PARALLEL HIGH-RESOLUTION CARBON AND SULFUR ISOTOPE RECORDS IN THE PALEOZOIC AND IMPLICATIONS FOR BIOSPHERIC OXYGENATION

Carbonate-associated sulfate (CAS) is a proven proxy for the sulfur isotope composition of sulfate in ancient seawater. Concentrations of CAS, typically at ppm levels, reflect substitution for the carbonate group in the lattices of calcium carbonate and dolomite. Comparisons with modern seawater and coeval gypsum confirm the fidelity of the CAS proxy. Past studies of Proterozoic carbonate rocks have revealed rapid shifts in the sulfur isotope composition of the ocean that suggest sulfate concentrations well below those present today. Since seawater sulfate is derived primarily from oxidative weathering on the continents, these lower concentrations reflect reduced atmospheric oxygen that may have persisted into the Paleozoic. Building from past Precambrian work, our ongoing study is centered on high-resolution CAS isotope records for Paleozoic rocks with large, 4 to 6 per mil, carbonate carbon isotope excursions spanning thicknesses of 60 to 400 meters. Specifically, our CAS data span the Upper Cambrian excursion (SPICE) and other excursions of similar magnitude within the Upper Ordovician, at the Silurian-Devonian boundary, and within the Lower Mississippian. Although preliminary, CAS data across the SPICE show over 10 per mil variability in phase with the carbonate carbon isotope shift. This sympathetic relationship may record parallel trends in enhanced burial of pyrite and organic matter as well as variability in oxygen-dependent continental weathering expressed on a global scale. Later in Paleozoic, the parallel relationship between the C and S isotope data seems to weaken, as expressed in muted and more complex S isotope variability. This mid-late Paleozoic shift in isotopic behavior suggests a decreased sensitivity to sulfate flux changes, which may be the result of increasing concentrations of sulfate in the ocean. Ultimately, this increasing sulfate and any corresponding decoupling of the C and S isotope records may be a product of progressive biospheric oxygenation during the Paleozoic.