Fluxes of particles and constituents to the eastern United States continental slope and rise: SEEP—I

Seventeen time-series sediment traps were deployed for a year from September 1983 to October 1984 along 70°55′W longitude in a two-dimensional array in water depths from 500 m on the upper continental slope to 2750 m on the upper continental rise as part of the SEEP—I (Shelf Edge Exchange Processes) experiment. Fluxes of total particulate mass, organic carbon and nitrogen, calcium carbonate and 210Pb were measured in the almost 300 samples that were recovered. Based on 210Pb inventories in the slope sediments, the total mass fluxes measured in the traps near the bottom of the slope water column are representative of the longer-term (∼100 y) accumulation rates of slope sediments. In addition to the vertical flux of particles coming from near-surface waters of the slope, both fresh biogenic particulate matter and resuspended sediment derived from the shelf and upper slope are transported downslope and added to the burden of particles settling vertically to the slope and upper rise sediments. The fluxes and composition of the trapped particles vary seasonally as a result of effects resulting from the spring bloom in plant biomass on the shelf, and as a result of the succession of the spring-bloom effects by carbonate-secreting organisms in the late summer and autumn. On a much shorter time scale variations in fluxes and composition are caused by more local biological events and by events that resuspend and transport sediment from the shelf across the shelf-break and down the slope. The scavenging of particle-reactive radionuclides (210Pb and 234Th) and, by analogy, particle-reactive pollutants, is closely tied to the flux of fresh biogenic debris. Comparison of the fluxes of organic carbon on the slope with the estimates of primary productivity over the shelf, indicates that <10% of the biogenic carbon fixed on the shelf is exported to the slope in the SEEP—I area. Comparison of the fluxes of 210Pb in the slope traps with the known atmospheric input to the shelf and slope surface waters implies that of the order of 20% of both biogenic and abiogenic particles from the shelf are exported to the slope on a time scale commensurate with the half life of210Pb. Comparison of the SEEP—I organic carbon and 210Pb fluxes to the slope at 1200 m with those fluxes ∼250km to the south at 975 m suggests that the shelf-to-slope export may increase by as much as three-fold for organic carbon particles and by two-fold for biogenic and abiogenic particles with which 210Pb is associated.