Much of our understanding of changes in Earth’s environments and the co-evolution of its biosphere is based on geochemical signatures within the rock record. This project specifically seeks to extract deeper insights into the environmental information preserved in one of the most commonly used geochemical proxies: the sulfur isotope composition of pyrite, a mineral commonly formed in marine sediments. It has become apparent that the traditional analysis of the bulk isotopic composition of pyrite in sediments is inadequate because it averages environmental information contained within individual pyrite grains. However, by analyzing a representative suite of individual pyrite grains in sediments, investigators propose that they can distinguish both the direct imprint of biological activity and the degree of subsequent overprinting acquired after sediment deposition. This grain-specific approach can transform the ability to distinguish between competing biological and environmental processes that together give rise to the more commonly measured bulk geobiological signatures. This work will include mentoring of an undergraduate as part of the Students and Teachers as Research Scientists (STARS) Program, which offers incoming high-school seniors an opportunity to work within a laboratory research setting. In addition to gaining experience using cutting-edge techniques, the students are taught to express the results and significance of their research orally and in a research paper.
Reconstructions of past environmental conditions and biological activity are often based on stable isotope proxies whose interpretations are inherently non-unique. In this project, researchers will develop and refine a new approach centered upon integrating traditional bulk (cm-scale) measurements of pyrite with micron-scale in-situ analyses in a petrographic context using secondary ion mass spectrometry (SIMS). They hypothesize that acquiring the individual sulfur isotopic values from a representative population of pyrite grains within each sample will enable a more rigorous reconstruction of depositional environments and, for the first time, distinguish between biological controls (i.e., isotopic fractionation during microbial sulfur cycling) and environmental controls (e.g., impact of sedimentation rate, organic carbon loading, etc.) that regulate the diffusive exchange between porewaters and the overlying water column. They seek to characterize and minimize any offset between the bulk value and the SIMS average, which could arise from several sources related to grain size, insufficient sampling density of individual pyrites, intragrain isotopic variability, or analytical artifacts in SIMS. They will demonstrate the technical feasibility of this approach and address the theoretical soundness of the hypothesis by analyzing samples from two contrasting systems: methane-seep sediments from Santa Monica Basin and mid-Pleistocene sediments from the Crotone Basin, Italy. In the former, the depositional environment has been stable and the porewater redox gradient is known, providing a control to investigate potentially variable biological fractionation and whether textural differences are diagnostic of different precipitation mechanisms. Samples in the latter location represent a wide range of depositional environments, exhibit large variations in the bulk slfur isotope values (-44 to +24‰), and include a wide range in petrographic textures and pyrite grain size and shape. Specific outcomes that will make this microanalytical approach more robust include: developing a mathematical algorithm to predict the sampling size (n) needed to accurately represent a sample distribution given the bulk sulfur isotope value and number of textures observed in SEM, refining their protocols to allow for analysing smaller grains while retaining < 1‰ precision, developing an iterative ion imaging-sputtering method to incrementally analyze the 3D structure of grains and isolate any oxidized rims that may contribute to artifacts. The successful development and testing of this approach will provide a framework that allows for the unique identification of biological fractionation during microbial sulfur cycling as well as the degree to which the associated sediments have been impacted by closed-system processes during pyrite formation. This transformative framework could be widely applied to a host of geobiological problems in modern environments and the sedimentary record.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
