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Notes: Abstract 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‰

Notes: Abstract Surficial sediments (0 to 12 cm) were removed from three lakes located within 20 km of a large siderite sintering plant which annually releases 140 000 tonne S and sinter fly ash up its stacks. A fourth lake located approximately 100 km upwind of the sintering plant was chosen as a control. Lake sediments were analyzed for sulphate, pyrite, organic S, total S and subfossil diatoms. The rate of lake acidification inferred from stratigraphic changes in diatom species composition indicated that one of the lakes located in a granitic basin near the sintering plant was undergoing rapid acidification. The other two study lakes located near the sintering plant were situated on carbonate-rich greenstones. Their pH as inferred from diatom stratigraphy indicated that both lakes were actually increasing in pH over the last 30 yr. The diatom inferred pH of the control lake's deeper sediments (4.8 ± 0.2) did not differ significantly from the lakes' present day observed pH (4.9). Lakes closest to the sintering plant had the highest S concentrations in their sediments. In addition, their surface sediments (0 to 2 cm) had significantly higher pyrite concentrations than their deeper sediments. Organic S and pyrite S comprised the major fraction of S in the sediments of all four lakes. The S isotope ratios (δ34S) in the various forms of S in the lake sediments were also measured. The δ34S values in the deeper (10 to 12 cm) sediments were found to be fairly uniform in all four lakes at + 4.0%., almost identical to those of the lake sulphates upwind from the Wawa sintering plant. This suggests little or no S isotope fractionation during sulphate reduction and precipitation of sulphides at the time of sediment deposition roughly 100 yr age. However, δ34S values in the surface sediments (0 to 2 cm) were shifted toward negative values. In the lakes downwind from the Wawa smelter, the isotope shift ranged from 14 to 16%, whereas in the upwind lakes the shift was very much smaller ranging from 1.5 to 5.0%. This shift in δ34S in the recent sediments wass presumed to be largely a result of isotope fractionation in the dissimilatory reduction of lake sulphate by bacteria stimulated by anthropogenic inputs of S. The possibility of anthropogenic inputs of S with high negative δ34S values to explain the shifts in isotopic ratios seems unlikely in view of known δ34S values for the ores in the area. Additional δ34S measurements on lake sediments, lake sulphates and ore samples (smelter effluent), etc., have been undertaken. This should make it possible to interpret the 534 shift in the sediments in terms of environmental changes since the Ambrosia rise (circa 1890).