Large S-33 Anomalies in Late Archean Carbonacous Shales

Multi-sulfur isotope ratios (³⁴S/³³S/³²S) were determined on sulfides extracted from two late Archean carbonaceous shale units, the Mt. McRae shale (~2.5 Ga) and the Jeerinah formation (~2.7 Ga), from the Hamersley Basin, Western Australia by using the CO₂-laser fluorination line at the Geophisical Laboratory. We have measured the largest Δ³³S anomaly yet reported for a terrestrial sample on sulfides from 22 m core section of the Mt McRae shale. The large positive Δ³³S, up to +6.9 ‰ , is found in the lower part of the core section; the Δ³³S shifts to negative upward in the section to as low as -1.9 ‰ . The age of the Mt. McRae shale is bracketed by 2470 and 2561 Ma. Therefore, the observed isotopic shift represents the maximum duration of 91 million years, but most likely less than 30 million years by assuming a constant sedimentation rate for the Mt. McRae shale that has average thickness of 60 m in the area. Sulfide sulfur from the Jeerinah formation also yield large Δ³³S anomalies ranging from -0.1 to +4.4 ‰ .

Our model fundamentally follows the one proposing a strong atmospheric influence in the Archean sulfur cycle by Farquhar et al. (2000). Our new data show the large and systematic variation between Δ³³S and δ³⁴S. This allows us to further speculate the isotopic compositions of Archean sulfur reservoirs and the manner in which the atmospheric signature was transferred to sediments. Our data are consistent with the late Archean seawater sulfate reservoir that has a negative Δ³³S of ~ -2 ‰ . Thus, pyrite formed via microbial sulfate reduction shows variable δ³⁴S and negative Δ³³S. The large positive Δ³³S of + 6.9 ‰ is likely to be a signature of deposition of sulfur aerosol, probably elemental sulfur. Our sulfur isotope data is best explained by mixing of those two components. Atmospheric deposition of elemental sulfur requires an anoxic atmosphere, and subsequent burial and preservation of the signature into pelagic sediments would have been favored in an anoxic and iron-rich ocean in the late Archean Earth.