Near-bottom particle flux in the abyssal northeast Atlantic

During a 17-month study at a site on the Porcupine Abyssal Plain of the northeast Atlantic (approx. 48 degrees N 20 degrees W), the downward flux of particulate material within and above the benthic nepheloid layer (BNL) was measured using sediment traps 1455 m above bottom (mab) (3100m depth) and 90 mab (4465 m depth). Flux at 90 mab is usually higher than the primary flux at 3100m depth, and this enhancement is especially pronounced during the winter. The additional material found in the near-bottom trap comprises recently deposited resuspended material (rebound flux), but with an admixture of refractory sediment. It is unlikely that scavenging of either BNL particles or dissolved material contributes greatly to the near-bottom flux. Fluxes of metal tracers ((232)Th and Al) and cyanobacteria into traps were used to examine the process of resuspension. The ratio of tracer flux at 90 mab to that at 3100m depth was taken as a measure of the strength of the resuspension process (the resuspension factor RF) and reflects clearly the enhanced resuspension in winter. This seasonal variation appears to be related to both the magnitude of near-bottom currents and to the wave height at the surface 40 days before. It is also possible that recently deposited material forms a partly cohesive blanket on the sediment surface that restricts resuspension to the benthic boundary layer. (C) 2000 Elsevier Science Ltd. All rights reserved.