Planktonic foraminifera, stable isotopes and sea surface temperature reconstruction of Southern Indian Ocean sediment cores

The paleoceanographic history of the Southern Indian Ocean is reflected by the movement of two prominent dynamical features of the Southern Ocean: the Subtropical Convergence (STC) and the Antarctic Polar Front (APF). These fronts, and their associated sea surface temperature (SST) signatures, are well delineated by planktonic foraminiferal faunas in surface sediments of the southern Indian Ocean. Using a transect of piston cores between 42°S and 48°S at about 90°E, we have reconstructed the latitudinal distribution of planktonic foraminiferal faunas over the past 500,000 years. These faunal variations imply changes in the paleolatitudes of the STC and APF and the surface isotherms associated with the fronts. Stratigraphic and chronologic control is provided by delta18O, %CaCO3, and biostratigraphy. Our reconstruction indicates that the STC has been equatorward of its present position (~40°S) for most of the past 500,000 years and has been poleward of that position for only four relatively brief (~10,000-year) intervals during that time. We estimate six equatorward excursions of the APF over the same period, with a maximum total range of about six degrees of latitude. The average paleo-position of the APF is about 46°S, about 4°north of its present position (~50°S). Ice-rafted debris, another indicator of APF position, occurs as far north as 45°S during glacial intervals. Latitudinal gradients in SST show little glacial-interglacial change between 42° and 48°S, suggesting that surface isotherms were displaced uniformly or that compression of SST gradients occurred outside the transect. Time series analyses of the SST records in this transect reveal statistically significant concentrations of variance in the primary orbital frequency bands. SST variations in these bands are coherent with orbital variations and with changes in delta18O. SST in the subantarctic Indian Ocean slightly precedes changes in delta18O occurring in the eccentricity, obliquity, and precession bands, and lag orbital variations in the obliquity and precession bands. These results place important constraints on possible mechanisms of interhemispheric climatic timing. The similarities in temporal patterns and timing among Southern Ocean SST, atmospheric CO2, and relative flux of North Atlantic Deep Water implicate atmospheric carbon dioxide and deep water circulation as possible interhemispheric pacing mechanisms.南印度洋的古海洋学历史，可通过南大洋两大显著动力特征的移动得以反映：亚热带辐合带（Subtropical Convergence, STC）与南极极锋（Antarctic Polar Front, APF）。这些锋面及其伴生的海表温度（sea surface temperature, SST）信号，可通过南印度洋表层沉积物中的浮游有孔虫动物群得到清晰勾勒。研究人员以约90°E经度、42°S至48°S纬度间的活塞岩芯剖面为研究载体，重建了过去50万年以来浮游有孔虫动物群的纬度分布格局。这些动物群的种群变化，暗示了STC、APF的古纬度位置，以及与锋面相伴随的地表等温线的变迁。地层与年代学约束依托于δ¹⁸O、碳酸钙百分含量（%CaCO3）以及生物地层学数据。本次重建结果显示，在过去50万年的绝大多数时段内，STC均位于其现今位置（约40°S）的向赤道一侧，仅在4个相对短暂的（约1万年）时段内处于向极一侧的位置。研究人员估算，同期APF共发生6次向赤道偏移，最大总纬度跨度约为6°。APF的平均古位置约为46°S，较其现今位置（约50°S）偏北约4°。作为APF位置的另一代用指标，冰筏碎屑在冰期时段可北迁至45°S附近。在42°S至48°S纬度区间内，海表温度的纬度梯度几乎未出现冰期-间冰期尺度的变化，这表明地表等温线发生了均匀位移，抑或是海表温度梯度的压缩发生在该岩芯剖面之外。对该剖面海表温度记录的时间序列分析显示，方差在主要轨道频率波段中呈现出具有统计学意义的集中分布。这些波段内的海表温度变化与轨道参数变化、δ¹⁸O的变化具有一致性。亚南极南印度洋的海表温度变化，略早于偏心率、黄赤交角及岁差波段内的δ¹⁸O变化，却滞后于黄赤交角和岁差波段内的轨道参数变化。上述结果为跨半球气候时序的潜在机制提供了重要约束。南大洋海表温度、大气CO₂浓度以及北大西洋深层水相对通量的时序模式与时间节点具有相似性，这表明大气二氧化碳含量与深层水环流可能是跨半球气候驱动的潜在调控机制。