长江三角洲DY03孔沉积物元素地球化学及其物源示踪意义

长江三角洲地区是寻找长江历史演化信息的关键地区之一。本文通过DY03孔长时间序列元素地球化学的研究，利用元素地球化学特征示踪物源变化，初步揭示长江三角洲地区物源变化的总体情况，分析讨论长江三角洲地区沉积记录对长江贯通的指示意义。研究发现，在晚上新世31Ma左右时期，钻孔中指示物源变化的 K₂O/Al₂O₃， SiO₂/Al₂O₃, Zr/Ti,Th/Sc，Th/Co，Th/Cr，Nb/Co等元素比值都发生由高到低的显著变化，而3.1Ma之后元素比值变化不大。化学风化A-CN-K图解也进一步说明在3.1Ma前后，沉积物具有不同的化学风化趋势。以上特征说明，在晚上新世时期本区物源曾发生重大变化，表现为沉积物来源由近源转变为远源，源岩由酸性长英质矿物为主转变为基性矿物为主。这次物源变化很可能说明最早在晚上新世长江上游物质已经通过长江三峡输送到现代长江三角洲地区。

INVESTIGATION OF SEDIMENTARY GEOCHEMISTRY OF CORE DY03 IN THE YANGTZE DELTA: IMPLICATIONS TO TRACING PROVENANCE

As one of the biggest rivers in the world，formation and evolution history of Yangtze River(Changjiang)has drawn lots of attention since in the early 1900s，however，there has been existing a controversy intensively disputed more than one hundred years over the age when the river ran though the Three Gorges area，which indicates the formation timing of the modern Yangtze River. The Yangtze Delta area is regarded as one of the crucial areas to resolve the scientific puzzle. A sediment core——Core DY03（30°582′N，131°258′E）drilled to the basement with depth of 24915m was recovered from the Yangtze Delta area，and was used to study the long term provenance of sediments and their implications to the history of Yangtze River.
With methods of paleo￣magnetism，magnetic susceptibility and lithological analysis，a reliable chronostratigraphic framework of the Core DY03 was established. According to paleo￣magnetism measurement，B/M (Q₂/Q₁)and M/G(Q₁/N₂)boundaries are found located at depth of 103.4m and 157.3m respectively. The rest boundaries are determined by lithological correlation and magnetic susceptibility characteristics，with Q₄/Q₃ at depth of 2484m and Q₃/Q₂ at depth of 618m. 
Through the investigation of long term elemental geochemistry of the Core DY03 and using geochemical characters to trace provenance change，we have acknowledged the general situation of provenance changes of this area，and have discussed the implications of sedimentary record of the Yangtze Delta to the formation of the Yangtze River. It is found that elemental ratios，including K₂O/Al₂O₃， SiO₂/Al₂O₃, Zr/Ti,Th/Sc，Th/Co，Th/Cr，Nb/Co，decrease obviously in the Upper Pliocene，approximately at 3.1MaB.P. However，the elemental ratios have kept stable after 3.1MaB.P. The plots of A-CN-K also show that there are two different weathering trends of sediments before and after 3.1MaB.P. All of the above mentioned indicates that the provenance had changed greatly since the Late Pliocene with sediment sources changing from the proximal to the distal，and the sedimentary composition changing from the felsic to the mafic. It is suggested by the provenance change that the earliest timing when sediments from the upper Yangtze area arrived at the Yangtze Delta area was possibly in the Late Pliocene.