Isotopic constraints on global marine iron cycling, ocean productivity and deoxygenation

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Dissolved iron is a key micronutrient limiting ocean primary productivity. The major source fluxes in the global oceanic iron budget are atmospheric deposition, sedimentary release, and hydrothermal input. They determine the amount of iron in the ocean but are nonetheless poorly constrained. A new emerging dataset of iron isotopes in the ocean provides additional constraints on the key iron sources since information on their source end-member signatures is known. In this project, we will implement iron isotope tracers in our global ocean biogeochemical model to provide an additional quantitative constraint on these source iron fluxes. In particular, we will develop a new module for ocean-sedimentary iron exchange, which currently represents the largest uncertainty in the marine iron budget. This new sedimentary iron flux component will explicitly account for both the organic matter driven reductive flux and the non-reductive seafloor dissolution flux. The reductive flux will be sensitive to bottom water oxygen concentrations according to a global observational dataset indicating enhanced sedimentary reductive iron fluxes as oxygen decreases. Our global ocean biogeochemical model will estimate how much the combination of increasing atmospheric iron pollution and warming-induced deoxygenation enhance iron fluxes to the surface ocean that fuels additional primary productivity, causing an amplifying feedback that drives additional deoxygenation. We hypothesize this enhanced source iron flux-deoxygenation amplifying feedback is a key missing feature causing global Earth System Climate models that provide projections for the Intergovernmental Panel on Climate Change to significantly underestimate the current rate of ocean deoxygenation.