南海南部巽他陆架氧同位素3期粘土矿物记录及其古环境意义

南海南部低海平面时期巽他陆架陆源碎屑的物源区变化,对于研究海陆变迁过程古环境演化具有重要意义.选择巽他陆架外缘对海平面变化影响非常敏感的氧同位素3期岩心记录(18282-2孔),开展高分辨率陆源碎屑粘土矿物分析,以期获得海平面变化影响下的物源区演化历史.研究结果显示,在氧同位素3期44.5～36.0 cal ka BP期间,巽他陆架外缘含有较高的蒙脱石(29%～44%)与中等含量的高岭石(21%～27%)、伊利石(19%～27%)和绿泥石(14%～20%).物源区分析表明,这个时期的蒙脱石主要由苏门答腊岛和泰国中部河流提供,高岭石主要由马来半岛、苏门答腊岛和婆罗洲西部河流提供,而伊利石和绿泥石主要由湄公河提供.在此期间,海平面长时间位于-80 m以下,巽他出露陆架上发育的大型古河流(北巽他河、古昭披耶河及古湄公河)可将来自周边物源区的陆源碎屑物质直接输送到陆架外缘.研究岩心的蒙脱石/(伊利石+绿泥石)比值与海平面变化具有良好的对应关系:当海平面下降时,蒙脱石/(伊利石+绿泥石)比值增大;反之亦然.这一现象表明海平面变化是影响巽他陆架外缘氧同位素3期沉积环境演化的最重要因素,即海平面升降...

Clay Mineralogical Record and Its Paleoenvironmental Significance during Marine Isotope Stage 3 on the Sunda Shelf,Southern South China Sea

Variation in source of terrigenous sediments on the Sunda Shelf in the southern South China Sea during the sea-level lowstand is of great significance to reconstruct the evolution of the land-sea configuration. Varying provenances response to sea-level fluctuations within Marine Isotope Stage 3 (MIS 3) was investigated by high-resolution clay mineralogy of Core 18282-2 recovered from the outer Sunda Shelf. The results indicate that clay mineral compositions on the outer shelf show relatively high contents of smectite (29%-44%), moderate contents of kaolinite (21%-27%), illite (19%-27%), and chlorite (14%-20%). The provenance analysis based on clay mineralogy suggests a mixture of individual clay mineral from various sources surrounding the southern South China Sea. Smectite originated mainly from Sumatra and middle Thailand. Kaolinite derived mainly from the Malay Peninsula, Sumatra, and western Borneo. Illite and chlorite were mainly contributed by the Mekong River. During 44.5-36.0 cal ka BP, the sea-level was below -80 m for most of the time, the vast paleo-river systems developed on the emerged shelf, namely the North Sunda River, the paleo-Chao Phraya River, and the paleo-Mekong River. Terrigenous sediments from the surrounding provenances were transported through these major ancient drainage systems directly to the outer shelf at that time. Variations in the smectite/(illite + chlorite) ratios correspond well to the sea-level fluctuations with relatively higher ratios developed during the sea-level lowering periods, implying that the sedimentary environmental evolution during the MIS 3 on the outer Sunda Shelf was dominantly controlled by the sea-level change. Modification of the land-sea configuration resulted from sea-level fluctuations, especially the migration of paleo-coastlines, could significantly affect the transport distance between the paleo-river mouths and the study area, resulting in variations in relative contributions of the surrounding provenances to the outer Sunda Shelf.