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.