Evaluating the Effects of Sediment Reworking on the Sulfur Isotopic Composition of Aqueous and Mineral Sulfides

Abstract:

The sulfur isotopic compositions of sulfide minerals preserved in the rock record are widely used in reconstructions of past ocean and atmospheric chemistry, but little is known about how syn- and post-depositional processes may affect these compositions. Furthermore, recent discoveries of pyrite more enriched in ³⁴S than coeval sulfate in the rock record (e.g., Ries et al., 2009) and in modern deltaic sediments (e.g., Aller et al., 2010) have indicated a need to develop a better understanding of the controls on isotopic fractionation during sulfur cycling in natural environments. Here, we report the results of a controlled laboratory experiment designed to simulate the repeated oxidative reworking of sediments using a series of sediment columns constructed with bulk carbonate mud from Florida Bay, Florida. A rapid decline in [H₂S] was observed in reworked sediments during the first 12 weeks of the experiment. Decreases in the δ³⁴S of sulfide phases and increases in Δ³⁴S_SO4-H2S (i.e., the difference between the δ³⁴S of sulfate, δ³⁴S_SO4, and that of hydrogen sulfide, δ³⁴S_H2S, at a given depth) were also documented across all columns. These results indicate a decline in labile organic matter concentrations within the sediments and a corresponding decrease in MSR rates with time. Complete reoxidation did not generate superheavy pyrite in this study, but did stimulate biological activity through the generation of sharp redox boundaries. Partial oxidation of aqueous sulfide and an associated increase in residual δ³⁴S_H2S must thus be key if oxidation itself is important to superheavy pyrite formation. These findings further suggest that organic matter lability may have played a more fundamental role than [SO₄^2-] in regulating MSR fractionation throughout Earth history and provide motivation for future research.