Sedimentology, foraminiferal assemblage, and isotope record of the Agulhas Bank region

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200-250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.非洲南部的厄加勒斯浅滩（Agulhas Bank）区域是海洋学研究中兼具重要性与复杂性的区域。温暖高盐的印度洋海水经由非洲南端渗漏进入南大西洋，这一过程是全球温盐环流（thermohaline circulation）的关键驱动因素之一。从厄加勒斯浅滩西部斜坡获取的岩芯MD962080顶部10米沉积物的有孔虫组合（foraminiferal assemblage）、稳定同位素（stable isotope）及沉积学数据，被用于重建过去45万年以来南大西洋东南部水团环流的演化历史。沉积学与浮游有孔虫数据清晰揭示了冷水侵入事件的信号。底栖稳定同位素记录构建了该岩芯的地层框架，并显示其涵盖了最近4个气候旋回（即追溯至海洋同位素阶段12（MIS 12））。浮游有孔虫组合呈现出明显的过渡型至亚南极型特征，其中膨胀拟抱球虫（Globorotalia inflata）和右旋厚壁新方抱球虫（Neogloboquadrina pachyderma (dextral)）为优势类群。冰期时段亚南极类群占比升高，表明该区域曾通过亚热带辐合带（Subtropical Convergence）的渗漏作用（伴随厄加勒斯环脱落过程）输入寒冷的亚南极表层水，同时伴随表层海水整体降温。热带/亚热带浮游有孔虫物种杜氏球室虫（Globoquadrina dutertrei）、红拟抱球虫白色变种（Globigerinoides ruber (alba)）及门氏圆球虫（Globorotalia menardii）的出现，指示厄加勒斯海流（Agulhas Current）携带的印度洋海水输入量存在变化，且最大渗漏量发生在冰消期。该岩芯中门氏圆球虫的持续存在表明，过去45万年以来南印度洋与南大西洋之间的水体交换从未完全中断。底栖碳同位素记录与沉积物结构数据反映了岩芯站位处底层水团的转变：从北大西洋深层水（North Atlantic Deep Water）转为南上部组成水（Upper Southern Component Water）。浮游有孔虫组合与沉积物组成显示，在约20万至25万年前的海洋同位素阶段7（MIS 7），该岩芯站位的表层水环境发生了显著转变：从混合的亚南极与过渡型水团，转为整体更温暖的表层水环境。