Evidence for lower productivity in the Antarctic Ocean during the last glaciation

Abstract

BOTH increased biological productivity and more efficient uptake of upwelled nutrients in high-latitude oceans have been proposed1–5 as mechanisms responsible for the glacial reduction in atmospheric concentrations of carbon dioxide deduced from ice-core measurements6–8. These glacial models invoke more efficient "biological pumping9 of carbon into the deep sea by increasing the uptake of 'excess' biolimiting nutrients in the Antarctic surface ocean9 or by reorganizing chemical circulation patterns within the ocean10,11. Here we challenge this conventional view with new evidence from tracers of palaeoproductivity preserved in Antarctic sediments. Records of the accumulation rates of diatom shells, the ratio of germanium to silicon in diatomaceous opal and the carbon isotope ratio in foraminiferal carbonate all suggest lower glacial productivity and less efficient uptake of nutrients. Although alternative interpretations are possible, our results support previous studies that indicate lower glacial productivity in the Southern Ocean12,13 and raise new questions about the role of ocean productivity in models of the causes (or remedies) for changes in atmospheric concentrations of carbon dioxide.