Collaborative research: Calibration of deep-sea coral paleoproxies for nutrients, carbonate ion, and temperature

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The ocean exchanges heat, moisture, and carbon dioxide with the atmosphere and plays a large role in earth's climate, both in the past and present day. This project seeks to understand and quantify physical and chemical properties of the deep ocean in the past by developing geochemical tools to analyze trace elements in the fossil skeletons of deep-sea corals. This approach has the potential to reveal how critical regions of the deep ocean changed in nutrient and carbon content, as well as temperature, over the last glacial cycle, in ways that have not been possible previously. The project will be carried out in collaboration with Spanish colleagues in Barcelona, who are experts in the maintenance of deep-sea corals in aquarium culture. The research team will grow living cultures of the solitary coral species Desmophyllum dianthus over a period of months to years, while carefully controlling the chemical composition and temperature of the seawater in which they grow. By calibrating the composition of the calcium carbonate skeletons of these organisms relative to tightly controlled seawater properties in the laboratory and in the modern ocean, the researchers will develop quantitative relationships that enable analysis of well-dated fossil corals and inference of the properties of the seawater in which the ancient corals grew. Such fossils can be collected from the sea bottom and dated. Large collections have been archived from the waters around New Zealand, a critical region for ocean-atmosphere interactions. Knowing the seawater properties from archived and new seawater measurements in this region will allow the principal investigators to test the conclusions found in the lab against relationships seen in corals growing in the natural ocean. The goal is to develop tools to establish the degree to which atmospheric carbon dioxide was controlled over the last glacial cycle by ocean productivity and gas exchange. The project will support a PhD student and facilitate international exchange in addition to training K-12 teachers. Overall, the project contributes to technical expertise development in the US and will allow an improved understanding of how our changing ocean affects global climate.In this collaborative project, a chemical paleoceanographer and a marine microgeochemistry expert will collaborate to carry out an extensive laboratory experiment that will take advantage of a large (150 individuals) collection of living D. dianthus, currently in culture at the Institut de Ciencies del Mar-Barcelona under automatically-controlled seawater chemistry and temperature. Laser ablation ICP-MS micro-geochemical techniques will be used to calibrate precisely the skeletal P/Ca (nutrient), Ba/Ca ("nutrient-type"), U/Ca (carbonate ion) and Mg/Li (temperature) proxies, and to investigate their secondary sensitivities to pH, feeding rate, calcification rate, and temperature. A previously unexploited collection of live-collected (dredged) D. dianthus corals, harvested over a broad range of hydrographically well-characterized subtropical and subantarctic waters around New Zealand, will also be analyzed for these proxies. The project will thus result in complementary laboratory and field-based calibrations of these new geochemical paleo-proxies, that can then be used in future efforts to analyze precisely dated collections of fossil corals. The unique set of samples from ongoing culture experiments and open-ocean archives, combined with the capabilities of a state-of-the-art laser ablation ICP-MS system and precision micro-milling techniques, will reduce significantly the uncertainties in calibrations for an important set of D. dianthus-based paleoproxies. This work will serve as a cornerstone for future coral paleo-reconstruction studies.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.