PATHWAYS OF CARBON OXIDATION IN CONTINENTAL-MARGIN SEDIMENTS OFF CENTRAL CHILE

Rates and oxidative pathways of organic carbon mineralization were det ermined in sediments at six stations on the shelf and slope off Concep cion Bay at 36.5 degrees S. The depth distribution of C oxidation rate s was determined to 10 cm from accumulation of dissolved inorganic C i n 1-5-d incubations. Pathways of C oxidation were inferred from the de pth distributions of the potential oxidants (O-2, NO3-, and oxides of Mn and Fe) and from directly determined rates of SO42- 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 NO 3- and depleted of O-2. Sediments at the four shelf stations were cove red by mats of filamentous bacteria of the genera Thioploca and Beggia toa. Carbon oxidation rates at these sites were extremely high near th e sediment surface (> 3 mu mol cm(-3) d(-1)) and decreased exponential ly with depth. The process was entirely coupled to SO42- reduction. At the two slope stations where bottom-water O-2 was > 100 mu M, C oxida tion rates were 10-fold lower and varied less with depth; C oxidation coupled to the reduction of O-2, 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 de pth-integrated rate, while the remainder of C oxidation was through SO 42- reduction. The depth distribution of Fe reduction agreed well with the distribution of poorly crystalline Fe oxides, and as this pool de creased with depth, the importance of SO42- reduction increased. The r esults 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 genera lly H2S-free despite high SO42- reduction rates, and precipitation of Fe sulfides dominated H2S scavenging during the incubations. A large N O3- pool was associated with the Thioploca, and the shelf sediments we re thus enriched in NO3- relative to the bottom water, with maximum co ncentrations of 3 mu mol cm(-3). The NO3- was consumed during our sedi ment incubations, but no effects on either C or S cycles could be disc erned.