Multiple Sulfur Isotope Applications in Diagenetic Models and Geochemical Proxy Records

Abstract

Many of the long-term geochemical fluxes influencing the surface sulfur cycle are microbially catalyzed and a substantial portion of active S cycling occurs in organic-rich continental margin sediments. Stable S isotopes historically provide the most powerful analytical tool for understanding these small and large scale fluxes and for relating them back to laboratory characterizations of microbial metabolisms—particularly that of dissimilatory sulfate reduction. A more recent expansion of stable S isotope geochemistry to include the minor isotopes (33S and 36S) has demonstrated the capacity to diagnose the presence of additional S metabolic processes and to further characterize the response of microbial sulfate reduction to environmental forcing. In particular, emerging work suggests that multiple S isotope signatures in laboratory experiments are dictated by the physiological rate of a metabolic process. This is especially true for sulfate reduction. In this thesis we expand the scope of minor S isotope geochemistry to include early diagenetic processes, exploring the fidelity of laboratory calibrations and how they translate both to the modern marine sediments as well as an S isotope proxy records of seawater sulfate through the Cretaceous and Cenozoic. Early diagenesis of organic carbon in marine sediments via sulfate reduction is a dominant microbial process, and leaves a characteristic isotopic imprint in pore water and solid phase S-bearing species. To place those S isotope signatures into a physical context, we construct reactive transport models that take sulfur, carbon, and in one case, iron cycling into account for two geochemically well characterized sedimentary environments: Alfonso basin and Aarhus bay. Alfonso basin is an anoxic-silled marginal basin in the Gulf of California and Aarhus bay is a well-oxygenated, shallow coastal system. We demonstrate in both cases that large S isotope fractionations during microbial sulfate reduction (34εSR = 70‰) are required to explain the pore water isotope signatures, and there is no need for a depth or rate-dependent fractionation relationship. Furthermore, in the case of Aarhus Bay, it is clear that that isotopic contribution from oxidative S processes is negligible. Both these results – an apparent fixed 34εSR in sediments and little to no isotopic sensitivity to oxidative reactions – should hold true across similar, common shallow water marginal sediments. In parallel to work targeting the behavior of modern marine sediments, we revisit a well known δ34S proxy record for Cretaceous and Cenozoic seawater sulfate. Using minor isotope techniques, we demonstrate that Δ33S and Δ36S values are isotopically homogeneous (Δ33SSO4 = 0.043±0.016‰ and Δ36SSO4 = -0.39±0.15‰) despite δ34S variability. These observations, the first of their kind, place upper limits on pyrite burial and evaporite dissolution over the last 120 million years. Together, this thesis highlights analytical advances in stable isotope geochemistry and complements those measurements with reactive transport environmental modeling. To date much attention has focused on the laboratory scale generation and controls on the production of microbial isotope signatures. Using high-precision minor isotope measurements, paired with the construction of the models incorporating multiple S isotope systematics, allows for the placement of quantitative constraints on sedimentary S cycling and for understanding the global scale consequences for seawater sulfate S isotope proxy records. More succinctly, this furthers our understanding of the imprint that microbial processes have on the multiple S isotope record.