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.
Reconstructing the long-term changes in Earth’s surface chemistry, particularly the timing and rate of oxygen accumulation in the atmosphere, remains one of the outstanding challenges in geobiology. These changes are critical to understanding the environmental controls on organismal development, how these may have changed over Earth history, and how biological activity, in turn, impacts the ambient environment (e.g., ocean chemistry or oxygen concentrations). While it is easy to assess modern oxygen concentrations by measuring the atmosphere, or to reconstruct relatively recent changes in oxygen abundance (e.g., by measuring oxygen trapped in gas bubbles in polar ice), it is challenging to reconstruct the longer-term record of oxygen deeper into Earth history. To do that, we need another geochemical record that can be related to oxygen concentrations, ideally one that can be preserved for long periods as a stable mineral. In the presence of water, sulfate (SO42-) is produced from the abiotic or biotic interaction of oxygen with hydrogen sulfide or sulfide minerals (such as pyrite, or "fool’s gold"). In fact, sulfate is the dominant pool of ‘oxidizing power’ present in the modern ocean. Sulfate can be preserved in minerals that form on the sea floor, including limestones (calcium carbonate minerals), such as the skeletons of corals and the shells of clams and snails. Known as carbonate-associated sulfate (CAS), the abundance and isotopic composition of this sulfate can be used to reconstruct the long-term change in ‘oxidizing power’ of the oceans. In recent decades, CAS has become a common tool for reconstructing biogeochemical change over geologic time. However, because CAS is only a trace element (comprising ~ 0.01 – 0.1% of these samples), large sample sizes of carbonates have traditionally been needed in order to generate enough material for analysis. Further, a typical carbonate sample consists of a mixture of fossil fragments, lime mud, and various marine and possibly later stage chemical precipitates (i.e., cements). Each of these may reflect a unique chemical composition and history. Bulk analyses, in turn, average over this variability and may bias the resulting CAS data. As part of research enabled by this grant, we developed new approaches to visualize micro-scale variations in CAS abundance using X-ray techniques (e.g., Figure 1). Using this approach, we can clearly see the difference in CAS abundance between major components of the carbonate (e.g., fossil fragments and limestone mud), as well as distinguish fairly subtle differences in sulfate concentrations from different fossil groups (e.g., echinoderms vs. snails vs. trilobites). These measurements, when combined with other elemental chemistry collected in parallel can possibly shed light on the mechanism by which these ancient organisms formed their skeletons or shells. Further, we can also distinguish between successive stages of cementation and identify any impact that post-depositional alteration may have had on this signal. A second component of the research supported by this grant involves the analysis of the stable isotopic composition (i.e., the ratio of two different forms of sulfur (32S and 34S), with slightly different masses) of sulfate preserved in these samples. We were able to develop a methodology using secondary ion mass spectrometry (SIMS) to analyze the isotopic composition of CAS at the micron-scale (an ~10,000 fold increase in spatial resolution). By enabling isotopic analysis at this spatial scale (e.g., within individual shell fragments), this provides a new window to investigate the origin and variability of sulfur isotopic signatures preserved in CAS. This isotopic record holds additional information regarding the operation of the global marine sulfur cycling, including about the oxidizing power of the ocean and the aggregate metabolic activity of organisms in marine sediments. These new analytical techniques allow us to identify primary signatures representative of ancient seawater records and can be used to revise and refine our understanding of how environmental conditions in the ocean have varied in sedimentary strata over geologic time.
