THE PRESERVATION OF NON-MASS DEPENDENT SULFUR ISOTOPE ANOMALIES IN A DEEP TIME ASTROBIOLOGY DRILL CORE FROM WESTERN AUSTRALIA

The history of atmospheric oxygenation is written in the geological record of redox sensitive elements preserved in ancient sediments. Recent discoveries of non mass-dependent sulfur isotope anomalies across the Archean-Proterozoic transition are also believed to record changes in pO₂ insofar as these result from UV photolysis of volcanogenic SO₂ in the absence of ozone. Due to these reactions sulfate aerosols and elemental sulfur are formed and represent a significant flux of sulfur to Earth's early surface environments. To resolve uncertainties about the cause of fluctuations in non-mass dependent sulfur isotope anomalies, we investigated both δ³⁴S and Δ³³S at high stratigraphic resolution using a new on-line combustion technique to analyze SO from bulk sediments. This method allows for the determination of both values from individual samples in six minutes with uncertainties of better than 0.3‰, and hence the ability to quickly construct high-resolution profiles through entire deep time drill cores. Results from the ~ 2.5 Ga McRae Shale sampled from the Astrobiology Drilling Program ABDP-9 core from Western Australia reveal significant stratigraphic variations in δ³⁴S and Δ³³S compositions. Sulfides extracted from these same samples and analyzed in the same manner are isotopically indistinguishable from the bulk measurements, with the exception of a few samples with very low sulfur concentrations. The general consistency between S_total and S_sulfide suggests that the organic S fraction in these sediments is isotopically negligible. In some instances bed-to-bed variations in both δ³⁴S and Δ³³S are rapid. We interpret these observations as the result of variable preservation of non mass-dependently fractionated atmospheric sulfur with terrestrial inputs, which may be mixed prior to or during bacterial pyrite formation. Superimposed on this variability are coherent longer term trends, including a correlated but transient rise in total sulfur and carbon contents, ³⁴S depletion, and the diminution of a non-mass dependent signal in samples between ca. 130 and 150 m depth in the McRae Shale. These observations are consistent with a short-lived rise in pO₂, enhanced oxidative weathering, and other associated biogeochemical perturbations in late Archean time.