Inverse Modeling of the Glacial Ocean

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Under this award the PIs will use an inverse modeling approach first pioneered by LeGrand and Wunsch (1995), to try to solve the origin of nutrient-rich waters below a depth of about 2.5 km in the Atlantic during the Last Glacial Maximum (around 21,000 years before present). This feature has been known for about two decades from measurements of d13C and Cd/Ca on fossil shells of benthic foraminifera. The inverse model will be used to test the validity of three possible mechanisms: a change in the deep circulation, a change in the nutrient concentration of the water mass end-members, and a change in the rates of organic matter remineralization at depth. Several critical extensions of the LeGrand and Wunsch will be undertaken: (i) the non-conservative character of the nutrients will be considered; (ii) benthic Cd/Ca and Zn/Ca data, in addition to benthic a 13C and a18O, will be incorporated; (iii) new benthic a13C data from the western Atlantic basin, which is a main flow path of deep waters, will be used; and (iv) the model domain will be extended to include the South Atlantic, where the nutrient proxy database has been largely improved since LW95.The project will include four major tasks: (I) a critical assessment of the existing nutrient proxy data, culminating with an estimate of the uncertainty of each datum; (II) the development of an inverse model of the circulation in the deep Atlantic; (III) the combination of the model with modern observations to test the model and to provide a reference state for hypothesis testing; and (IV) the combination of the model with data to test various hypotheses of nutrient enrichment in the glacial abyssal Atlantic. The project will involve a graduate student and a post-doc investigator and will contribute to fill the gap that exists today between the community of paleoceanographers and the community of physical oceanographers. It will bring together a modeler, a paleoceanographer and a physical oceanographer.