南海北部沉积物常量元素变化、碳酸盐旋回及其古环境意义

通过对南海北部ODP1145站沉积物的常量元素和碳酸盐分析,探讨了早更新世南海北部沉积物常量元素和碳酸盐含量变化特征及其古环境意义.结果表明,南海北部沉积物中主量元素CaO、Fe2O3、K2O、Na2O、MnO和P2O5受碳酸盐含量影响相对较小,而SiO2、MgO、Al2O3和TiO2受到碳酸盐含量影响相对较大.碳酸盐校正后的常量元素变化可以划分为3种类型,即PC1、PC2和PC3,它们的主要特点有:PC1元素组合包括了SiO2、CaO、Na2O、P2O5、Fe2O3和K2O,主要反映了与季风强度有关的生源物质产率变化,指示了距今2.5~1.5Ma时段生产力的逐渐降低; PC2元素组合(Al2O3、TiO2)代表了陆源物质的输入,指示了在约2Ma开始陆源物质输入的明显增加; PC3元素组合(MgO、MnO)可能与海洋自生作用有关,反映了海底氧化/还原环境的相对变化.早更新世南海北部CaCO3含量呈阶段性降低,一方面受陆源物质稀释作用的影响,另一方面可能更与生物生产力逐渐降低有关.主量元素组合及碳酸盐含量在约2Ma和1.7Ma左右均发生显著变化,可能指示了早更新世时期东亚冬季风的两次增强.

VARIATIONS OF MAJOR ELEMENTS AND CARBONATE CYCLE OF THE NORTHERN SOUTH CHINA SEA SEDIMENTS AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE

The South China Sea (SCS) sediments have been investigated intensively to reconstruct paleoceanographic and paleoclimatic changes during the Cenozoic in recent decades. Long-term evolution and short-term variability of the Asian monsoon have been inferred from mulitple sedimentological, mineralogical, geochemical and isotopic proxies of the SCS sediments. However, previous studies suggested that variation of carbonate content in South China Sea sediments could affect the elemental compositions to various degrees, which hampers further paleoclimatic interpretations of the elemental compositions. In this study, based on paleomagnetic results, a total of 368 samples within meters composition depth 137~191m, which spanning the Early Pleistocene(2.5~1.5Ma)were selected from the Ocean Drilling Program Site 1145(19°35.04'N, 117°37.86'E; water depth 3175.5m), located at the base of the northern continental margin of the South China Sea, and were analyzed for the contents of major elements and carbonate. Samples have been taken at a 15cm resolution. Our results show that major elements including Fe2O3, K2O, Na2O, MnO, and P2O5 are slightly influenced by carbonate dilution, while concentrations of SiO2, MgO, Al2O3 and TiO2 are highly influenced by the carbonate content. Based on principal component analysis, the carbonate-free basis major element composition can be subdivided into three principal components: PC1(SiO2, CaO, Na2O, P2O5, Fe2O3 and K2O), PC2(Al2O3 and TiO2) and PC3(MgO and MnO). PC1 mainly reflects changes in monsoon-related biogenic productivity, characterized by a gradual decrease during 2.5~1.5Ma. PC2 represents the input of terrigenous material and displays an increased variability since 2Ma(169m). PC3 is likely associated with oceanic autogenous conditions, reflecting the redox environmental variations. In the Early Pleistocene, gradual decreasing of the carbonate content was attributable to the dilution of increased terrigenous input and gradually decreased biological productivity. Both major element compositions and carbonate content show significant shifts at about 2Ma(169m) and 1.7Ma(151m), likely indicating stepwise strengthening of the East Asian winter monsoon in the Early Pleistocene.