Barium and cadmium stable isotopes as tracers for carbon export, remineralization strength and post-deposition diagenesis in the Peruvian oxygen minimum zone

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Oxygen minimum zones (OMZs) associated with eastern boundary upwelling systems are important economic areas where intense upwelling of nutrient-rich waters to the surface ocean sustains high marine primary production and supports >50% global fisheries. Vertical expansion and intensification of OMZs is predicted as a consequence of climate change because less oxygen dissolves in warmer waters and strengthened ocean stratification reduces oxygen supply to the ocean interior. These changes of OMZs directly restrict fish habitats to the surface, increase the risk of over-fishing, and may lead to future mass extinctions in coastal marine ecosystems. Variabilities of OMZs will also affect nutrient availability to the surface ocean and the marine carbon cycle it supports, but the exact mechanisms are not fully understood. Understanding how nutrient supply and marine biota responded to past changes of OMZ intensities will provide important insights on how marine biogeochemical and carbon cycles respond to future ocean deoxygenation. In this proposed research, I will use two novel proxies, stable cadmium (Cd) and barium (Ba) isotopes, from the Peruvian OMZ to 1) constrain processes that affect surface ocean productivity and subsurface remineralization in OMZs, and 2) assess how future ocean deoxygenation may impact the global carbon cycle. Both dissolved Cd and Ba isotopes in seawater are tightly coupled to biological processes and water mass mixing. In marine sediments, Cd and Ba isotopes are also affected by authigenic precipitation of these elements and post-depositional diagenesis, whereas the extent of isotope fractionation among these processes are poorly quantified. I will measure Cd and Ba isotope ratios in seawater, marine sinking particulates, sediments and pore waters to constrain their isotope fractionation effects during different biological, chemical and physical processes. With these results, I will quantify the sources and sinks that fractionate both isotope systems in the modern ocean and assess how post-depositional diagenesis affects Cd and Ba isotope fractionation. The ultimate goal of this proposed work is to develop sedimentary Cd and Ba isotopes as reliable proxies for past changes in export production and remineralization strength.