Factors affecting dissolved organic matter dynamics in mixed-redox to anoxic coastal sediments

Mixed-redox (suboxic, or oscillating between oxidizing and reducing conditions) to anoxic marine sediments from the Raritan –New York Bay complex and the Inner New York Bight of the eastern U.S. were studied to investigate the factors controlling the accumulation of pore-water dissolved organic carbon (DOC). DOC increased with depth at each of four study sites, but accumulation was generally limited in the mixed-redox zone relative to the anoxic zone. Humic-like fluorescence intensity also differed between mixed-redox and anoxic zones of the sediment, such that anoxic pore waters were relatively enriched in fluorescent, humic-like compounds. A pore-water DOC model was tested for its capacity to explain these geochemical patterns. Model results for a heavily irrigated and a non-bioturbated site both suggest that, excluding the uppermost few cm of the sediment column, pore-water DOC is predominantly comprised of poorly-reactive material with estimated degradation rate constants on the order of 10 3 yr . Model results are also consistent with the suggestion that DOC accumulation is suppressed in the mixed-redox compared to the anoxic zones of the sediment due to rapid oxidation of high-molecular-weight DOC, and limited production and enhanced oxidation of the less reactive polymeric low-molecular-weight component of the DOC pool. An assessment of sorptive behavior of DOC in the surface sediments of the study area suggests that sorption can influence pore-water DOC distributions under conditions of high sedimentation and low bioirrigation. Copyright © 2004 Elsevier Ltd