Deep-water deposition in response to sea-level fluctuations in the past 30 kyr on the northern margin of the South China Sea

Abstract

Sea-level fluctuations exert a major influence on deep-water depositional processes through changing sediment supply and oceanographic processes. By utilizing the bathymetric map and six piston cores (P1 and P5–P9) from the two major canyon systems (i.e., Pearl River Canyon (PRC) and Shenhu Submarine Canyon Group (SSCG)) within the Baiyun Sag, northern South China Sea (SCS), this study investigated the deep-water depositional responses to the sea-level fluctuations in the past 30 kyr, during which the last glacial maximum (LGM) shelf edge remained farther (>60 km) seawards of the contemporary shoreline and had water depths greater than 120 m. In the past 30 kyr, the Baiyun Sag was dominated by mixed sedimentary processes involving in both turbidity and bottom currents. Coarse-grained turbidites were mostly deposited at the upper-middle segments of the slope channel within the PRC and the canyon heads of the SSCG from the LGM to the middle deglacial stage (ca. 11.5 ka); while the fine-grained contourites were continuously and widely deposited within the Baiyun Sag. It is noted that the contourites show fining- and coarsening-upwards trends before and after 6.7 ka, respectively.The turbidity currents in the past 30 kyr were mainly triggered by oceanographic processes (i.e., contour currents and internal waves) within the Baiyun Sag. Internal waves exerted greater effects on the occurrences of turbidity currents flowing within the canyons of the SSCG. The near-seafloor intensities of contour currents and internal waves around the shelf edge weakened with sea level rising. Internal waves were responsible for triggering high-frequency turbidity currents to deposit the multiple thin-bedded turbidites at the slope canyon heads of the SSCG. The fining-upwards trend of the contourites during the period of 29.5–6.7 ka was mainly attributed to the decreasing sediment supply from the Pearl River with sea level rising, whereas the coarsening-upwards trend in the past 6.7 kyr was resulted from the circulation intensification, in response to the reopening of all straits around the SCS. This study provides a new insight into the significance of varying oceanographic processes for deep-water deposition in response to sea-level fluctuations on sediment-starved continental margin with wide shelf.