Bottom water oxygen reconstructions Iberian Margin sediment core MD95-2042 between 0 and 150,000 years BP

During the last and penultimate glacial maxima, atmospheric CO2 concentrations were lower than present, possibly in part because of increased storage of respired carbon in the deep oceans (Sigman and Boyle, 2000, doi:10.1038/35038000). The amount of respired carbon present in a water mass can be calculated from its oxygen content through apparent oxygen utilization; the oxygen content can in turn be calculated from the carbon isotope gradient within the sediment column (McCorkle and Emerson, 1988, doi:10.1016/0016-7037(88)90270-0). Here we analyse the shells of benthic foraminifera occurring at the sediment surface and the oxic/anoxic interface on the Portuguese Margin to reconstruct the carbon isotope gradient and hence bottom-water oxygenation over the past 150,000 years. We find that bottom-water oxygen concentrations were 45 and 65 µmol/kg lower than present during the last and penultimate glacial maxima, respectively. We calculate that concentrations of remineralized organic carbon were at least twice as high as today during the glacial maxima. We attribute these changes to decreased ventilation linked to a reorganization of ocean circulation (McManus et al., 2004, doi:10.1038/nature02494) and a strengthened global biological pump (Kohfeld et al., 2005, doi:10.1126/science.1105375). If the respired carbon pool was of a similar size throughout the entire glacial deep Atlantic basin, then this sink could account for 15 and 20 per cent of the glacial PCO2 drawdown during the last and penultimate glacial maxima.