Glacial to interglacial changes in surface nitrate utilization in the Indian Sector of the Southern Ocean as recorded by sediment δ15N

Abstract

We present a new approach for paleoceanographic reconstruction of surface nutrient utilization in the southern ocean. It has been observed in the contemporary ocean that, due to preferential uptake of 14NO3− during photosynthesis, the δ15N of planktonic organic matter increases with increasing nitrate depletion in surface waters. Our results demonstrate that the δ15N signal produced in surface waters is reflected in the underlying surface sediments; core top δ15N is inversely correlated with surface nitrate concentration along a transect across the Subtropical Convergence and the Polar Front in the southeast Indian Ocean. These results are consistent with a four‐box model showing that the nitrogen isotopic composition of sinking organic matter depends on percent nitrate utilization in the euphotic zone. By comparing the δ15N of surface sediments with that measured in the glacial sections of several cores, we infer changes in the intensity and latitudinal distribution of nitrate uptake in this region during the last glacial maximum. These preliminary results argue against increased biological uptake of nutrients in southern polar waters as a major mechanism for glacial lowering of atmospheric CO2. They also suggest that Subantarctic waters in the SE Indian sector became more nutrient depleted as they migrated northward. Increased nitrate depletion might have also occurred slightly south of the glacial Polar Front. We use a six‐box model to explore the possible impact of this observation on atmospheric CO2.