Vertical and temporal variability of redox zonation in the water column of the Cariaco Basin:: implications for organic carbon oxidation pathways

Temporal and vertical variability in the concentrations of major electron acceptors and donors (O-2, NO3-, particulate Mn and Fe, dissolved Mn2+ and Fe2+, and HS-) were investigated in the water column of the Cariaco Basin between 1995 and 1999. Vertical profiles of the redox species during upwelling and non-upwelling periods reflected the zonation of aerobic respiration, denitrification, metal reduction, and sulfate reduction in the water column and varied widely over time scales of months to years. Comparisons of these profiles with distributions of organic carbon production and acetate uptake suggest that the rate of organic carbon supply was the major control on the variability. Inhibition and enrichment experiments suggested that sulfate reduction was not a dominant pathway for organic carbon oxidation near the oxic/anoxic interface. Instead, the reduction of metal oxides, elemental sulfur, and/or thiosulfate is likely to be more important. Estimates of vertical fluxes showed that vertical diffusive transport of the major electron acceptors (oxygen, nitrate, and metal oxides) only accounted for less than 10% of the oxidation of organic carbon produced near the suboxic/anoxic transition zone. Thus, horizontal oxic mid-water intrusions coupled with strong internal redox recycling of the major redox species occur to supply additional electron acceptors for organic carbon oxidation near the redox transition zone. (C) 2004 Elsevier B.V. All rights reserved.