ESTABLISHING RATES OF SEDIMENT ACCUMULATION ON THE MEKONG SHELF: RADIOCHEMICAL APPROACHES BASED ON 210Pb AND 14C DISTRIBUTIONS

Thirty four kasten cores, collected from the Mekong continental shelf, have been analyzed for their excess ²¹⁰Pb 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 meters, and stations sampled topset, foreset, and bottomset beds (water depths 7-21 m). Apparent excess ²¹⁰Pb sediment accumulation rates ranged from >10 cm/y (no down-core decrease over 300 cm core length) near the Song Hau river mouth, to 1-3 cm/y in topset and foreset beds between 20 and 50 km of the river mouth, to rates as low as 0.5 cm/y in cores from bottomset beds. ¹⁴C measurements of the bulk organic carbon fraction also have been used to assess rates of sediment accumulation in 10 of these kasten cores. Although ¹⁴C enables longer timescales to be characterized than ²¹⁰Pb, the assumption of constant ¹⁴C_org age at the sediment-water interface is less robust than the assumption of constant surface excess ²¹⁰Pb activity because of variable organic carbon sources near the river mouth (i.e., old soils from land and young marine organic matter from offshore). The variable organic carbon sources can be resolved by measuring down-core delta ¹³C values or C/N ratios in the bulk organic carbon fraction. The radiochemical sediment accumulation rates are being assessed in the context of their clinoform setting as revealed by extensive CHIRP seismic profiles from the study area.