Glacial to interglacial changes in surface nitrate utilization in the Indian Sector of the Southern Ocean as recorded by sediment δ 15 N (original) (raw)

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 515N of planktonic organic matter increases with increasing nitrate depletion in surface waters. Our results demonstrate that the 515N signal produced in surface waters is reflected in the underlying surface sediments; core top 515N 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 515N 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 maxilnum. 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 Copyright 1992 by the American Geophysical Union. Paper number 92PA01573. 0883-8305 / 92 / 92PA-01573510.00 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. Francois et al.: Southern Ocean Sediment Such findings prompted an extensive search in the sedimentary record of the southern ocean for evidence of the biogeochemical changes predicted by these models. Changes in surface nutrient concentrations were traced by 5•3C [Labeyrie and Duplessy, 1985; Charles and Fairbanks, 1990; Keigwin and Boyle, 1989] and Cd/Ca [Boyle, 1988; Keigwin and Boyle, 1989] in planktonic foraminifera and by Ge/Si in biogenic silica [Froelich et al., 1989; Mortlock et al., 1991], while estimates of past changes in primary production were obtained from accumulation rates of opal in sediments [Mortlock et al., 1991; Charles et al., 1991]. As a whole, however, results from these studies remain inconclusive. Calcite 5•3C in planktonic foraminifera was found to decrease during the last glacial maximum [Labeyrie and Duplessy, 1985; Charles and Fairbanks, 1990], suggesting higher nutrient levels in southern ocean surface waters during that period. On the other hand, Cd/Ca in the same foraminifera species stay relatively constant with time [Boyle, 1988; Keigwin and Boyle, 1989], suggesting no significant changes in surface nutrient concentration during deglaciation. Ge/Si ratios in biogenic opal suggest higher glacial silicate concentrations in Antarctic surface waters [Froelich et al., 1989; Mortlock et al., 1991], in apparent