A NEW APPROACH TO THE SULFUR ISOTOPE GEOCHEMISTRY OF SHALES

Temporal trends in sulfur redox chemistry, and the associated relationships to atmospheric oxygen, have traditionally been modeled using sulfur isotope data for sulfides and sulfate and the isotopic offset between the two. These data are seldom available for precisely coeval samples, however, and models are thus compromised by sample availability and age control. We are presently exploring the use of carbonate-associated sulfate (CAS) to measure sulfate and sulfide isotopic values for single shale samples through analysis of reduced S in pyrite and sulfate within the associated carbonate fraction. Importantly, this method eliminates reliance on published evaporite-based isotope curves.

Although primary pyrite-S isotope trends are often preserved despite complex burial histories, diagenetic effects are a concern for CAS. Our recent work in modern and ancient settings, however, has shown that interactions with meteoric or low-sulfate fluids appear to affect only sulfate concentrations and have little, if any, influence on primary S isotope compositions. Our preliminary efforts also suggest that secondary carbonate precipitation and the presence of pyrite do not isotopically compromise CAS in organic-rich sediments, although the analytical artifacts remain poorly known.

An ideal setting for defining and refining shale CAS methods is the Pennsylvanian cyclothems of midcontinent North America. The repetitive nature of the cyclothems conveniently yields calcareous mudstones and clean limestones that are interbedded across narrow stratigraphic gradients in paleoredox and in concentrations of organic matter and pyrite. Furthermore, sulfate in biogenic and authigenic apatite spanning the facies, particularly the black shales, provides independent control on seawater sulfate and diagenetic overprints. Our ability to measure S isotope offsets between sulfate and sulfide within individual samples should speak to sulfate concentrations, reservoir dynamics and specific microbial pathways of sulfate reduction as linked to oxygen concentrations in the ocean/atmosphere system. The latter has particular relevance to Precambrian paleoredox trends. In this talk, we will outline the CAS approach and emphasize our recent work with calcareous, fine-grained siliciclastic lithologies.