Award: OCE-1657832

This project was built upon the overarching goal of constraining the physical and chemical processes that lead to element accumulation, mobilization, and composition in modern marine sediments. Under this broad goal, we targeted our research along the California and Mexico continental margins. These sites have deep water chemical compositions and organic matter accumulation rates that vary widely compared to the more static conditions observed in the abyssal deep sea. The combination of factors at our study sites creates varying sediment chemical characteristics that support our goal for understanding how chemical signatures are emplaced within the sediment record and how sediment records can be used to predict how and why ocean chemistry may have changed in the past. The major activities on our expedition focused on collecting a series of sediment cores to extract pore fluids that would allow us to evaluate the composition of both the sediment solid phases as well as their associated fluids. Our analyses were accompanied by a range of measurements, such as concentrations of trace metals and the composition of organic carbon and nitrogen, as well as an array of other analytes that help define environmental conditions for metal behavior. In addition to collecting samples from multiple sites in this region we have been continuing some long-standing collaborations with colleagues, which have added value to our long-term studies of elements that can be used as chemical tools. Although the work within this project largely focused on chromium and nickel, the chemistry of a variety of other elements including uranium, cadmium, and potassium among others were also explored as they were germane to the overall goals of our work. These additional analyses served as either background data, complementary data, or represent new fundamental opportunities for our research endeavor. One of the key outcomes of our study is that chromium behavior within the sediments depends on (1) oxidation-reduction processes and (2) biological processes. Although the sediment conditions can create characteristic distribution patterns for chromium in sediments and pore fluids, these patterns can undergo modification during sediment decomposition, which potentially alters the chromium isotope compositions recorded by the sediment. At sites where the deep water has higher oxygen levels, the efficiency of chromium burial is lower, presumably due to the lower delivery of chromium to the sediments and the loss of dissolved chromium to the bottom water during sediment decomposition. At sediment locations underlying low oxygen waters, the mobilization of chromium during organic matter decomposition is small relative to the high rate of chromium delivery to the sediments. Importantly, in terms of using chromium isotopes as a chemical tool, these data suggest that the isotope composition of chromium captured under these conditions will be well preserved in the sediments. Thus, although chemical reactions within the sediments have the potential to alter the primary chromium composition, under conditions of low bottom water oxygen, chromium signatures are likely reflecting the primary ocean conditions. In support of this idea, we observe a strong positive correlation between organic carbon and chromium in the organic and reactive sediment fractions of the sediments. We also evaluated a suite of chemical extraction methodologies that we applied to our samples, with a focus on repeatability and within-method consistency as part of an undergraduate internship during parts of 2019 and 2020. In combination, our techniques highlighted that there is significant amount of chromium associated with the reductive sediments as compared to more oxidizing sediments. The primary impact in terms of human resources is the significant training provided to this undergraduate student who has been able to work with the project for two years; an opportunity afforded partly because the first year of the internship was followed by a second year whereby much of the intern’s work could be done remotely during the pandemic conditions that prevailed during that time. In the case of nickel chemistry, there is a strong correlation between organic carbon and the fraction of nickel associated with particular pools of reactive sediments. Furthermore, nickel chemistry is strongly influenced by iron and manganese chemistry. For cadmium, the environments studied are important depot centers for cadmium and the isotope composition of this cadmium is notably light. Similarly, our potassium work is showing that the active formation of potassium-rich clays during the earliest phases of diagenesis drives K-isotope fractionation in continental margin sediments. This project is likely to make an impact on furthering our understanding of trace metal behavior under a variety of chemical conditions. These impacts are likely to contribute to our understanding of the importance of the chemical processes that set the chemical signature of seawater. The impact of this project is secondarily targeted at understanding how to interpret the geologic record and, in particular, how we interpret chemical signatures that are preserved in the marine sedimentary record. Last Modified: 06/04/2022 Submitted by: James Mcmanus