Oxygen isotope and Mg/Ca record of planktonic foraminifera of sediment core MD01-2390

Changes in the local freshwater budget over the last 22,000 years have been estimated from a sediment core located in the southern South China Sea (SCS) using a combined approach of Mg/Ca and oxygen isotopes on the planktonic foraminifera Globigerinoides ruber (white) sensu stricto (s.s.). Core MD01-2390 (06°28,12N, 113°24,56E; water depth 1591 m) is located near the glacial paleo-river mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers that drained the exposed Sunda Shelf. The delta18Oseawater record reveals lower average values (-0.96±0.18 per mil) during the Last Glacial Maximum (LGM) when compared with modern values (-0.54±0.18 per mil). Low salinity during the LGM is interpreted to reflect a higher freshwater contribution due to a greater proximity of the core site to the mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers at that time. A general deglacial increasing trend in salinity due to the progressive landward displacement of the coastline during deglacial shelf flooding is punctuated by several short-term shifts towards higher and lower salinity that are likely related to abrupt changes in the intensity of the East Asian summer monsoon. Thus, the deglacial delta18Oseawater changes reflect the combined effects of sea-level-induced environmental changes on the shelf (e.g. phases of retreat and breakdown of the shelf drainage systems) and East Asian monsoon climate change. Lower salinity than at present during the Early Holocene may be attributed to an increase in summer monsoonal precipitation that is corroborated by previous marine and terrestrial studies that report a Preboreal-Early Holocene monsoon optimum in the Asian monsoon region.过去22000年以来的局地淡水收支变化，可通过南海（South China Sea, SCS）南部的一处沉积物岩芯（sediment core），结合浮游有孔虫（planktonic foraminifera）白氏红拟抱球虫严格型（Globigerinoides ruber (white) sensu stricto, s.s.）的Mg/Ca比值与氧同位素分析方法进行估算。该岩芯编号为MD01-2390，站位坐标为06°28.12′N、113°24.56′E，水深1591米，位于曾暴露的巽他陆架的冰川时期古河口附近，即巴拉姆河（Baram）、拉让河（Rajang）与北巽他/莫伦格拉夫河（North Sunda/Molengraaff Rivers）的古河口周边。该站位的海水氧同位素（delta18Oseawater）记录显示，末次冰盛期（Last Glacial Maximum, LGM）的平均数值为-0.96±0.18‰，较现代值（-0.54±0.18‰）更低。末次冰盛期期间的低盐度特征被解释为：彼时岩芯站位距上述三条河流的河口更近，因此获得了更高的淡水输入量。冰消期阶段，随着陆架海侵导致海岸线逐步向陆迁移，盐度整体呈现上升趋势，但其间穿插了数次短期的盐度升降波动，这些波动大概率与东亚夏季风（East Asian summer monsoon）强度的突变事件相关。因此，冰消期的海水氧同位素变化，反映了海平面变化引发的陆架环境变化（如陆架排水系统的退缩与瓦解阶段）与东亚季风（East Asian monsoon）气候变化的共同调控作用。早全新世期间的盐度较现代更低，这一现象可归因于夏季季风降水的增加，该结论得到了此前海洋与陆地研究的佐证——这些研究均报道了亚洲季风区存在前北方期-早全新世季风适宜期（Preboreal-Early Holocene monsoon optimum）。