ACCUMULATION RATES AND CS-137 DISTRIBUTION IN SEDIMENTS OFF THE PO RIVER DELTA AND THE EMILIA-ROMAGNA COAST (NORTHWESTERN ADRIATIC SEA, ITALY)

Sediment accumulation rate studies were conducted using Pb-210 and Cs-137 with the aim of understanding recent and present sedimentation patterns on the Italian Adriatic continental shelf. Since there is no simple model governing the delivery of radionuclides to coastal sediments a variety of approaches were used to estimate dry mass accumulation rates. At many sites there was little divergence between the various estimates, and the mean value is taken to be a reliable estimate of the net sediment accumulation rate. At other sites greater variation was observed. Reasons for these variations can include both secular changes in accumulation rates, and interruptions to the process of sedimentation arising from, for example, sediment mixing, scouring and focussing. At these sites best estimates of accumulation rates are given on the basis of an assessment of the most appropriate dating technique. Highest mass accumulation rates are found near the Po river (up to 1.80 +/- 0.29 g cm-2 year-1), reflecting the large supply of sedimentary materials from this source. South of the delta accumulation rates were much lower, ranging from 0.22 +/- 0.06 to 0.39 +/- 0.06 g cm-2 year-1. At a number of sites the Cs-137 activity versus depth profile had a well defined peak (resulting from weapon fallout) which served to identify the depth corresponding to 1963, providing a valuable check on the Pb-210 derived accumulation rates. At other sites this peak was too broad or confused to be a valuable reference. The distributions of Pb-210 and Cs-137 reflect their different geochemical behaviour and sources, and provide information on the process of sediment accumulation.