210Pb activities in and fluxes to sediments of the Washington continental slope and shelf

Abstract

Surface sediments of the Washington coast have 210Pb activites which average 104 ± 48dpm/g for submarine canyon and slope regimes and 18 ± 12dpm/g for the continental shelf regime. 210Pb sedimentary fluxes are also higher in canyons, averaging 18 ± 13dpm/cm 2 per yr, compared to 5.2 ± 3.1 dpm/cm 2 per yr for slope and 4.8 ± 1.8dpm/cm 2 per year for shelf regions. These 210Pb activities and fluxes are 2–7 times greater than those reported for other coastal regions. Inputs from the atmosphere and the Columbia River are not sufficient to supply the 210Pb, but advection of seawater containing dissolved 210Pb produced in situ from 226Ra provides an input several times larger than the sedimentary fluxes. The sedimentary 210Pb flux is limited by scavenging reactions rather than by supply of dissolved 210Pb. Calculations of maximum biological uptake and fluxes of 210Pb and ‘selective’ chemical leaching experiments all show that the primary scavenging processes are due to hydrous Mn and Fe oxides rather than biological phases. The pattern of higher 210Pb depositional fluxes in canyons than in nearby open slope areas of comparable water depth is most reasonably explained by enhanced scavenging of dissolved 210Pb near the sea floor, rather than by processes operating throughout the water column. Relatively rapid removal of dissolved 210Pb from the near bottom nepheloid layer to slope and canyon sediments is shown by its mean residence time of less than two years in this layer.