Stable isotope record and sea surface reconstruction of sediment core MD88-770

Hydrographical changes of the southern Indian Ocean over the last 230 kyr, is reconstructed using a 17-m-long sediment core (MD 88 770; 46°01'S 96°28'E, 3290m). The oxygen and carbon isotopic composition of planktonic (N. pachyderma sinistra and G. bulloides) and benthic (Cibicidoides wuellerstorfi, Epistominella exigua, and Melonis barleeanum) foraminifera have been analysed. Changes in sea surface temperatures (SST) are calculated using diatom and foraminiferal transfer functions. A new core top calibration for the Southern Ocean allows an extension of the method developed in the North Atlantic to estimate paleosalinities (Duplessy et al., 1991). The age scale is built using accelerator mass spectrometry (AMS) 14C dating of N. pachyderma s. for the last 35 kyr, and an astronomical age scale beyond. Changes in surface temperature and salinity clearly lead (by 3 to 7 kyr) deep water variations. Thus changes in deep water circulation are not the cause of the early response of the surface Southern Ocean to climatic changes. We suggest that the early warming and cooling of the Southern Ocean result from at least two processes acting in different orbital bands and latitudes: (1) seasonality modulated by obliquity affects the high-latitude ocean surface albedo (sea ice coverage) and heat transfer to and from the atmosphere; (2) low-latitude insolation modulated by precession influences directly the atmosphere dynamic and related precipitation/ evaporation changes, which may significantly change heat transfer to the high southern latitudes, through their control on latitudinal distribution of the major frontal zones and on the conditions of intermediate and deep water formation.

过去23万年以来南印度洋的水文变化，通过一根长17米的沉积物岩芯（MD 88 770；46°01'S 96°28'E，水深3290米）得以重建。本研究对浮游有孔虫（N. pachyderma sinistra 与 G. bulloides）及底栖有孔虫（Cibicidoides wuellerstorfi、Epistominella exigua 和 Melonis barleeanum）的氧、碳同位素组成开展了分析。海表温度（Sea Surface Temperature, SST）的变化通过硅藻与有孔虫转换函数计算得出。针对南大洋的全新岩芯顶部校准方法，可将北大西洋开发的古盐度估算方法（Duplessy 等，1991）进行拓展应用。年龄标尺的构建方案如下：近3.5万年以来，通过对N. pachyderma s.进行加速器质谱（Accelerator Mass Spectrometry, AMS）14C测年建立；更早时段则采用轨道年龄标尺。海表温度与盐度的变化明显早于深水环流变化（提前3至7千年），因此深水环流变化并非南大洋表层对气候变化产生早期响应的成因。我们认为，南大洋的早期增温与降温至少由两类作用于不同轨道频段与纬度的过程驱动：（1）受黄赤交角调控的季节变化，会影响高纬度海洋表层反照率（海冰覆盖度）以及海洋与大气之间的热交换；（2）受岁差调控的低纬度日射量，可直接影响大气动力过程及相关的降水/蒸发变化，通过控制主要锋面带的纬度分布以及中层水与深水形成的条件，显著改变向高南纬度传递的热量。