CARBON BUDGET FOR THE MID-SLOPE DEPOCENTER OF THE MIDDLE ATLANTIC BIGHT

A mass budget was constructed for organic carbon on the upper slope of the Middle Atlantic Bight, a region thought to serve as a depocenter for fine-grained material exported from the adjacent shelf. Various co mponents of the budget are internally consistent, and observed differe nces can be attributed to natural spatial variability or to the differ ent time scales over which measurements were made. The flux of organic carbon to the sediments in the core of the depocenter zone, al a wate r depth of similar to 1000 m, was measured with sediment traps to be s imilar to 65 mg C m(-2) day(-1), of which 6-24 mg C m(-2) day(-1) is b uried. Oxygen fluxes into the sediments, measured m with incubation ch ambers attached to a free vehicle lander, correspond to total carbon r emineralization rates of 49-70 mg C m(-2) day(-1). Carbon remineraliza tion rates estimated from gradients of C-org within the mixed layer, a nd from gradients of dissolved ammonia and phosphate in pore waters, s um to only similar to 4-6 mg C m(-2) day(-1). Most of the C-org remine ralization in slope sediments is mediated by bacteria and takes place within a few mm of the sediment-water interface. Most of the C-org dep osited on the upper slope sediments is supplied by lateral transport f rom other regions, but even if all of this material were derived from the adjacent shelf, it represents <2% of the mean annual shelf product ivity. This value is further lowered by recognizing that as much as ha lf of the C-org deposited on the slope is refractory, having originate d by reworking from older deposits, Refractory C-org arrives at the se a bed with an average C-14 age 600-900 years older than the pre-bomb C -14 age of DIC in seawater, and has a mean life in the sediments with respect to biological remineralization of at least 1000 years. Labile carbon supplied to the slope, on the other hand, is rapidly and (virtu ally) completely remineralized, with a mean life of <similar to 1 year . Carbon-14 ages of fine-grained carbonate and organic carbon present within the interstices of shelf sands are consistent with this materia l acting as a source for the old carbon supplied to the slope. Winnowi ng and export of reworked carbon may contribute to the often-described relationship between organic carbon preservation and accumulation rat e of marine sediments.