Determination of pore-water dissolved organic carbon fluxes from Mexican margin sediments

Sediment dissolved organic carbon (DOC) fluxes were determined in the oxyge n minimum zone along the northwestern Mexican margin using five different m ethods: in situ benthic chambers, on-deck incubations, slicing, dialysis sa mpling (peepers), and sipping. For each of the five methods, replicates (n = 6-12) were made. Directly determined fluxes (whole-core incubations and b enthic chambers) and calculated fluxes (sliced and dialysis-sampled cores) agree well (0.41 +/- 0.09, 0.36 +/- 0.04, 0.25 +/- 0.05, and 0.25 +/- 0.05 mmol C m(-2) d(-1), respectively). On the Mexican margin, the DOC flux was 8% of the sedimentary carbon input, suggesting that it is a significant com ponent to the local carbon budget. Extrapolations of this flux to the total global margin suggest that shelf and slope sediments contribute 96 Tg C yr (-1). The residence time of oceanic DOC based on this flux is consistent wi th measurements of the deep-water (DOC)-C-14 age. Profiles were also constr ucted from sip-isolated pore waters and yield consistently lower DOC profil e gradients and DOC fluxes (0.06 +/- 0.02 mmol C m(-2) d(-1)). We propose t hat the consistently observed discrepancy between sip-isolated profiles and other isolation techniques is a result of sampling different reservoirs of pore water present in the heterogeneous sediment matrix.