Pathways of carbon oxidation in continental margin sediments off central Chile

Rates and oxidative pathways of organic carbon mineralization were determined in sediments at six stations on the shelf and slope off Concepcion Bay at 36.5 degrees S. The depth distribution of C oxidation rates was determined to 10 cm from accumulation of dissolved inorganic C in 1-5-d incubations. Pathways of C oxidation were inferred from the depth distributions of the potential oxidants (O2, NO3-, and oxides of Mn and Fe) and from directly determined rates of SO4(2-) reduction. The study area is characterized by intense seasonal upwelling, and during sampling in late summer the bottom water over the shelf was rich in NO3- and depleted of O2. Sediments at the four shelf stations were covered by mats of filamentous bacteria of the genera Thioploca and Beggiatoa. Carbon oxidation rates at these sites were extremely high near the sediment surface (>3 micromol cm-3 d-1) and decreased exponentially with depth. The process was entirely coupled to SO4(2-) reduction. At the two slope stations where bottom-water O2 was > 100 microM, C oxidation rates were 10-fold lower and varied less with depth; C oxidation coupled to the reduction of O2, NO3-, and Mn oxides combined to yield an estimated 15% of the total C oxidation between 0 and 10 cm. Carbon oxidation through Fe reduction contributed a further 12-29% of the depth-integrated rate, while the remainder of C oxidation was through SO4(2-) reduction. The depth distribution of Fe reduction agreed well with the distribution of poorly crystalline Fe oxides, and as this pool decreased with depth, the importance of SO4(2-) reduction increased. The results point to a general importance of Fe reduction in C oxidation in continental margin sediments. At the shelf stations, Fe reduction was mainly coupled to oxidation of reduced S. These sediments were generally H2S-free despite high SO4(2-) reduction rates, and precipitation of Fe sulfides dominated H2S scavenging during the incubations. A large NO3- pool was associated with the Thioploca, and the shelf sediments were thus enriched in NO3- relative to the bottom water, with maximum concentrations of 3 micromol cm-3. The NO3- was consumed during our sediment incubations, but no effects on either C or S cycles could be discerned.