DEVELOPMENT OF A PAIRED KEROGEN-PYRITE SULFUR ISOTOPE PROXY TO TRACK MARINE BIOGEOCHEMICAL CYCLING

The sulfur isotope ratios of barite, carbonate-associated sulfate (CAS), and pyrite in marine sediments have been studied to help reconstruct the redox history of Earth’s oceans. While useful, a significant limitation has been that the S isotope record of pyrite convolves the fractionation between pyrite and sulfate associated with microbial sulfate reduction (ε_MSR) with the porewater isotopic gradient (open- or closed-system behavior). Sulfur bound into organic molecules is also ubiquitous, and constitutes another potential isotopic archive. We are working to develop a new proxy from modern International Ocean Discovery Program (IODP) records based on the difference in sulfur isotope composition between coeval kerogen sulfur and pyrite (εk-p), that we believe will help us reconstruct past changes in carbon and sulfur cycling. Here, we will present data from two marine cores collected off the northwestern coast of Australia on IODP cruise 363. We measured total organic carbon (TOC), Fe speciation, and paired kerogen and pyrite δ³⁴S values. Despite similar average TOC (0.7 and 0.9 wt%), total iron (1.21 and 1.41 wt%), pyrite (0.51 and 0.63 wt%), and organic sulfur (0.02 wt% for both) contents, εk-p varied significantly between the two sites. Site U1482 εk-p values range from 13 to 26‰, whereas values are more consistent in U1483, ranging from 13 to 18‰. U1482 has a slower average sedimentation rate (< 8 cm/kyr) and deeper sulfate methane transition zone (SMTZ) (~100 m), relative to U1483 (8-12 cm/kyr and ~60 m SMTZ). We hypothesize that the greater variation in εk-p is driven by related differences in the depth distribution of sulfur cycling.