末次冰期以来南海西北部沉积物源和风化演变的Sr-Nd同位素与稀土元素证据

The Sr-Nd isotopic and REE geochemical evidence for the provenance and weathering evolution of sediments in the NW South China Sea since the last glacial age

研究边缘海沉积物地球化学组成与来源是揭示大陆边缘沉积物源-汇过程的关键环节。本文基于南海西北部琼东南盆地KT2站位岩心沉积物的稀土元素(REE)和Sr-Nd同位素分析,研究了28 ka以来沉积物源的演变和化学风化作用。结果表明,研究区沉积物源主要为红河,且28 ka以来没有发生明显改变。KT2站位岩心陆源沉积物的稀土元素特征参数(La/Yb)UCC、(Gd/Yb)UCC以及(LREE/HREE)UCC,均受到了母岩组成和风化过程的共同影响。根据Sr-Nd同位素对物源的限制,本文对影响REE组成的因素进行了评估,并认为化学风化作用可能是造成沉积物REE组成偏离源区母岩组成的主要原因。REE指标指示了冰期沉积物比全新世具有更强的化学风化强度,可能暗示冰期海平面降低导致北部湾陆架暴露发生进一步风化而造成了冰期化学风化作用的增强。

The research of geochemical compositions and sources of marginal marine sediments is the key link to reveal the source-sink process of the continental margin sediments.Here we analyzed composition of rare earth elements(REE)and Sr-Nd isotopes of sediments from cores of Hole KT2 in the Qiongdongnan basin in the northwestern South China Sea,in order to study the provenance evolution and chemical weathering of sediments since the last glacial age of 28 ka.The results show that the main sediments in the study area had been sourced from the Red River since 28 ka.The REE characteristic parameters(La/Yb)ucc,(Gd/Yb)ucc and(LREE/HREE)ucc of sediments from cores of Hole KT2 are influenced by both compositions and weathering processes of parent rocks.On the basis of constrains on provenances of sediments by using the Sr-Nd isotopes,we have evaluated the influences of multiple factors on REE compositions of sediments and have suggested that chemical weathering was possibly the major factor causing the deviation of REE compositions of sediments from those of parent rocks in the source regions.The REE proxies indicate that the glacial sediment has a stronger chemical weathering intensity than the Holocene sediment,possibly suggesting that the enhancement of chemical weathering of glacial sediments could be caused by the exposure of the Beibu Gulf shelf led by the decrease of sea level during glacial age.