The effect of sediment mixing on Pb-210 accumulation rates for the Washington continental shelf

Abstract

Nine cores from the Washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from Pb-210 profiles. Th-234 profiles indicate mixing coefficients of 140 cm 2 yr −1 for the seabed offshore from the Columbia River and 47 cm 2 yr −1 for the seabed of the Mid-Shelf Silt Deposit (75 km north of the Columbia River). These large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer (∼ 10 cm) within one year after emplacement at the seabed surface. Observed depths of Cs-137 penetration within the seabed are compared with depths predicted from the surface mixed-layer thickness and the Pb-210 accumulation rate. The observed and predicted depths agree well at all but one station. At this station a combination of Th-234, Co-60, Pb-210 and Cs-137 profiles suggests that active mixing occurs below the intensely mixed surface layer. The agreement of Pb-210 and Cs-137 data at the other stations, however, indicates a general absence of deep mixing on the Washington shelf. Thus, the apparent accumulation rates calculated from Pb-210 profiles (below the intensely mixed surface layer) on the Washington shelf generally reflect the true rates of sediment accumulation.