Variability in oxygen concentrations in Atlantic intermediate waters during the last deglaciation

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Oxygen is a primary component of the biological and chemical cycles of the ocean. Its involvement in photosynthesis and respiration links the changes of oxygen concentration to that of carbon in the ocean, and ultimately in the atmosphere. As the oceans take up more and more carbon, oxygen concentrations in the ocean are declining, with the potential to reach levels low enough to reduce biodiversity. The last ice age provides a natural experiment to investigate how oxygen concentrations may have varied under different climate conditions in order to help forecast what might happen in the future. Most previous studies have only been able to distinguish the relative change in oxygen concentrations (higher or lower), but in this project a new application will be developed that can quantitatively constrain oxygen concentrations in the past. In regions like the tropical Atlantic, where oxygen minimum zones (OMZs) approach but do not reach a total absence of oxygen today, it is critical to pinpoint the environmental conditions that can cross that threshold into an absence of oxygen. This project will promote training and learning for several participants, including one early-career scientist without previous NSF support and one undergraduate summer student.This project will produce high quality U-Th isotope and trace element data in a suite of intermediate depth sediment cores from the western tropical Atlantic and one site under the eastern tropical OMZ. These data will be used to test hypotheses regarding the evolution of bottom water oxygen concentrations (BW [O2]) during the last deglaciation in response to changes in the intensity of the eastern tropical OMZ. Specifically this project seeks to understand how the spatial extent and absolute magnitude of the OMZ has varied , and how these changes may relate to intermediate water ventilation and carbon storage. The first component of this proposal involves method development of a quantitative proxy system for BW [O2]. It parameterizes sedimentary [O2] with authigenic uranium and organic carbon fluxes with Baxs fluxes to generate a comprehensive approach to reconstructing BW [O2]. This component focuses on the Demerara Rise, where preliminary data indicate a high probability for successful application, and then it combines the new data from the Demerara Rise with additional work in the Arabian Sea and previously published data from the Pacific to prospect the global utility of the approach. The second component of this proposal investigates how and why oxygen concentrations in tropical intermediate waters may have varied in the past in response to changes in zonal circulation, meridional circulation, and organic matter fluxes.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.