NSF GEO-NERC: Constraining the oxic marine sink of novel metal isotope proxies to underpin paleoceanographic reconstructions

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The evolution of life on Earth is closely tied to conditions, including oxygen levels, in the oceans. Understanding when the oceans first became oxygenated, and how oxygen levels have varied through time, is important to understanding the history of our planet. Chemical clues in marine sediments can be useful in deciphering past conditions. Tools that are currently being investigated as indicators of past oxygen levels include changes in the isotope content of certain trace elements, including thallium, molybdenum, uranium, and zinc. Before these tools can reliably be applied to past sediments, it is important that we understand how these elements are cycled in the present-day ocean, and how they are incorporated into sediments. While most work to date has looked at how these elements and isotopes are incorporated into oxygen-poor sediments, this study would measure these isotopes in a collection of oxygen-rich sediments that spans the world’s oceans. This study would provide essential bounds on marine isotope cycles to better understand modern ocean chemistry and, ultimately, how life and ocean geochemistry co-evolved over Earth history. This study would contribute to the professional development and training of three early-career scientists, and foster international collaboration. This is a project that is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own country.This project seeks to understand the modern day cycling of thallium (Tl), molybdenum (Mo), uranium (U) and zinc (Zn) isotopes in the ocean and improve interpretations of paleoceanographic proxies by constraining the oxic deep-sea sediment sink. This goal will be achieved by analyzing the Tl, Mo, U and Zn isotope compositions of the hydrogenous component in a newly assembled set of 73 marine sediment core top samples collected across every ocean basin. The authigenic component of the sediments will be isolated using partial dissolution techniques aimed at only dissolving authigenic minerals. The robustness of these methods will be thoroughly tested using bulk sediment mineral assemblages (obtained by XRD) as well as major and trace element compositions of bulk and partially dissolved samples. The generated dataset will enable an evaluation of the marine oxic output flux magnitude (constrained with 230Th) and stable isotope fractionation for Tl, Mo, U and Zn. A pilot study suggests that the isotope fractionation of these elements is different between Fe-oxides and Mn-oxides within the sediment, which are also unique from ferromanganese crusts and nodules. By expanding the pilot study to a global scale, we will be able to evaluate how previously published records of Tl, Mo, U and Zn isotopes in seawater may need to be reinterpreted due to diverse isotope fractionation patterns in the oxic marine sink.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.