孟加拉湾晚第四纪的碳酸盐溶解旋回

孟加拉湾由于陆源物质大量输入产生稀释效应,使碳酸盐含量表现为冰期时减小、间冰期时增大的“大西洋型”旋回。通过对该区四支活塞岩芯的有孔虫溶解指数、浮游有孔虫沉积通量、CaCO₃ 沉积通量和 >16 0 μm粗组分沉积通量的分析发现,研究区碳酸盐溶解作用强烈,表现出冰期减弱而间冰期增强的总趋势,且溶解作用滞后于浮游有孔虫壳的氧同位素旋回,尤以氧同位素 4、5期的高溶解度情况与印度洋和太平洋氧同位素 5期晚期至 4期为CaCO₃溶解高峰一致。因此,孟加拉湾地区的CaCO₃ 旋回是大西洋型稀释作用与太平洋型溶解作用两者叠加的产物.

Carbonate Dissolution Cycles during Late Quaternary in the Bay o f Bengal

The Bay of Bengal is of a deep-sea environment. Samples of 4 cores (MD77181, MD 77183, MD77190 and MD81349) from this area were studied in detail. As a result, it is found that the carbonate content in this area was high during the intergl acial interval and low during the glacial interval, and thus, shows the dilution cycles of “Atlantic Type”. 　　The analysis of the carbonate dissolution of the 4 cores mentioned above shows t he dissolution was intense in this area. 　　Core MD77190 is nearly 3800m in depth. The preservation situation of planktonic foraminifers indicates a distinct erosion of the shell. The foraminiferal dissol ution index(FDX) curve shows that in core MD77190 located place the dissolution extent is high. FDX are mostly above 5. The dissolution extent is the lowest in oxygen isotope stage 2. The dissolution extent is high in oxygen isotope stages 1, 5 and the early stage 4. Core MD81349 is located in the 90 Sea Mountain and about 2 500 m in depth. The FDX curve shows that in the MD81349 core located place the dissolution extent is relatively low and no big change. 　　The mass accumulation rates (MAR) of planktonic foraminifers and CaCO3 change are entirely in accordance with each other in core MD77190, both reach their hig hest in oxygen isotope stage 2. The MARs of both planktonic foraminifers and CaC O3 in oxygen isotope stage 5 are lower than that in oxygen isotope stages 1, 2 and 3, and reach their lowest in oxygen isotope stage 4. The MAR curves of CaCO 3 and coarse fraction(>160 μm) from cores MD77181 and MD77183 show that the M ARs of CaCO3 and coarse fraction are both the highest in oxygen isotope stage 2, the lowest in oxygen isotope stage 4 and 5. This indicates a low dissolution extent or a high productivity or both in oxygen isotope stage 2 and a high disso lution extent in oxygen isotope stages 4 and 5 in the studied area. 　　To sum up, the dissolution in the studied area is relatively weak in oxygen isot ope stage 2, but became intensified in oxygen isotope stages 4 and 5. 　　In the studied area, the accumulated amount of CaCO3 is low and the dissolutio n extent is high in oxygen isotope stages 4 and 5. The dissolution extent is low , and the accumulate amounts of CaCO3 and planktonic foraminifers are both rel atively high in oxygen isotope stage 2. This accords with the fact that CaCO3 dissolution extent reaches its peak in the late oxygen isotope stage 5 through 4 in Indian and Pacific oceans. The fact that the CaCO3 cycle in the Bay of Ben gal was expressed as of the Atlantic type dilution cycle due to the dilution eff ect of terrestrial material, is actually the result of the combined effects of t he Atlantic type dilution and the Pacific type dissolution.