Nitrogen cycling on the Namibian shelf and slope over the last two climatic cycles: Local and global forcings

In the light of new surface sediment δ¹⁵N data collected over the Namibian shelf and slope, we examined glacial-interglacial variations of N inventory in the area of Lüderitz (25°6S) by deciphering δ¹⁵N signals of three cores distributed from the upper to the lower continental slope. The lower slope cores display low δ¹⁵N during cold periods and high δ¹⁵N during climatic optima, akin to many other records from the world ocean, whereas the upper slope core displays a high-frequency low-amplitude δ¹⁵N signal without obvious glacial-interglacial variability. This dissimilarity results from the segregation of the upwelling structure in two cells, decoupling nutrient dynamics of the shelf from those beyond the shelf-edge. The δ¹⁵N signal of the coastal cell is relatively constant irrespective of wind strength variations and shows that nitrate was never depleted in the surface water. For the deeper cores, comparisons between N isotopic signals and indicators of paleoproductivity (total organic carbon) and upwelling intensity (sea surface temperature and dust grain size) reveal that, over Milankovitch cycles, nitrate delivery to the photic zone was driven by the nutrient richness of the South Atlantic Central Water (depending, in turn, on Aghulas water inflow and denitrification at a global scale) rather than by atmospheric forcing. We propose that the δ¹⁵N signals of the deeper cores do not only mirror changes in relative nitrate utilization, as it seems the case over annual timescales, but are arguably influenced by global ocean changes in middepth nitrate δ¹⁵N.