Benthic fluxes and porewater concentration profiles of dissolved organic carbon in sediments from the North Carolina continental slope

Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment-water inte rface suggest that there may be a significant flux of organic carbon from s ediments to the water column. Furthermore, accumulation of DOC in the porew ater may influence the burial and preservation of organic matter by promoti ng geopolymerization and/or adsorption reactions. We measured DOC concentra tion profiles (for porewater collected by centrifugation and "sipping") and benthic fluxes (with in situ and shipboard chambers) at two sites on the N orth Carolina continental slope to better understand the controls on porewa ter DOC concentrations and quantify sediment-water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bi oturbation rates, organic carbon content, and mineral surface area) that al low us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a do wnslope transect (300-1000 m water depth) follow a trend consistent with ot her porewater constituents (Sigma CO2 and SO42-) and a tracer of modern, fi ne-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due t o differences in the depth of the active remineralization zone, which in tu rn is largely controlled by the intensity of bioturbation. Comparison of po rewater DOC concentrations, organic carbon burial efficiency, and organic m atter sorption suggest that DOC levels are nor a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic f luxes are difficult to detect, but suggest that only 2% of the dissolved or ganic carbon escapes remineralization in the sediments by transport across the sediment-water interface. Copyright (C) 1999 Elsevier Science Ltd.