Australia: The Land Where Time Began

A biography of the Australian continent 

Glacial-Interglacial Bottom Water Oxygen Content Changes on the Portuguese Margin

Atmospheric CO₂ concentrations were lower than at present during the last and penultimate glacial maxima, possibly partly because of the increased storage of respired carbon in the deep oceans (Sigman & Boyle, 2000). It is possible to calculate the amount of respired carbon in a water mass from its oxygen content through apparent oxygen utilisation, and the oxygen content can in turn be calculated from the carbon isotope gradient within the sediment column (McCorkle & Emerson, 1988). In the study reported in this paper Hoogakker et al. analysed the shells of benthic foraminifera that are present at the surface of the sediment and the oxic/anoxic interface on the Portuguese margin which they used to reconstruct the carbon isotope gradient and therefore the oxygenation of bottom water over the past 150,000 years. Their results suggested that the oxygen concentrations of the bottom water are 45 and 65 μmol/kg lower during the last and penultimate glacial maxima respectively than they are at present. Hoogakker et al. calculated that the concentration of remineralised organic carbon was at least twice as high during the glacial maxima than they are at the present. They attributed these changes to ventilation that was decreased linked to a reorganisation of circulation of the ocean (McManus et al., 2004) and an increased strength of the biological pump (Kohfeld et al., 2005). If the pool of respired carbon remained of similar size throughout the entire glacial deep Atlantic basin, this sink could have accounted for 15-20 % of the glacial Pco₂ drawdown during the last and penultimate glacial maxima.

Sources & Further reading

Hoogakker, B. A. A., H. Elderfield, G. Schmiedl, I. N. McCave and R. E. M. Rickaby (2015). "Glacial-interglacial changes in bottom-water oxygen content on the Portuguese margin." Nature Geosci 8(1): 40-43.