Regional distribution of the Quaternary Red Clay with aeolian dust characteristics in subtropical China and its paleoclimatic implications.

摘要

Grain-size and geochemical characteristics of 18 Quaternary Red Clay (QRC) profiles mainly located in the lower and middle reaches of the Yangtze River in subtropical China were studied. The results showed that all the QRC profiles can be classified into three types: Type A, mainly distributed between 29 degrees N and 31 degrees N, share the similar characteristics with the Xiashu Loess, with the uniformity of grain-size and elemental compositions and the enrichment of the basic dust fraction (10 micro m-63 micro m), which may originate from aeolian dust deposits and is hence called a loess-like QRC. Type B, mainly distributed between 25 degrees N and 28 degrees N, shows alluvial features, with many coarse grains and large-scale vertical variation of grain-size and chemical compositions. Type AB, distributed between Type A and Type B, approximately 28 degrees N-29 degrees N, shows double-unit structure: The upper part is similar to Type A; the lower, however, shows alluvial features like Type B. The loess-like QRC is mainly distributed along the Yangtze River, but gradually declines southwards and is rarely seen south of 28 degrees N. The existence of the loess-like QRC is strong evidence for the occurrence of heavy dustfalls in the modern red-soil areas in subtropical China during the Quaternary glacial periods. The region distributed with the loess-like QRC is at least 3 degrees latitude south of the Xiashu Loess, implying that the loess/red clay boundary in the Yangtze Valley had once oscillated with the alternations between glacial and interglacial climates during the Quaternary period. The loess-like QRC actually includes both glacial and interglacial paleoclimatic information. We suggest that the Yellow-brown Earth (YBE) in the upper QRC was formed during the last glacial period; the underlying red clay, with the aeolian dust characteristics but being strongly weathered, was initially deposited during the second last glacial or earlier glacial stages but heavily weathered and rubefied in the subsequent interglacial events. No multi-cyclical paleoclimatic records were found in the QRC profiles due to the overlapped information. But the weathering degree of the profiles clearly show a trend of decreasing upwards, which suggests that the climate in the Yangtze Valley has become colder and drier since the late Quaternary period.