Nutrient regime at the siliceous belt of the Atlantic sector of the Southern Ocean during the past 660 ka

Sediment cores from Ocean Drilling Program sites 1094 and 1093 hold the most extensive and most detailed records of carbon and nitrogen isotopes of diatom-bound organic matter in the Atlantic sector of the Southern Ocean over the past 660 ka. These records were combined with summer sea surface temperatures and winter sea ice that were deduced from diatom transfer functions to reconstruct the nutrient regime at the northern and southern boundaries of the siliceous belt at latitudes 53 degrees S and 50 degrees S, respectively. The strong coupling between carbon and nitrogen isotope records at site 1093 suggests that the diatom productivity was influenced mainly by changes in nutrient availability. The anticorrelation between the carbon and nitrogen isotopes at site 1094 suggests that the diatom productivity was closely linked to global temperature changes, sea ice coverage, and dissolved CO2 concentration. A detailed comparison of the nitrogen isotope records between the two sites shows three time intervals that represent different nutrient regimes. The nutrients at site 1093, which is close to the Polar Front, were dominated by eddies and meanders that sustained relatively high productivity at times of stratification or reduced ventilation, whereas at site 1094 the nutrients were influenced by ventilation processes.