Mathematics – Logic
Scientific paper
Jun 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980orli...10..127t&link_type=abstract
Origins of Life, Volume 10, Issue 2, pp.127-136
Mathematics
Logic
2
Scientific paper
The stable isotopes of sulphur are fractionated in equilibrium and unidirectional processes in the earth's crust and biosphere. By far the most important of these processes occur in the biological sulphur cycle characterized by the activity of sulphur oxidizing and reducing microbiota. In particular, the dissimilatory reduction of sulphate to hydrogen sulphide by anaerobic bacteria leads to isotope effects of from 0 to ˜60‰, the magnitude of the effect depending largely on metabolic rates. Actual isotope ratio (δ3 4S) patterns in sediments depends, therefore, on environmental conditions and the nature of sulphate reservoirs during reduction. Sulphur isotope ratios can and have been used to trace environmental conditions, sources, and modes of formation of certain Phanerozoic deposits. These studies which have been extended to late and early Precambrian sediments provide a potential source of information about very early sediment deposition environments and early life. Recent carbon and sulphur isotope data for the low grade metamorphosed banded iron-formations of the Michipicoten area in Ontario (2.7 b.y. old) provide strong evidence for the existence of autotrophic organisms and reducing bacteria in late Archean times. Sulphur isotope ratios (δ3 4S) have now been obtained for samples from the Isua area of West Greenland. The δ3 4S of the Isua sediments (3.7 b.y. old), including the various facies of the banded iron-formations, have a very narrow spread with their mean close to zero ‰ C.D.T. (0.45 ± 0.5). This comes extremely close to the respective means yielded by the Isua tuffaceous amphibolites (+0.3±0.9‰) and by the somewhat younger, 3.1 to 3.7×109 yr, basaltic Ameralik dykes of the region (+0.6±1.1‰). These results indicate a complete absence of isotopic evidence for ‘sulphate reducers’ in the Isua sediments (early Archean) in contrast to the banded iron-formations of the late Archean, where δ3 4S varies from -2-to +20‰
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