Sedimentary responses to sea-level rise and Kuroshio Current intrusion since the Last Glacial Maximum: Grain size and clay mineral evidence from the northern South China Sea slope

Abstract

We analyzed the terrigenous fraction of a sediment core from the northern slope of the South China Sea, to better understand the depositional history of the region since the Last Glacial Maximum (LGM). Temporal variations in the grain size and clay mineral composition revealed a complex set of overlapping controls. Approximately 90% of terrigenous sediment throughout the core ranged in size from 5.5 to 8.5 Φ, with the 5.25–6.25 Φ size class being particularly sensitive to environmental changes. The proportion of this fraction decreased during deglacial, indicating a reduction in the supply of coarser grains due to coastline retreat. As the sea level rose toward its present position, the relative proportion of clay derived from Taiwan increased; however, the mass accumulation rate of Taiwan-derived clay decreased, suggesting a reduction in the westward bottom current intensity. In contrast, the relative contribution of clay from the Pearl River decreased from ~40% during the LGM, to ~5% when the sea level reached ~70m below its present position. The Pearl River contribution has remained consistently low throughout the Holocene, indicating that shore-parallel circulation patterns on the shelf resembled their present configuration by 13ka. Compared to Taiwan and the Pearl River, the proportion of clay derived from the area east of Pearl River (EPR) was relatively minor, and less variable. A local peak in the relative contribution from this area occurred during a Melt Water Pulse Event (MWP-1A), suggesting that a reservoir of smectite-rich clays residing in coastal bays was remobilized by this marine transgression. Spectral analysis of the relative contributions of the three major clay sources revealed a significant periodicity of ~380years, which may reflect changes in solar insolation modulating the flux of clay minerals from Taiwan and the Pearl River to the deposition site on a centennial scale.