THE ROLE OF RELATIVE SEA LEVEL IN MICROBIAL SULFATE REDUCTION AND THE LOCAL S-ISOTOPE RECORD

The sulfur (S) isotope composition of marine rocks, archived in reservoirs of sedimentary pyrite and gypsum/anhydrite, is a primary tool for reconstructing the oxygenation history of Earth’s surface. This proxy is based on the premise that variability in fractionation (δ³⁴S_SO4 - δ³⁴S_py) is broadly a function of marine sulfate concentrations [SO₄^2-]. However, recent studies reinvigorated an older hypothesis: that the local δ³⁴S_pyr record might be heavily influenced by environmental and depositional conditions (i.e., sedimentation rate). One key test of this hypothesis would be measurements in an onshore-offshore transect. In this study, we measure δ³⁴S_py at three sites from the mid-Cretaceous Western Interior Basin (WIB) of North America. The geometry of the broad and shallow seaway provides sedimentary records especially sensitive to changes in relative sea level (RSL), interpreted to preserve the rhythmic tempo of Milankovitch cycles. Because these deposits include datable volcanic ash layers, a floating astronomical time scale, established through spectral analysis of the orbital signal, is anchored by in situ radioisotopic ages, thus facilitating a high-resolution reconstruction of sedimentation rates.

Our current δ³⁴S_py dataset indicates a strong positive relationship between δ³⁴S and sedimentation rate associated with RSL change: we observe a >30‰ change between RSL highs and lows at the most proximal site in our onshore-offshore transect. This suggests that high RSL, associated with a decrease in local sedimentation rates, allows microbial sulfate reduction and attendant S-isotope fractionation to more closely approach theoretical limits (~60‰). Alternatively, as RSL drops, enhanced transport of sediment from the continent increases local sedimentation rates, interfering with the connection between pore waters and the overlying seawater sulfate reservoir, and resulting in reservoir effects and progressively heavier δ³⁴S_py values. The strength of the correlation between sedimentation and δ³⁴S decreases basinward, as changes in sedimentation rates between RSL highs and lows diminish. We are investigating other local changes in depositional environment (total organic carbon content, weight percent pyrite, and changes in reactive iron availability) to further delineate controls on the δ³⁴S_py record.