210Pb-Excess and Sediment Accumulation Rates at the Iberian Continental Margin

Sediments from the continental shelf, slope, and rise at the continental margin of northern Portugal and the adjacent Iberian abyssal basin were analysed for 210Pb, 226Ra, 137Cs and 14C. Pb-210 derived sedimentation rates at the continental shelf off the Portuguese coast were 0.2-0.6 cm/a. In some cores from fine sediment deposits at the outer shelf, the 210Pb excess continuum was interrupted and sediment layers were missing, suggesting that events such as sediment slides could have occurred. Higher sedimentation rates were determined in locations at the rise of the continental slope, confirming enhanced deposition by sediment slides. In the deeper Iberian Abyssal Basin, using the 14C age of sediment layers the sedimentation rate was determined at 3.2 cm/ka, thus four orders of magnitude lower than at the continental shelf. The spatial distribution of sedimentation rates determined by radionuclide based chronologies, suggested that fine sediments from river discharges are deposited mainly at the outer continental shelf. These deposits may became unstable with time and, occasionally, originate sediment slides that are drained by the canyons and reach the deep sea. The Iberian abyssal basin receives some advective contribution of these sediment slides and the sedimentation rate is one order of magnitude higher than in other abyssal basins of the NE Atlantic Ocean. (author)