Clay mineralogy indicates the Holocene monsoon climate in the Changjiang (Yangtze River) Catchment, China

The sediments from two cores (CM97 and LGZ) in the Changjiang (Yangtze River) Delta were collected for clay mineral analyses, with the aims of examining the origin of the clay and the application of clay mineralogy to the reconstruction of monsoon climate variability during the Holocene. The clay mineral assemblages of the Holocene sediments are similar overall to clays deposited in the modern Changjiang, albeit with large fluctuations in depositional environments. Peak contents of smectite occurring at ca. 13–11.5 ka are primarily due to the significant contribution of clays weathered from the upper Changjiang catchment under the impact of an enhanced Indian Summer Monsoon. The regular variations of crystallographic indices including illite crystallinity, chlorite crystallinity and chemical index of illite in the cores show close correspondence with the well-known oxygen isotopic curves of stalagmites in China. This suggests that the evolution of the Asian Summer Monsoon in the Changjiang catchment can be reliably reconstructed from variation in the weathering characteristics of the detrital clay component of sediments, despite the complex sediment source-to-sink transport patterns and the changes of depositional environments. For short timescales, the degree of crystallinity of clay minerals is more sensitive to chemical weathering and climate change than their absolute abundance. This study sheds new light on the reconstruction of paleoclimatic change in large drainage basins through the clayey sediments accumulated in estuarine and deltaic areas where land and sea interact.