Fluxes of particulate matter on the slope of the southern Middle Atlantic Bight: SEEP-II

Abstract

An array of 17 sediment traps was moored in two cross-isobath transects during the SEEP-II experiment, one north of the Washington Canyon, and the other south of the Norfolk Canyon off the Delmarva Peninsula in the southern Middle Atlantic Bight. The overall duration of the experiment was 15 months, during which the moorings were deployed three times, and the traps collected the vertical flux of particles during a total of 30 sampling intervals of average duration 12.8 days. The 483 particulate samples successfully recovered were each analyzed for total mass flux and the abundance of major biogenic phases (organic carbon, nitrogen, calcium carbonate and opal) and for 210Pb. A smaller number were also analyzed for 234Th. The distribution of fluxes in space requires transport of particles both downslope from the shelf-slope break and out into the water column, and alongslope transport. The region of maximum in particulate-matter flux to the slope, the depocenter, is from 400 to 1000 m, and the position of this maximum coincides with the maximum in alongslope current activity as expressed by the average total kinetic energy density. The total mass flux and the flux of all components to the slope depocenter increases between the northern Middle Atlantic Bight region, studied during the SEEP-I experiment, and SEEP-II sites in the southern Middle Atlantic Bight by about a factor of three to four. The temporal variability of the fluxes measured is dominated by storm-driven resuspension events on the shelf and shelf-slope break, but this extreme, short-term variability is not seen in the depocenter traps, the fluxes to which are very constant in magnitude and in composition. Canyons are suggested to serve both as conduits for transport of sediment from the shelf to the slope, and as a reservoir that feeds shelf-derived particles to the slope depocenter on a regular basis, even when sediment resuspension on the shelf is subdued or absent.