Isotope rates and Cd/Ca ratios in benthic foraminifera from the Southern Ocean

South Atlantic benthic foraminiferal Cd/Ca shows no glacial-interglacial variation, suggesting that the glacial contribution of North Atlantic Deep Water to the Southern Ocean was not much different than at present. In contrast, Cd/Ca in southeast Indian Ridge cores show lower glacial bottom water Cd, comparable to levels in intermediate depths of the North Atlantic and significantly lower than in the deep South Atlantic. Low glacial Cd/Ca was also recorded in planktonic foraminifera, suggesting a substantial decrease in the nutrient concentration of Subantarctic surface water during the glacial maximum which most likely was caused by increased biological productivity. The Cd data are inconsistent with low glacial benthic foraminiferal d13C which suggest higher nutrient concentration. We propose that the low Cd/Ca in the Southeast Indian Ridge records reflects a local source of nutrient-depleted deepwater, formed during the last glacial maximum by open-ocean convection near the Antarctic Polar Front, downstream of the Kerguelene Plateau. If this source was limited to the southeast Indian basin then its impact on the overall chemistry of glacial Circumpolar Deepwater was rather small. However, if during glaciations open-ocean convection became the dominant mode of bottom water formation, it might have had a greater impact on CPDW chemistry.南大西洋底栖有孔虫镉钙比（Cd/Ca）未表现出冰期-间冰期变化，表明冰期时段北大西洋深层水（North Atlantic Deep Water）向南大洋的输送贡献量与现代并无显著差异。与之形成对比的是，东南印度洋脊岩芯中的底栖有孔虫Cd/Ca值显示，冰期底层水镉含量更低，该水平与北大西洋中层水域的镉含量相当，且显著低于南大西洋深层水域。浮游有孔虫的Cd/Ca记录同样呈现冰期低值，这暗示冰盛期亚南极表层水的营养盐浓度大幅降低，其成因极大概率为生物生产力提升。上述镉数据与冰期底栖有孔虫δ¹³C记录存在矛盾——后者指示当时营养盐浓度更高。我们提出，东南印度洋脊的低Cd/Ca值记录，反映了一种局地形成的贫营养深层水团；该水团于末次冰盛期在凯尔盖朗高原（Kerguelene Plateau）下游的南极极锋（Antarctic Polar Front）附近通过远洋对流作用形成。若该水团仅局限于东南印度洋盆地，则其对冰期环极深层水（Circumpolar Deepwater）整体化学特征的影响相对有限。然而，若冰期期间远洋对流成为底层水形成的主导模式，则其对环极深层水化学特征的影响可能更为显著。