长江河口区第四纪沉积物中的地球化学元素 分布特征及其古环境意义*

文章通过对上海浦东机场孔(Pd)第四纪地层中地球化学微量元素B,Ga,Sr和Ba的观测,结合该钻孔粒度、古地磁和微体古生物等分析资料,综合剖析了这些微量元素及其比值在不同沉积相中的分布特征及其古环境演变的意义。研究发现,微量元素分布与本区各种沉积相有着十分密切的关系。通常,B和Sr及其比值B/Ga和Sr/Ba在冲积相、河流相沉积中较低,在溺谷-浅海相、三角洲相沉积中较高。而Ba与之相反,一般在陆相沉积环境中比较高。Ga元素在第四纪沉积物中的分布波动不明显,但极大值还是偏向陆相环境。本研究还发现微量元素B和Sr及其比值B/Ga,Sr/Ba的分布自下而上存在7个(1~7)高值阶段。阶段1~3高值与海侵无关,认为反映了上新世-早更新世干旱气候的产物;阶段4~7高值则与本区中更新世以来4次海侵事件密切相关。

THE DISTRIBUTION OF GEOCHEMICAL TRACE ELEMENTS IN THE QUATERNARY SEDIMENTS OF THE CHANGJIANG RIVER MOUTH AND THE PALEOENVIRONMENTAL IMPLICATIONS

In the Changjiang River mouth of Eastern China, Quaternary sediment and stratigraphy has been a key field in examining the paleoenvironmental evolution and interaction between land and sea, closely associated with global change. Geological and sedimentological analyses are adequate, but, geochemical means seem weaker.The present study examines 4 geochemical trace elements (Boron, Gallium, Strontium, Barium) in a 360m long Quaternary Borehole (Pd) recovered from the Changjiang River mouth. On the basis of paleomagnetic, petrologic, and biologic analyses, including grain-size and microfossils (Foraminifera and Ostracoda), this study has revealed trace element distribution in relation to various sedimentary facies. The results found that marine microfossil evidence of Boron and Strontium are marine-origin, and Barium and Gallium are more of terrigenous. Boron ranges from 47.22×10^-6 to 55.38× 10^-6 in mean value from alluvial-braided to meandering riverine sediments, reaches 97.69×10^-6 in average in the drowned valley and shallow marine sediments, and 108.75×10^-6 in the deltaic section. Strontium, 81.20×10^-6 and 80.39×10^-6 in average respectively in alluvial-braided section and meandering section, is increased to 174.56×10^-6 in the drowned valley and shallow marine sections, and slightly down to 135.71×10^-6 in the deltaic section. B/Ga and Sr/Ba are 2.63 and 0.15 in mean value respectively in alluvial-braided section, reach 2.86 and 0.19 in turns in meandering section, while each increased to 5.58 and 0.35 in average in the drowned valley and shallow marine sediments, and then 5.64 and 0.27 in each in deltaic facies. That is to say that Boron, Strontium, B/Ga, and Sr/Ba appear to be lower in alluvial to fluvial sediments of Pliocene, Early to Middle Pleistocene, and higher in marine influenced sediments of Late Quaternary age, chiefly including the drowned valley, shallow marine, and delta facies. On the contrary, Barium is higher in distribution in riverine sediments, and lower in marine influenced sections, which is evidenced by the fact that the mean value is generally decreased from 555.25×10^-6 to 504.15×10^-6 from alluvial-braided deposits to deltaic facies. Gallium seems vague in distribution in the different facies, but shows a slightly higher value in the non-marine sediments than that of marine sediments. In a word, Boron and Strontium are marine-origin, and Barium and Gallium are more of terrigenous.Furthermore, there are 7 stages of high-value Boron, Strontium and B/Ga, Sr/Ba identified from the Pliocene and Quaternary sediments, closely in association with the regional paleoclimate evolution and global sea level oscillation. These high values of stage 1 (330~320m), stage 2 (230~220m), and stage 3 (190~180m) probably result from the dry paleoclimate setting in Pliocene and Early Pleistocene, during that time no marine fossils were found in the sediments and the former landscape of study area remained as a high topography. The higher values of Boron, Strontium and B/Ga, Sr/Ba of stage 4, 5, 6, and 7 (140~135m; 90~50m; 47~30m; <29m) took place coincidently with marine microfossil distribution in the Late Quaternary sediments, implying the controls by marine transgressions primarily in the Late Pleistocene and Holocene.