Nitrogen cycling on the Namibian shelf and slope over the last two climatic cycles: Local and global forcings
Nitrogen cycling on the Namibian shelf and slope over the last two climatic cycles: Local and global forcings
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过去两个气候周期中纳米比亚陆架和斜坡上的氮循环：局部和全球强迫
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发表时间：
2005
期刊：
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通讯作者：
K. Emeis
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作者：
L. Pichevin;Philippe Martinez;P. Bertrand;R. Schneider;J. Giraudeau;K. Emeis
[1] 根据在纳米比亚陆架和斜坡上收集的新表层沉积物 δ15N 数据，我们通过破译从上大陆坡到下大陆坡分布的三个岩心的 δ15N 信号，研究了吕德里茨（南纬 25°6）地区氮库存的冰期-间冰期变化。下坡岩心在寒冷时期显示出低 δ15N，在气候最适宜时显示出高 δ15N，类似于世界海洋中的许多其他记录，而上坡岩心则显示出高频低振幅 δ15N 信号，没有明显的冰期-间冰期变异。这种差异是由于两个单元中的上升流结构的分离造成的，从而将陆架的营养动态与陆架边缘以外的营养动态脱钩。无论风力如何变化，沿海单元的 δ15N 信号都相对恒定，这表明地表水中的硝酸盐从未耗尽。对于更深的核心，氮同位素信号与古生产力（总有机碳）和上升强度（海面温度和尘埃颗粒大小）指标之间的比较表明，在米兰科维奇循环中，硝酸盐输送到透光区是由南大西洋中央水的营养丰富度驱动的（反过来，取决于全球范围内的阿古拉斯水流入和反硝化作用），而不是大气强迫。我们认为，更深地核的 δ15N 信号不仅反映了相对硝酸盐利用率的变化，就像年度时间尺度上的情况一样，而且可能受到全球海洋中深度硝酸盐 δ15N 变化的影响。
[1] 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 Luderitz (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.