PB-210 BALANCE AND IMPLICATIONS FOR PARTICLE-TRANSPORT ON THE CONTINENTAL-SHELF, US MIDDLE ATLANTIC BIGHT

Supply of (210)pb to the continental shelf off the northeastern United States is dominated by the deposition from the atmosphere, the rate o f which is reliably known from previously published work. Excess (210) pb inventories in the shelf sediments show accumulations that are near ly in balance with the supply, even in areas of relict sands where it is believed that no net accumulation of sediment presently occurs. The (210)pb distributions in shelf and slope water indicate that the two- way fluid exchange at the shelf-slope front and the net transport in t he alongshore Row make comparatively small contributions to the shelf (210)pb budget: The near balance between supply and decay of (210)pb o n the shelf implies a limit to the particle export flux. It is conclud ed that the export of particulate organic carbon does not exceed 60 g m(-2) y(-1) (similar to 25% of primary production) and is probably low er. The hypothesis is advanced that fine particulate matter introduced to the continental shelf is detained in its transit of the shelf beca use of bioturbational trapping in the sediment due to benthic animals. Distributions of (210)pb in suspended particulate matter and in the f ine fraction of shelf sediments suggest that the average fine particle must undergo several cycles of deposition-bioturbation-resuspension-r edeposition and requires a number of decades for its transit and ultim ate export from the shelf. Thus, only the most refractory organic matt er is likely to be exported.