南海中部全新世以来海山深潜区有孔虫的地球化学记录及反映的气候变化

Geochemical record of foraminifera and its reflection on climate change in the central South China Sea since Holocene

  • 摘要: 有孔虫在生长发育过程中能够捕获或黏附周围海水中的钙质或硅质形成自己的壳体,因此有孔虫的地球化学特征能够记录古气候、古海洋和古环境信息。以2017年蛟龙号第136潜次在南海珍贝海山底部约2 500 m水深位置精准采集的柱状样品为研究对象,对其中的G.ruber以及G.sacculifer两类浮游有孔虫壳体的Mg/Ca以及碳氧同位素组成进行了分析测试。结果表明自12.6 ka以来该海域表层海水温度(SST)的变化范围为24.4~29.3 ℃,平均温度为26.2 ℃,并能够识别出明显的气候突变事件,在时间范围上与新仙女木事件和全新世东亚夏季风突变事件大致吻合。这些突变事件可能受控于ENSO活动和热带辐合带(ITCZ)平均位置的纬向移动,并与北大西洋冰筏事件具有遥相关。此外,发现全新世期间浮游有孔虫G.sacculiferG.ruber的碳同位素分馏值Δ13CG.sacculifer-G.ruber与SST变化有关,SST降低时,Δ13CG.sacculifer-G.ruber呈负偏;而在SST升高时,Δ13CG.sacculifer-G.ruber呈正偏。

     

    Abstract: Foraminifera form their crust by absorption or capture of calcium or silica from the seawater they live in. As the results, the geochemical features of the crust are the efficient indicators of palaeoclimate, palaeooceanography and palaeoenvironments during their life. In the year of 2017, columnar samples of a pushcore were collected by the “Jiaolong” submersible precisely at the foot of the Zhenbei seamount near the Huangyan Island in the Middle of South China Sea. The Mg/Ca ratios and the carbon and oxygen isotope compositions of the planktonic foraminifera Globigerinoides ruber and Globeriginoides sacculifer shells were measured to trace the history of sea surface temperatures (SST) and influence parameters. The results show that the SST in the sea area varied from 24.4 to 29.3 ℃ since 12.6 ka with an average of 26.2 ℃. Some cold events can be the obviously identified, which could be correlated with the events of Younger Dryas and Holocene East Asian summer monsoon. These events may be controlled by the ENSO activities and the shift of the mean location of ITCZ, and even influenced by the North Atlantic ice rafting events. In addition, we found that during the period of Holocene the fractionation of Δ13CG.sacculifer-G.ruber between the carbon isotope compositions of G.sacculifer and G.ruber may also be constrained by SST since the Δ13CG.sacculifer-G.ruber values was negatively biased when SST decreased and vice versa.

     

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