Sediment discharge and storage over the last deglacial highstand period on the central Vietnam shelf off Nha Trang

Abstract

A sediment budget for the central Vietnam shelf off Nha Trang over the last deglacial Holocene highstand period has been investigated on the basis of shallow seismic and sediment core data and empirical equations. The annual suspended sediment discharge to the Nha Trang shelf ranges from: 4.3 to 5.4 Mt/year. Estimates based on published empirical equations suggest that the sediment discharge by three main local mountainous rivers (the Cai, Dinh and Van Phong rivers) that enter the Nha Trang shelf ranges between 1.7 and 4 Mt/year, which implies that the local rivers discharge approximately 75% of the total annual sediment input to the shelf. The annual sediment supply of the Cai River is approximately 2 and 6 times higher than that of the Dinh and Van Phong rivers, respectively. The highstand sediment depocentre of the Nha Trang shelf is mostly attached to the local river outflows, indicating their importance as the principal sediment supply sources to the shelf. Additional sources of sediment supply to the Nha Trang shelf can probably be related to along-shore transport from the nearby shelves. Calculations based on seismic and sediment core data indicate that the net sediment volume storage on the Nha Trang shelf is approximately 2.15 Mt/year. Approximately 50% of the total sediment yield supplied to the shelf is probably transported along-shore to the south. The sediment budget model for highstand deposits on the Nha Trang shelf is typical for a small mountainous river basin, which is significantly different from that of the large river delta systems in Vietnam such as the Mekong and Red rivers where 90% of the river sediments are captured on the delta plain/subaqueous part and only 10% of the river sediments are transported to the nearby shelf. In contrast, most of the sediments supplied by small mountainous rivers off Nha Trang are transported to the mid-shelf, forming a shore-parallel mud depocentre.