Collaborative Research: Calibration, Validation, and Application of Barium Isotopes in Marine Barite as a Tracer of the Marine Carbon Cycle

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The ocean's biological pump refers to a set of chemical, physical, and biological processes that redistribute carbon and nutrients from the sunlit upper ocean to the deep ocean and seafloor. Globally, this pump has an important role in modulating the air-sea balance of carbon dioxide and can thus impact Earth's climate. However, reconstructing fluctuations in the strength and efficiency of the oceans biological pump cannot be done directly because there are so many variables in play and the ocean is so large. Thus, geochemical indicators, called proxies, that have a specific measurable response to a complex process are used. Most geochemical proxies for the ocean biological pump are sensitive only to local conditions. This research involves the exploration, calibration, and validation of a novel geochemical tracer (the stable isotopes of barium in the mineral barite) that is hypothesized to be sensitive to regional-scale biogeochemical processes. To examine this tracer, barite will be separated from marine sediment cores from the Equatorial Pacific. Barite's occurrence is widely interpreted as reflecting local biogeochemical processes related to the ocean's biological pump. Thus, the proposed proxy has the benefit of simultaneously revealing information on both local- and regional-scale processes, though reliable interpretation of proxy signals first requires calibration and validation in modern settings. This research sets out to do the calibration and validation of this proxy in a set of cores from the Equatorial Pacific Ocean. It is anticipated that the barium isotope approach will bridge the biogeochemical scaling divide and enable more accurate reconstructions of the biological carbon pump throughout Earth's history. Broader impacts of the work include student training, building infrastructure for science by developing a new biogeochemical proxy, engagement in the research of students from groups underrepresented in the sciences, support an early career scientist, and support of a researcher whose gender is underrepresented in the sciences. This research focuses on examining the utility of barium-isotopic analyses of marine barite to reconstruct the ocean's carbon cycle over regional spatial scales. The project involves a two pronged approach. Phase one of the research entails calibration of the proxy itself by means of an extensive survey of modern core-top sediments from the Equatorial Pacific. Samples of co-located sediments and pore waters will be analyzed. Barite will be separated from the cores via acid digestion and other separation techniques and examined for purity using a scanning electron microscope and its integrated chemical analyzer. Pore waters will be analyzed by standard wet-chemical techniques. After ensuring purity of the barite samples, the barium stable isotopes of the barite will be measured for each of the core tops. Phase two of the research assesses the fidelity of the proxy during diagenetic alteration down core via the analysis of co-located mineral separates and pore water geochemistry. This stage of the work tests whether and how much barium-isotopic modification occurs after burial on the seafloor. Results from phase one and two will be used to establish whether the barium-isotopic composition of marine barite faithfully records ocean biogeochemistry. If so, this project will have successfully calibrated and validated a novel means to reconstruct regional-scale marine carbon cycling, which will have utility over a range of timescales.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.