Organic matter decomposition pathways and oxygen consumption in coastal marine sediments

In this study, we investigate the possible use of ∑CO2 production, determined from anoxic incubation experiments, and natural solute (O2, NH4+, SO4–) distributions to infer the pathways of organic matter decomposition in nearshore marine sediments. Integrated ∑CO2 production from anoxic incubation experiments agreed well with direct measurements of ∑CO2 fluxes across oxic sediment-water interfaces in Long Island Sound and Flax Pond salt marsh. This implies that anoxic incubations give reliable estimates of organic carbon remineralization rates. Therefore, anoxic ∑CO2 production, as a function of depth, was used to derive the relative roles of different electron acceptors in the organic matter decomposition process. These calculations indicate that the maximum contribution of O2 to total organic matter decomposition in the environments investigated ranges from 3–14%, whereas the minimum contribution of SO4– to total decomposition ranges from 65–85%. One-dimensional models of O2 distributions in sediments indicate that O2 reduction can be a significant component of organic matter decomposition (\u3e20%) only in benthic environments characterized by: (1) low total organic matter decomposition rates, (2) extremely rapid attenuation of organic matter decomposition with depth, and/or (3) very intense bioturbation. These results bring into question the generality of previous assertions that O2 accounts for 50% or more of total organic matter decomposition in reactive nearshore marine sediments