Planktonic foraminifera of sediment core MD01-2390

Sea-surface temperature (SST) estimates in the sediment core MD01-2390 based on planktonic foraminiferal species abundances using five different transfer function techniques suggest nearly unchanged or unusually higher temperatures in the tropical southern South China Sea (SCS) during the Last Glacial Maximum (LGM) relative to modern temperatures. These results are in contrast to substantial cooling of 2-5 °C inferred by geochemical (Uk'37, Mg/Ca ratios) and terrestrial proxies from the western tropical Pacific region. Using multivariate statistics we show that the glacial southern SCS harboured unique planktonic foraminiferal assemblages that have no modern analogs. Analyses of faunal variation through the core reveal that planktonic foraminiferal assemblages responded to temperature changes inferred from Mg/Ca data but that this signal is subdued by superimposed variations in the relative abundance of Pulleniatina obliquiloculata and Neogloboquadrina pachyderma (dextral). These species occur in glacial samples at proportions that are not observed in the calibration data set. The glacial high abundance of N. pachyderma (dextral) are interpreted to reflect a seasonal (winter) inflow of cold surface water from the northeast via the Bashi Strait due to the combined effects of an intensified winter monsoon, a southward shift of the polar front and the eastward migration of the Kuroshio Current. In contrast, processes controlling the high relative abundances of P. obliquiloculata during the LGM may be unique to the southern SCS. We propose a scenario involving a stronger (winter) mixing or enhanced upwelling due to an intensified winter monsoon that prevented shallow-dwelling, warm indicators to establish larger populations during the LGM. Our results indicate that a no-analog behaviour of planktonic foraminifera faunas is responsible for the warm glacial conditions in this part of the western Pacific warm pool as implied by foraminiferal transfer functions and that a more significant surface cooling in the region as implied by terrestrial and geochemical (Mg/Ca ratios; alkenone unsaturation index) marine proxies is a more likely scenario.

基于5种不同转移函数技术（transfer function techniques），通过浮游有孔虫（planktonic foraminifera）物种丰度重建的MD01-2390沉积物岩芯海表温度（Sea-surface Temperature, SST）记录显示，相较于现代温度，热带南海南部海域在末次冰盛期（Last Glacial Maximum, LGM）的海表温度几乎未发生变化，甚至异常偏高。该结果与热带西太平洋区域通过地球化学指标（Uk'37、Mg/Ca比值）及陆生代用指标所推断的2~5℃显著降温结果相悖。通过多元统计分析（multivariate statistics），我们发现冰期南海南部存在独特的浮游有孔虫组合，该组合并无现代类比物。对岩芯内群落变化的分析表明，浮游有孔虫组合对Mg/Ca数据反演的温度变化存在响应，但该信号被斜室普林虫（Pulleniatina obliquiloculata）与右旋厚壁新球虫（Neogloboquadrina pachyderma (dextral)）相对丰度的叠加变化所削弱。这两个物种在冰期样品中的占比并未出现在校准数据集当中。冰期右旋厚壁新球虫的高丰度被认为反映了因冬季季风增强、极锋南移以及黑潮（Kuroshio Current）东移共同作用，导致冬季冷水通过巴士海峡从东北方向季节性注入的结果。相反，控制末次冰盛期斜室普林虫高相对丰度的过程可能仅存在于南海南部区域。我们提出一种情景：由于冬季季风增强，引发更强的冬季混合作用或上升流增强，这阻止了浅栖暖水指示类群在末次冰盛期形成更大种群。我们的研究结果表明，正如有孔虫转移函数所暗示的那样，浮游有孔虫群落的无现代类比特征是西太平洋暖池该区域冰期暖态的成因；而陆生及地球化学（Mg/Ca比值、藻烯酮不饱和度指数）海洋代用指标所暗示的该区域更为显著的表层降温，才是更合理的情景。