南海表层海水Sr-U同位素记录及其环境意义

STRONTIUM AND URANIUM ISOTOPIC RECORDS FOR THE SURFACE WATER OF THE SOUTH CHINA SEA AND THEIR ENVIRONMENTAL IMPACT

利用热电离质谱方法分析了取自南海不同海域的现代珊瑚和全新世珊瑚样品的Sr、U同位素组成。结果表明 ,南海珊瑚代表的礁区表层海水87Sr/86Sr比值介于 0 70 890 2— 0 70 940 1之间 ,分布相当不均一 ,而δ2 34U则主要介于 1 70— 1 80之间 ,明显高于开放大洋 1 44的结果 ,反映周边大陆物质输入对南海表层海水组成具有重要影响。

Coral paleoclimate reconstruction is one of the methods to retain high resolution records of the climatic and environmental change, and it is one of the major subjects of the globe change study. There are a number of successful results in such studies in the ocean, but fewer in the marginal marine environment. However, there is perhaps some differences between the environmental factors, such as the component of the seawater, and those of the ocean. These may probably cause some different response of the coralline records to the climate in the marginal area, compared with that in the ocean. In this case, strontium and uranium isotopic compositions of the seawater are good indicators to mark the special component of the seawater. In this paper, strontium and uranium isotopic compositions of some modern coral samples and some Holocene corals, which represent the isotopic composition of the seawater when these corals formed, had been analyzed by thermal ionization mass spectrometry method. The results indicate that the Sr and U isotopic compositions of the reef water of the South China differ from those of the ocean at least from 8 000a to present. The 87 Sr/ 86 Sr ratios of the coral samples from the South China, ranging from 0 708 902 to 0.709 421 (relative to NBS 987=0 719 310),were not as homogeneous as those from the ocean, which range from 0 709 200 to 0 709 270. δ 234 U of the modern coral samples and δ 234 U (T) , the initial ratios of the Holocene corals, mostly lie between 170 and 180, which were significantly higher than 144, the homogeneous ratio of the ocean. These observations indicate that the run-off from the continent and nearby islands plays an important role to the isotopic compositions of the surface water of the South China Sea. The discussion above implies that when reconstructing the paleoclimatic records from the coral of the South China Sea, the special component of the seawater of the South China Sea may not be neglected.