长江三角洲晚第四纪地层沉积物源特征及其对季风气候变化的响应

物源分析可以反演河流系统演化过程中,构造运动、气候变化与海平面升降等因素的影响。选择长江三角洲现代、中全新世和末次盛冰期3个代表性时代的沉积物样品,进行重矿物组合、锆石U—Pb年龄谱系等分析,以揭示不同气候条件下长江三角洲河口沉积物源的变化。与末次盛冰期相比,中全新世沉积物中榍石、锆石含量较低,重矿物GZi指数较大,古元古代锆石含量较高,但白垩纪、新元古代锆石含量较低。这些特征表明中全新世长江河口沉积物有更多源自上游地区的物质,这种变化可能与流域受夏季风影响的降雨量变化有关。初步分析认为,末次盛冰期研究区季风很弱,降雨量和径流量均下降,上游物质较少被搬运至河口;全新世季风加强后,降雨量和径流量均上升,此时河口沉积物更多源自上游,尤其是在中全新世夏季风最强盛期。

Provenance characteristics of the late Quaternary in the Yangtze River Delta and its response to monsoon climate change

River systems are particularly sensitive to tectonic movements,climate and sea-level changes,and the history of these processes can be decoded by provenance analysis. Samples of modern surface sediments,the Mid-Holocene and the Last Glacial Maximum(LGM)core sediments in the Yangtze River Delta were analyzed with heavy-mineral composition and zircon U-Pb age spectra,aiming to decode the variation in estuarine sediment provenance in response to different climate conditions. In comparison with the LGM sediments,the Mid-Holocene sediments have less sphene and zircon content,higher GZi value,fewer zircons from the Cretaceous and Neoproterozoic but more zircons from the Paleoproterozoic. It was suggested that there were more sediments sourced from the upper reach in the Mid-Holocene than in the LGM, and this phenomenon may be linked with variations of summer monsoon precipitation in the catchment. Less sediments from the upper reach were transported into the sea during the LGM weak monsoon period because of the sharp decrease in precipitation and runoff,while they significantly increased in the Holocene with the strengthening monsoon and the consequent precipitation and runoff,typically during the Mid-Holocene Climatic Optimum with the strongest summer monsoon.