OCE-PRF-Development of a paired kerogen-pyrite sulfur isotope proxy for organic remineralization

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).Dr. Selva Marroquin will start an NSF OCE funded Postdoctoral Fellowship to develop a new method to track the recycling of organic matter in ocean sediments. The project will compare the chemical composition of sulfur within co-occurring organic matter and sulfur bearing minerals (e.g., pyrite or fool’s gold) within archived ocean sediment drill cores. The burial and incorporation of sulfur into these different forms is closely tied to key aspects of carbon cycling within the marine system that are integral to reconstruct. The results of this study will allow us to better understand the outstanding questions regarding the chemical and biologic response of the oceans to rising carbon dioxide levels associated with global climate change. The research will be carried out at Caltech with Dr. Alex Sessions in collaboration with Dr. Will Berelson at the University of Southern California. With this funding, Dr. Marroquin will provide research opportunities and mentorship to students at different education levels (i.e. high school and community college) to help build the STEM identities of minoritized students and broaden their participation in the field. The sulfur isotope ratios (δ34S values) of marine minerals have been studied for decades to help reconstruct the global redox history of Earth’s oceans. A significant limitation has been that the δ34S record of pyrite convolves the fractionation between pyrite and sulfate associated with microbial sulfate reduction and the porewater isotopic gradient, both local controls. The primary goal of this work is to improve our understanding of reduced δ34S records by developing a new proxy based on the difference in δ34S values between coeval organic sulfur (kerogen) and pyrite from modern marine sediments. The environmental controls on this proxy remain unclear, though it is undeniably linked to key aspects of the marine system that we seek to reconstruct (e.g., organic carbon delivery, sulfate reduction, sulfurization, and carbon burial). To address these uncertainties, measurements of TOC, paired kerogen and pyrite δ34S, and Fe speciation will be made on samples from IODP cores representing a variety of depositional environments. This work will help determine how the variable depositional conditions correlate with the recorded δ34S offset between kerogen and pyrite. The results of this work will refine this proxy for application in the geologic record which is integral for improving our understanding of how climate change will impact the future of our ocean system.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.