SIMS Analysis of Carbonate-Associated Sulfate: Toward Building a d34S Record of Individual Carbonate Grains and Fossils

Washington University, Saint Louis MO

Investigators

Abstract

Sulfur isotope ratios (δ34S) have become one of the most useful geobiological tools for probing sedimentary rocks with regard to: the evolution of the sulfur cycle, the first appearance and subsequent rise(s) (and falls) of atmospheric oxygen, and the redox balance of buried sedimentary phases (pyrites vs. sulfates). Moving beyond evaporite-based studies, the increasing application of carbonate-associated sulfate (CAS) has enabled us to fill in the temporal record of seawater sulfate (δ34SSW) over much of Earth history. When compiled, however, these data yield several surprises, often recording high-frequency stratigraphic variation in δ34SCAS and large differences in δ34SCAS between spatially distant but coeval sections, which are inferred to reflect conditions of low (~mM) marine sulfate concentrations for much of the Ediacaran and early Paleozoic. As a bulk-rock proxy, δ34SCAS can be influenced by the complex histories that ancient sediments often have experienced, combining detrital, pelagic, benthic, diagenetic, and metamorphic components. Deciphering the multiple origins of sulfate within carbonate minerals is critical to extract meaningful information from δ34SCAS about the depositional and diagenetic environment. This project will develop a method to analyze δ34SCAS using secondary ion mass spectrometry (SIMS) to investigate the variability of δ34SCAS between multiple co-existing phases, including analysis of individual grains and fossils, at a scale (~10μm) previously unobtainable. Preliminary δ34SCAS data have been collected on a Cameca 7f Geo to demonstrate the feasibility of this method. With proper development, this approach can dramatically improve our ability to a primary seawater sulfate δ34S curve through time (e.g., from unaltered brachiopods), much the same way that micro-drilling and analysis of δ13Ccarb, δ18Ocarb, and 87Sr/86Sr from individual well-preserved fossils advanced our understanding of the evolution of these biogeochemical proxies over the Phanerozoic. The ability to extract precise (sub-permil precision) δ34SCAS data from individual grains or fossils would allow us to robustly document spatial variations in marine δ34S (e.g., between benthic and pelagic organisms, or from basin to basin), without the uncertainties associated with complex bulk-rock proxies. By developing a new analytical tool (SIMS analysis of δ34SCAS) that can be widely applied to geobiological questions throughout Earth history, this project will open up a new field of microanalytical work that can be fruitfully explored by many researchers for decades to come. Analysis of δ34SCAS by SIMS will be a central component of undergraduate, graduate, and post-graduate research in the PIs lab, providing them with research experience using cutting-edge analytical tools and techniques. The PI?s course ?Methods in Biogeochemistry? currently has a section on SIMS applications with hands-on instrument use, to which this technique could profitably be added.