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

Sediment dissolved organic carbon (DOC) fluxes were determined in the oxygen minimum zone along the northwestern Mexican margin using five different methods: in situ benthic chambers, on-deck incubations, slicing, dialysis sampling (peepers), and sipping. For each of the five methods, replicates (n = 6-12) were made. Directly determined fluxes (whole-core incubations and benthic 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 component 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 with measurements of the deep-water (DOC)-C-14 age. Profiles were also constructed from sip-isolated pore waters and yield consistently lower DOC profile gradients and DOC fluxes (0.06 +/- 0.02 mmol C m(-2) d(-1)). We propose that 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.