Award: OCE-1657690

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. We targeted our research along the California and Mexico continental margins where our selected sites represent a wide range of bottom water chemical compositions and organic matter accumulation rates. These boundary conditions 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 therefore how sediment records can be used to reconstruct past changes in ocean chemistry. 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 and 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, thallium and vanadium among others were also explored as they were closely related to the overall goals of our work. One of the key outcomes of our study is that chromium behavior within the sediments depends on (1) the availability of oxygen and (2) biological processes. Although some if not most of the chromium is originally incorporated into by particles as they settle through the water column, the particles chemical and isotope composition can undergo modification during sediment decomposition and burial. At sites where the deep water has higher oxygen levels, less chromium is incorporated into sediments, partly due to the loss dissolved chromium from sediments 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. Thus, although chemical reactions within the sediments have the potential to alter the primary chromium composition, under conditions of low bottom water oxygen, chromium isotope 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. During the summer 2019 and 2020 we worked with an undergraduate as part of an internship program to evaluated a suite of chemical extraction methodologies that we applied to our samples. The student made significant contributions to our research and her findings highlighted that there is significant amount of chromium associated with the reductive sediments as compared to more oxidizing sediments. After graduation, the same student was hired by us as a research assistant for several months to continue in the development of methods for chromium isotope analysis. 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 three years; an opportunity afforded partly because the first year of the internship was followed by a second year whereby much of the interns work could be done remotely during the pandemic conditions that prevailed during that time. The student is now perusing a PhD at Woods Hole Oceanographic Institution. Further work on nickel, cadmium, thallium and vanadium refined our understanding of their biogeochemical cycling in sediments with implications for their isotope compositions. Specifically, we were able to parse out the relative roles of water colum uptake onto sinking organic particles, adsorption onto manganese oxide (which occurs under oxic conditions) and the role of sediment uptake under anoxic conditions. Based on our isotope measurements for all four elements we were able to differentiate isotope signatures that originate in the water column versus isotope signatures that are modified during organic matter decomposition in the sediments. Understanding these processes forms the basis for the use of these elements isotope compositions in ancient marine deposits to infer the past presence or absence of oxygen in seawater. 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: 02/21/2024 Submitted by: SilkeSevermann