Glacial–interglacial changes in bottom-water oxygen content on the Portuguese margin

Abstract

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 oceans¹. 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 column². 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⁻¹ 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 circulation³ and a strengthened global biological pump⁴. 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 $P_{{\text{CO}}_2}$ drawdown during the last and penultimate glacial maxima.