Determining rates of sediment accumulation on the Mekong shelf: Timescales, steady-state assumptions, and radiochemical tracers

Thirty-two kasten cores, collected from the proximal Mekong continental shelf, have been analyzed for their excess Pb-210 distributions in an effort to establish rates of sediment accumulation over the past 100 years. The length of the cores varied from 0.5 to 3 m, and stations sampled topset, foreset, and bottomset beds (water depths 7-21 m). Apparent excess Pb-210 sediment accumulation rates ranged from > 10 cm/y (no down-core decrease of excess activity over 300 cm core length) near the Song Hau river mouth, to 1-3 cm/y in topset and foreset beds within 20-50 km of the river mouth, to rates as low as 0.4 cm/y in cores from bottomset beds. The Pb-210 sediment accumulation rates yield an overall sediment burial rate of 6.1 x 10(13) g/y for the proximal deltaic deposits, which corresponds to 43% of the total modern Mekong sediment burial on the southern Vietnam shelf (1.4 x 10(14) g/y; based on our Pb-210 and seismic data and Pb-210 data from the literature). This shelf burial rate is in reasonable agreement with current long-term estimates of Mekong River sediment discharge (1.3-1.6 x 10(14) g/y) from the literature. The inventory of excess Pb-210 in the proximal Mekong deltaic deposits indicates that the shoreward flow of offshore water (entrained during river/ocean mixing) is approximately twice the flow of the Mekong freshwater discharge. Organic-carbon C-14 ages were measured on 10 cores from the proximal Mekong delta and compared to Pb-210 sediment accumulation rates in the same core. The Pb-210 accumulation rates in all 10 cores were considered to be more robust and accurate than the C-14 geochronologies, primarily because of down-core variations in the source of organic carbon deposited on the seafloor (old terrestrial carbon versus younger marine carbon). Variations in the source of organic carbon accumulating in the seabed were resolved by measuring the delta C-13 value of the seabed organic carbon.