Modern sediments and sediment accumulation rates on the narrow shelf off central Vietnam, South China Sea

摘要

The narrow shelf along the coast of central Vietnam is seasonally supplied by large amounts of sediment from the adjacent mountainous hinterland following monsoonal precipitation. This study examines the fate of these sediments, and their accumulation rates along two transects across the shelf, based on analyses of radionuclides (Pb-210, Cs-137), sediment texture and structure, as well as carbonate content. The inner shelf is covered by sands, and probably serves as bypass zone for fine sediments transported offshore. Sediment characteristics suggest that the transport to the mid and outer shelf is related to flood events. Averaged over the last century, the Pb-210-based mud mass accumulation rates on the mid and outer shelf vary between 0.25 g cm (-2) and 0.56 g cm (-2) year (-1) (corresponding to linear sediment accumulation rates of 0.20-0.47 cm year (-1)). Along with high excess Pb-210 inventories, these high accumulation rates suggest a significant sediment depocentre on the mid shelf. The Pb-210-derived sediment accumulation rates were found to be several times higher than C-14-derived rates previously reported for the Holocene, at the same location on the outer shelf. This is probably due to the incompleteness of the Holocene record, and an overestimation of the modern rate. Another explanation would be increased erosion within the rivers' drainage basins, due to 20th century deforestation. This hypothesis is supported by the difference between recent (less sand, more lithic grains in the sand fraction) and older sediments. In terms of modern sedimentation processes and rates, the central Vietnam shelf, although being a part of a narrow passive continental margin, is similar to active flood-dominated continental margins.