Late Quaternary planktonic foraminifera of ODP Hole 175-1087A

Planktonic foraminiferal assemblages from the upper Pleistocene part of Hole 1087A (0 to 12.1 meters below seafloor) are investigated to assess the role of global and local climate changes on surface circulation in the southern Benguela region. The benthic stable isotope record indicates that the studied interval is representative of the last four climatic cycles, that is, down to marine isotope Stage (MIS) 12. The species assemblages bear a clear transitional to subpolar character, with Neogloboquadrina pachyderma (d), Globorotalia inflata, and Globigerina bulloides, in order of decreasing abundance, as the dominant taxa. This species association presently characterizes the mixing domain of old upwelled and open ocean waters, seaward of the Benguela upwelling cells. Abundance variation of the dominant foraminiferal species roughly follows a glacial-interglacial pattern down to MIS 8, suggesting an alternation of upwelling strength and associated seaward extension of the belt of upwelled water as a response to global climate changes. This pattern is interrupted from ~250 ka down to MIS 12, where the phase relationship with global climate is ill defined and might be interpreted as a local response of the southern Benguela region to the mid-Brunhes event. Of particular interest is a single pulse of newly upwelled waters at the location of Site 1087 during early MIS 9 as indicated by a peak abundance of sinistral N. pachyderma (s). Variable input of warm, salty Indian Ocean thermocline waters into the southeast Atlantic, a key component of the Atlantic heat conveyor, is indicated by abundance changes of the tropical taxon Globorotalia menardii. From this tracer, we suggest that interocean exchange was hardly interrupted throughout the last 460 k.y., but was most effective at glacial terminations, particularly during Terminations I and II, as well as during the upper part of MIS 12. This maximum input of Indian Ocean waters around the southern tip of Africa is associated with the reseeding of G. menardii in the tropical Atlantic.本研究针对1087A钻孔（Hole 1087A）更新世上段（海底以下0至12.1米）的浮游有孔虫组合（planktonic foraminiferal assemblages）展开系统调查，旨在评估全球与局地气候变化对南本格拉区域表层环流的调控作用。底栖稳定同位素记录显示，本次研究的层段对应末次四个气候旋回，即可追溯至海洋同位素阶段（Marine Isotope Stage, MIS）12。识别出的物种组合呈现显著的亚极地过渡特征，优势类群依次为厚壳新方球虫右旋型（Neogloboquadrina pachyderma (d)）、膨胀圆辐虫（Globorotalia inflata）与布氏抱球虫（Globigerina bulloides），丰度随顺序递减。该物种组合目前是本格拉上升流单元外侧、老化上升流水与开阔大洋水混合区域的典型特征。优势有孔虫物种的丰度变化在至MIS 8之前大致遵循冰期-间冰期旋回模式，表明上升流强度及其伴生的上升流水带向海延伸的交替过程，响应于全球气候变化而发生改变。该模式在约25万年至MIS 12期间被打断，此时其与全球气候的相位关系尚不明确，或可解释为南本格拉区域对中布容事件的局地响应。尤为值得关注的是，在MIS 9早期，左旋型厚壳新方球虫（Neogloboquadrina pachyderma (s)）丰度出现峰值，指示1087站位区域出现单次新生上升流水脉冲事件。热带类群门氏圆辐虫（Globorotalia menardii）的丰度变化，表明印度洋温跃层暖盐水团以可变通量输入东南大西洋——这一过程是大西洋热输送带的关键组成环节。基于该示踪指标，我们认为在过去46万年中，跨洋交换几乎未曾中断，但在冰期终止事件（尤其是第一、第二冰期终止事件以及MIS 12上段）期间最为显著。非洲南端周边印度洋水团的这一最大输入事件，与热带大西洋门氏圆辐虫种群的重新定植存在关联。