Pathways of carbon oxidation in continental margin sediments o central Chile

Rates and oxidative pathways of organic carbon mineralization were determined in sediments at six stations on the shelf and slope off Conception Bay at 36.5”s. The depth distribution of C oxidation rates was determined to 10 cm from accumulation of dissolved inorganic C in l-5-d incubations. Pathways of C oxidation were inferred from the depth distributions of the potential oxidants (0,, N03-, and oxides of Mn and Fe) and from directly determined rates of SOd2- 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,- and depleted of OZ. 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 pmol cmm3 d-l) and decreased exponentially with depth. The process was entirely coupled to SOd2-reduction. At the two slope stations where bottom-water O2 was> 100 PM, C oxidation rates were lo-fold lower and varied less with depth; C oxidation coupled to the reduction of 02, N03-, 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 SOd2- 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 SOd2- reduction increased