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

Abstract

[1] In the light of new surface sediment d 15 N 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 d 15 N signals of three cores distributed from the upper to the lower continental slope. The lower slope cores display low d 15 N during cold periods and high d 15 N during climatic optima, akin to many other records from the world ocean, whereas the upper slope core displays a high-frequency low-amplitude d 15 N 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 d 15 N 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 d 15 N 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 d 15 N.Citation: Pichevin, L., P. Martinez, P. Bertrand, R. Schneider, J. Giraudeau, and K. Emeis (2005), Nitrogen cycling on the Namibian shelf and slope over the last two climatic cycles: Local and global forcings,

Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ15N-signals of suspended matter and surface sediments

“…N sediment beyond the shelf agrees with earlier studies and is usually ascribed to the steady 15 N-enrichment of the nitrate pool as it is drawn down by phytoplankton growth (Holmes et al 1996(Holmes et al , 1998(Holmes et al , 1999Pichevin et al 2005). Still, the translation of d 15 N sediment -values into nitrate consumption remains debatable.…”

“…Second, the central Namibian shelf exhibits pronounced nitrate deficits in coastal proximity (Tyrrell & Lucas 2002;Kuypers et al 2005). The nitrate deficits (i) in combination with the coastal-parallel belt of high d (Figs 4A, 5), and the oxygen depletion in the bottom waters (iv), unanimously support the influence of denitrification in the source waters (see also Pichevin et al 2005;Emeis et al 2009;Lavik et al 2009). Based on a number of nutrient profiles taken at selected stations during Meteor cruise M57-3, Brçchert et al (2004) were able to locate the zones of denitrification along the water column.…”

“…Wada & Hattori 1978;Wada 1980;Montoya 1994;Waser et al 1998 N-records were extensively used as a proxy of relative nitrate utilisation in the photic zone, sometimes holding valuable clues about ocean circulation and trade wind pattern (e.g. Francois et al 1992;Altabet & Francois 1994;Montoya 1994;Montoya & McCarthy 1995;Voss et al 1996;Holmes et al 1998Holmes et al , 1999Holmes et al , 2002Pichevin et al 2005).…”

“…Since then, various authors have put the hypothesis of shelf edge upwelling further forward (e.g. Bang 1971;Barange & Pillar 1992;Brçchert et al 2004;Pichevin et al 2005;Summerhayes et al 2005;Emeis et al 2009). Emeis et al (2009) reported a narrow band of elevated nitrate concentrations situated above the shelf break between latitude 22 and 24 S. Nutrient replenishment via shelf edge upwelling not only accounts for reduced relative nitrate consumption and a corresponding decline in d 15 N in the primary product, but also fits the increase in primary production, as inferred from the minor peak in [TOC] above the outer shelf and upper continental slope (Fig.…”

The potential distortion of sedimentary δ15N and Corg/N ratios by NH4+ and the effects of pre-analysis sample treatment

“…A dilution by higher trophic levels as well as terrestrial detritus (i.e. clay minerals) is considered negligible (Holmes et al 1998;Pichevin et al 2005).…”