长江河口区晚新生代以来沉积化学元素分布及物源指示意义

Geochemical Finger Prints on China's Yangtze River Channelization into the Sea and Its Provenance Implications

通过对长江三角洲平原上海浦东机场孔(PD钻孔)沉积物的化学元素分析,结合判别函数和因子分析等手段,探讨了元素富集规律随时间的演变特征,揭示了该区物源的演化过程。研究结果显示,PD钻孔沉积物可以划分为Ⅰ~Ⅴ带:Ⅰ带(上新世—早更新世早期)主要富集Pb,Th,U,Ba,La和Ce等元素,此时长江三角洲逐渐由区域性台地向沉降盆地转变,较封闭的局地物源控制了该区沉积特征;Ⅱ带(早更新世早期—早更新世中期)中Fe,K,As和Rb含量增高,表明此时长江中游物质加入进来,镇江峡口被切穿;Ⅲ带(早更新世末期—中更新世)由Ti,Ⅴ和Cr等主导,这些元素代表了上游峨眉山玄武岩,也表征该区物源进一步扩展至长江上游;Ⅳ~Ⅴ带(中新世末—更新世—全新世)元素特征与Ⅲ带类似,但以Br高含量为特征,表明此时该区地势进一步降低,并遭受多次海侵。PD钻孔的地球化学资料很好地揭示了在我国东西地势倒转的大背景下古长江溯源侵蚀,逐级切穿峡口的演化历程。结合测年资料,推断长江贯通东流至现今河口地区的时间不晚于1.0~1.2 Ma(Ⅲ带底部)。

This study investigated sediment source to sink relating the channelization of the Yangtze River into the sea. A sediment borehole （PD） on the river coast, penetrating thick Quaternary sediments, to thin sediments of late Pliocene age until the bedrock recorded a change in sediment provenances through time. Geochemical elements and magneto-stratigraphy helped identify five zones. Zone Ⅰ （the late Pliocene-the Early Pleistocene）, characterized by Pb, Th, U, Ba, La, Ce, Nd, Hf, Y, Zr, Nb and Mn, indicated a local sediment provenance. This means that the study area was a localized sub-basin. Zone Ⅱ （the Early Pleistocene-the mid-stage of Early Pleistocene）, with remarkable high Fe, K, As and Rb implied a new sediment provenance joining to the sub-basin from the middle Yangtze reach after the opening of Zhenjiang Gorge. Zone Ⅲ （ the mid-stage of Early Pleistocene-the Middle Pleistocene）, featured by Ti, V, Cr, St, Sc, Cu, Co, Ni, Mg, Ca, Na and P suggested a further extension of sediment provenance to the upper Yangtze reach, where a large block of the E＇ mei basalt occurs. This hinted the channelization of the Three Gorges valley linking the upper and middle Yangtze reaches. Zone Ⅳ-Ⅴ （ the Middle Pleistocene-the Holoeene） showed their geochemical similarity to Zone Ⅲ. Discrimination ratio f（ Cr, Th）, f（La） and f （K, La）, a new approach developed for tracing sediment provenance, confirmed a basin-wide sediment source through Zone III-Ⅴ. These together witness a progressive extension of the sediment provenance towards the upper Yangtze reach, corresponding to the long-term tilting effect of the Cenozoic Topographic Reversal of the eastern China continent. The timing of the Yangtze River channelizing into the continental shelf of East Asia appeared at ca. 1.0- 1.2 Ma （bottom of Zone Ⅲ）.