231Pa/230Th ratios in sediments as a proxy for past changes in Southern Ocean productivity

Abstract

THE biological productivity of the oceans is sensitive to changes in climate, which can affect essential factors such as nutrient and light availability. In turn, ocean productivity may influence climate by regulating the partitioning of carbon dioxide, a greenhouse gas, between the ocean and the atmosphere. Investigators have attempted to link variations in atmospheric CO2 content, recorded in ice cores1,2, to the productivity of the Southern Ocean3–6, but an unambiguous means of assessing past changes in ocean productivity has been lacking. Here we exploit established relationships between 231Pa/230Th ratios and particle flux7–12 to infer, from the analysis of dated sediment cores, variability through time of fluxes of particulate biogenic material exported from surface waters. Records from two cores in the Atlantic sector of the Southern Ocean indicate that ocean productivity during glacial periods was lower than at present south of the Antarctic polar front, and support earlier conclusions13–16 that the zone of maximum productivity migrated northwards during glacial conditions. Although further work at other sites is needed for an assessment of changes in total Antarctic productivity, our technique has the potential to provide this information while avoiding some of the limitations of other productivity proxies.