New insights into Archean sulfur cycle from mass-independent sulfur isotope records from the Hamersley Basin, Australia

We have measured multiple sulfur isotope ratios (S-34/S-33/S-32) for sulfide sulfur in shale and carbonate lithofacies from the Hamersley Basin, Western Australia. The A 13S values (Delta(33)Sapproximate todelta(33)S-0.515 X delta(34)S) shift from -1.9 to +6.9parts per thousand over a 22-m core section of the lower Mount McRae Shale (similar to2.5 Ga). Likewise, sulfide sulfur analyses of the Jeerinah Formation (similar to2.7 Ga) yield Delta(33)S values of -0.1 to +8.1parts per thousand over a 50-m section of core. Despite wide variations in Delta(33)S and delta(34)S, these two shale units yield a similar positive correlation between Delta(33)S and delta(34)S. In contrast, pyrite sulfur analyses of the Carawine Dolomite (similar to2.6 Ga) yield a broad range in delta(34)S (+3.2 to +16.2parts per thousand) but a relatively small variation and negative values in Delta(33)S (-2.5 to - 1.1parts per thousand). The stratigraphic distribution of delta(33)S, delta(34)S, and Delta(33)S in Western Australia allows us to speculate on the sulfur isotopic composition of Archean sulfur reservoirs and to trace pathways in the Archean sulfur cycle. Our data are explained by a combination of mass-independent fractionation (MIF) in the atmosphere and biological mass-dependent fractionation in the ocean. In the Archean, volcanic, sulfur-bearing gas species were photolysed by solar ultraviolet (UV) radiation in an oxygen-free atmosphere, resulting in MIF of sulfur isotopes. Aerosols of S-8 (with Delta(33)S > 0) and sulfuric acid (with Delta(33)S < 0) formed from the products of UV photolysis and carried mass-in dependently fractionated sulfur into the hydrosphere. The signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments. Pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass-dependent fractionation on mass-independent atmospheric effects. Multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur-metabolizing organisms. (C) 2003 Elsevier Science B.V. All rights reserved.