Geochemistry and foraminiferal stable isotoe record of sediment core SCS90-36

Changes in the Southeast Asia monsoon winds and surface circulation patterns since the last glaciation are inferred using multiple paleoceanographic indicators including planktic foraminifer faunal abundances, fauna and alkenones sea-surface temperature (SST) estimates, oxygen and carbon isotopes of planktic and benthic foraminifers, and sedimentary fluxes of carbonates and organic carbon obtained from deep-sea core SCS90-36 from the South China Sea (SCS) (17°59.70'N, 111°29.64'E at water depth 2050 m). All these paleoceanographic evidences indicate marked changes in the SCS ocean system over the last glacial toward the Holocene. Planktic foraminiferal faunal SST estimates show stable warm-season SST of 28.6°C, close to the modern value, throughout the glacial-interglacial cycle. In contrast, cold-season SST increases gradually from 23.6°C in the last glacial to a mean value of 26.4°C in the Holocene with a fluctuation of about 3°C during 13-16 ka. SST estimates by UK'37 method reveal less variability and are in average 1-3°C lower than the fauna-derived winter-season SST. These patterns reveal that the seasonality of the SST is not only higher by about 3-4°C in the glacial, but also a function of the winter season SST. Sedimentation rates decrease from the last glacial-deglacial stage to the Holocene due to a reduction in supply of terrigenous components, which led to an increase of carbonate contents. Total organic carbon (TOC) contents of primarily marine sources decrease from the last glacial-deglacial to the Holocene. The last deglaciation is also characterized by high surface productivity as indicated by increased ketones abundances and high mass accumulation rates (MAR) of the TOC and carbonates. The gradient of planktic foraminifer ocygen and carbon isotopes of between surface dwellers and deep dwellers increases significantly toward Termination I and Holocene, and is indiscernibly small in the carbon isotope gradient of between 14 and 24 ka, revealing a deep-mixing condition in surface layers prior to 10 ka. The glacial-interglacial fluctuation of the carbon isotope value of a benthic foraminifer is 0.61%. which is significantly larger than a global mean value. The large carbon isotope fluctuation indicates an amplification of marginal-sea effects which is most likely resulted from an increase in surface productivity in the northern SCS during the last glacial-deglacial stage. The multiple proxies consistently indicate that the last glacial-deglacial stage winter monsoon in the Southeast Asia was probably strengthened in the northern SCS, leading to a development of deep-mixing surface layer conditions and a more efficient nutrient cycling which supports more marine organic carbon production.本研究依托南海（South China Sea, SCS）深海岩芯SCS90-36（站位：17°59.70'N，111°29.64'E，水深2050 m）获取的多套古海洋学指标，重建了末次冰期以来东南亚季风环流与表层环流格局的演化历史。所用指标包括：浮游有孔虫（planktic foraminifer）种群丰度、基于浮游有孔虫群落与烯酮类化合物（alkenones）的海表温度（sea-surface temperature, SST）估算结果、浮游与底栖有孔虫（benthic foraminifer）的氧碳同位素组成，以及碳酸盐与有机碳的沉积通量。所有古海洋学证据均显示，末次冰期至全新世期间，南海海洋系统发生了显著改变。基于浮游有孔虫群落的海表温度估算结果表明，冰期-间冰期旋回内暖季海表温度始终稳定在28.6℃，与现代观测值接近。与之形成鲜明对比的是，冷季海表温度从末次冰期的23.6℃逐渐升高至全新世的平均26.4℃，并在13~16 ka期间出现约3℃的波动。采用UK'37法（UK'37 method）得到的海表温度估算结果变异性更低，且平均比基于群落的冬季海表温度估算值低1~3℃。上述特征揭示，冰期南海海表温度的季节差较现代增大约3~4℃，且该季节差异主要由冬季海表温度变化主导。由于陆源组分（terrigenous components）供给减少，沉积速率从末次冰期-冰消期阶段向全新世逐步降低，进而导致碳酸盐含量上升。源自海洋生源的总有机碳（total organic carbon, TOC）含量同样从末次冰期-冰消期向全新世持续降低。末次冰消期同样表现出较高的表层生产力，具体体现为烯酮类化合物丰度升高，以及TOC与碳酸盐的质量堆积速率（mass accumulation rates, MAR）显著增加。表层与深层栖息浮游有孔虫的氧、碳同位素梯度在向冰消期终止点I（Termination I）及全新世演化的过程中显著增大，而在14~24 ka期间该碳同位素梯度几乎无法被观测到，这表明10 ka之前南海表层水体存在强烈的混合作用。底栖有孔虫碳同位素值的冰期-间冰期波动幅度达0.61‰，显著高于全球平均值。如此大的波动幅度表明边缘海效应被放大，这极有可能是末次冰期-冰消期阶段南海北部表层生产力升高所导致的。多指标重建结果一致表明，末次冰期-冰消期阶段东南亚冬季风在南海北部有所增强，进而促使表层水体混合作用加剧、营养盐循环效率提升，最终支撑了更高的海洋有机碳生产。