence stems from the fact that the carbon atom has two stable isotopes, carbon-12 and carbon-13. The 12C/13C ratio in abiotic mineral compounds is 89. In biological syntheses, the processing of carbon [in] CO2 and carbonates gives a preference to the lighter carbon isotope and raises the ratio to about 92. Consequently, the carbon residues of previously living matter may be identified by this enrichment in 12C. A compilation has been made of the carbon isotopic composition of over 1,600 samples of fossil kerogen (a complex organic macromolecule produced from the debris of biological matter) and compared with that from carbonates in the same sedimentary rocks. This showed that biosynthesis by photosynthetic organisms was involved in all the sediments studied. In fact, this enrichment is now taken to be one of the most powerful indications that life on Earth was active nearly 3.9 billion years ago because the sample suite encompasses specimens right across the geological time scale" (Brack 2002).
Mojzsis et al. 1996 (p. 55) claims to have identified biological carbon isotope signatures from >n;3,800 Ma aged, chemically precipitated sediments, including banded iron formations (BIFs) and chert, on Akilia Island, southwestern Greenland. However, this interpretation has been challenged by Fedo & Whitehouse (2002) who regard the contested unit as a younger hydrothermal vein. The original claim has been vigorously defended and the final conclusion is as yet unresolved.
More certain is the report from Rosing (1999) of a biological carbon isotope signature from ~3,780 Ma (3,779 ± 81 Sm-Nd date) greywackes and slates with well-preserved sedimentary structures from the Garbenschiefer Formation in the Isua belt.
Some molecular clock analyses suggest an even earlier origin: Hedges 2002 estimates the divergence of Bacteria and Archaea at >n;4 Ga, noting however, that "the fidelity of genetic replication and repair systems in the early history of life is unknown, and the different environment of early Earth might have affected rates of molecular change. It is for these reasons that we have less confidence in the time estimates for the earliest splitting events" (p. 842). A phylogenetic tree constructed from highly conserved portions of the iron/manganese superoxide dismutase enzyme sequence (Kirschvink et al. 2000, p. 1404) suggests an age for this divergence of 3 to 4 Ga.
- Chris Clowes, Peripatus - Hadean Era
Links GeoWhen Database - Eoarchean; Wikipedia, Peripatus - Hadean Eon (also pertains to Eoarchean)
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