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

We have measured multiple sulfur isotope ratios (34S/33S/32S) for sulfide sulfur in shale and carbonate lithofacies from the Hamersley Basin, Western Australia. The Δ33S values (Δ33S≈δ33S−0.515×δ34S) shift from −1.9 to +6.9‰ over a 22-m core section of the lower Mount McRae Shale (∼2.5 Ga). Likewise, sulfide sulfur analyses of the Jeerinah Formation (∼2.7 Ga) yield Δ33S values of −0.1 to +8.1‰ over a 50-m section of core. Despite wide variations in Δ33S and δ34S, these two shale units yield a similar positive correlation between Δ33S and δ34S. In contrast, pyrite sulfur analyses of the Carawine Dolomite (∼2.6 Ga) yield a broad range in δ34S (+3.2 to +16.2‰) but a relatively small variation and negative values in Δ33S (−2.5 to −1.1‰). The stratigraphic distribution of δ33S, δ34S, and Δ33S 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 S8 (with Δ33S>0) and sulfuric acid (with Δ33S<0) formed from the products of UV photolysis and carried mass-independently 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.