The cause of glacial/interglacial (G/IG) variations in atmospheric carbon dioxide concentrations remains among the most important unresolved challenges in Earth sciences. Recent modelling studies suggest that the ultimate explanation of the full G/IG atmospheric CO2 change relies in a combination of physical and biogeochemical processes that are involved in the regulation of the deep ocean carbon reservoir. The Southern Ocean (SO) ventilates the largest fraction of the deep ocean, and represent the biggest open ocean region where the extraction of nitrate and phosphate by marine phytoplankton is incomplete, allowing for the leakage of deeply sequestered carbon back to the atmosphere. Consequently, an increased SO nutrient utilization during glacial stages has been proposed as a plausible mechanism to increase the efficiency of the ocean’s biological pump that could explain the lower atmospheric CO2 levels during ice ages. Recent estimates of the nutrient status of the glacial SO using a newly developed technique based on the analysis of the isotopic composition of nitrogen bound within microfossil shells (and hence potentially free of sedimentary artefacts), have offered promising new insights on the spatial and temporal response of nutrient consumption in the Antarctic region during glacial stages. However, the available microfossil-bound dataset is limited to the last glacial cycle and far too sparse to provide a clear picture of the nutrient status of the SO during ice ages. This proposal seeks to provide key constrains on this mechanism by applying a multidisciplinary approach that combine (i) the generation of new G/IG microfossil-bound N isotopes records from previously unexplored regions of the SO; (ii) its extension back in time beyond the last glacial stage; and (iii) the use of state-of-the-art biogeochemical and ocean circulation models to gauge the effect of the reconstructed changes in SO nutrient utilization on atmospheric CO2 concentrations.
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