VARIATION OF GEOCHEMICAL RECORDS IN CORE A7 SEDIMENTS FROM MIDDLE OKINAWA TROUGH DURING THE PAST 18 kaBP AND ITS RESPONSE TO PALEO-ENVIRONMENTAL CHANGES
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摘要: 对取自冲绳海槽中部的A7孔沉积物进行了元素地球化学的测试与分析,配合高分辨率测年和氧同位素数据,探讨了冲绳海槽中部18 000年以来的元素地球化学变化特征及其控制因素。使用R型因子分析方法,将分析的20种常量与微量元素分为4个主要的因子:因子I主要由Al、Ti、Th、Rb、Mg、U和Zn等元素组成,代表了陆源物质有关的元素组合,可作为陆源物质输入的指示因子;因子Ⅱ主要由Ca、Sr、Ba、P等元素组成,代表了与生物作用有关的元素组合,可作为古生物生产力指示因子。根据各元素及其组合随时间的变化特征,整个A7孔沉积物可明显分为4段:18~15 kaBP (Ⅰ),15~8 kaBP (Ⅱ),8~2 kaBP (Ⅲ)和2 kaBP以来(Ⅳ),前3段中都表现出陆源物质先增加再减少的变化趋势,古生物生产力的变化相对复杂,规律性不强,但总体表现出与陆源物质输入相反的变化趋势;第4段中则表现出2 kaBP年以来陆源物质的输入有一定的加强,而古生产力则没有大的变化。在7.3 kaBP左右发生的火山物质快速堆积事件对柱状沉积物的地球化学特征产生了较大的影响,也使陆源物质输入和古生物生产力急剧降低。自18 kaBP年以来,元素地球化学组成的变化主要受温度、海平面变化、黑潮水的"摆动"的影响,而火山沉积也造成了一定的影响。Abstract: A piston core retrieved from the Middle Okinawa Trough was studied of the geochemical variation since the last 18 cal. kaBP on the basis of the high-resolution geochemical analysis as well as detailed AMS 14C dating. R-mode Factor analysis method was used to reunite the whole analyzed 20 element contents into four factors, in which Factor I mainly composing of Al, Ti, Rb, Th, Mg, U, Zn, etc. was regarded as the index of terrigenous detrital components, while Factor Ⅱ with high positive loadings of Ca, Ba, P and Sr was suggested to be the paleo-productivity proxy. Based on variation pattern of the geochemical records, the whole strata were divided into 4 sections, namely, 18~15 kaBP (I), 15~8 kaBP(Ⅱ), 8~2 kaBP (Ⅲ) and 2 kaBP to present (IV). They are all characterized by the distinct geochemical compositions, terrrigeous or biogenetic. The volcanic sedimentation event that occurred during 7.3 cal kaBP played an important role in the fluctuation of the geochemical records in Core A7 sediments, in which the terrigenous and biogenetic contributions were very smaller compared with counterparts from other intervals. We relate these geochemical variations to the coupling effect of temperature change, sea-level change, and the shifts of Kuroshio Currents, which were the main factors controlling the variations of the geochemical composition in Middle Okinawa Trough. However, there are also some exceptions, such as the lowered terrigneous input centering near 15 cal. kaBP. and 2 cal. kaBP. The reason is uncertain because they all coincided with the lowered temperature. Moreover, we also believe that there are tight tele-connections between the Okinawa Trough region and the global climatic change, when comparing the geochemical records of this study with the climatic records from North Atlantic and Greenland.
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