南海北部晚中新世-上新世期间碎屑物质输入对青藏高原隆升与季风演化的响应

青藏高原在中新世晚期隆升到一定高程之后，它对东亚气候系统和风化作用的影响，通过对高原东北缘干旱、半干旱区域沉积记录的研究取得了重要的认识；然而在整体处于温暖湿润气候状态下的高原的东南缘区域，风化作用过程对高原活动的响应方式则需要更多的工作去揭示。文章选择位于南海北部琼州海峡的钻孔沉积物(QZ6，钻孔总长200.15 m，顶部3.05 m以上未取样)，尝试在利用磁性地层学确定地层年代框架的基础上，探讨高原东南缘的风化作用自中新世晚期以来的基本规律。磁性地层学结果揭示，约197 m厚的沉积物发生在约7.20~3.10 Ma之间，沉积速率在约5.25 Ma从早期较低快速突变至最高，这一变化与高原东北缘红粘土堆积和河湖沉积体系的沉积速率变化一致，但是与接受高原南缘碎屑输入的南海南部和孟加拉扇的沉积速率变化相反，说明高原在中新世末期可能存在向东扩展、而南缘相对稳定的特征。风化作用的强度以4.75 Ma为界，前后两个阶段具有不同的控制因素，早期受高原构造活动影响，物理风化、剥蚀作用强烈，而后期与东亚夏季风的强度相关，化学风化作用增强。

Response of terrigenous flux to the uplift of the Tibetan Plateau and summer monsoon evolution during the Late Miocene-Pliocene in the northern South China Sea

In the Late Miocene, with the uplift of the Qinghai-Tibet Plateau to a certain elevation, its influence on the climate system and weathering process reached a critic understanding in the arid and semi-arid regions of East Asia, the northeastern margin of the plateau. However, in the southeastern margin of the plateau, characterizing with a warm and humid climate conditions, more works are also very important to reveal the response pattern of the weathering process to plateau activities. This paper attempts to explore the general rules of weathering process on the southeastern margin of the plateau, by selecting the sediments of core QZ6(20°14′N, 109°49′E) (sediments of top 3.05 m were abandoned), which locate in the Qiongzhou Strait in the northern part of the South China Sea, to carry out the paleomagnetic works. The sediments of about 197 m in length were composed mainly of light-grey clay and silty clay interbedded with silts and fine sands layers. Some debris of the marine biology, such as corals and shells, were contained in the silts or fine sands. Total 1649 samples were used to measure the magnetic properties and analyze the Characteristic Remanent Magnetization(ChRM)inclinations, which presents eight reverse polarity and seven normal polarity intervals respectively. The rock magnetic properties of sediments, associated with a less than 35 mT remanence coercivity and ca. 580 ℃ Curie temperature, suggest that the pseudosingle domain(PSD)magnetite dominated the remanece. The relative stable magnetic minerals in assemblage and grain size clarify the reliable geomagnetic polarity intervals. The stratigraphic chronological framework can be determined by the magnetostratigraphy combined a nannofossil distribution. The results display the deposition sequence ca. 197 m in length occurred during the period of about ca. 7.20~3.10 Ma. The deposition rate changes rapidly from the relative low in early stage to the highest at ca. 5.25 Ma, which is consistent with the red clay accumulation and river-lake sedimentary system in the northeastern margin of the plateau, but contrary to the sequence in the southern South China Sea and Bengal fan, which accepts the terrigenous flux from the southern margin of Tibet. These features hint that the plateau may expand eastward at the end of Miocene and the southern margin is relatively stable. The intensity of weathering process presents two stage evolution bounded at the age of ca. 4.75 Ma. In the early stage(7.20~4.75 Ma)with a high terrigenous flux in general, the relative fine sediments are response to the strong summer monsoon periods while the relative coarse sediments relate to the weak summer monsoon in the later stage(4.75~3.10 Ma)with a low terrigenous flux on the time scale of hundreds of thousands of years. This different corresponding patter suggests that the varying forcing mechanism may control the weathering intensity. Plateau tectonic activities probably exert the significant role to physical weathering and strong exhumation when the summer monsoon increasing the chemical weathering process in the early stage, however, the strong summer monsoon enhances lateral erosion and river transport capacity in the later stage.