Carbon isotopic evidence for coupled sulfate reduction methane oxidation in Amazon Fan sediments

Carbon isotope ratios of dissolved inorganic carbon were measured on porewaters from Amazon Fan sediments collected on Ocean Drilling Program Leg 155. Values as low as -49.6 parts per thousand vs. PDB were recorded, with the lowest values coinciding with the I,ase of the sulfate reduction zone at each site. The values indicate that on the order of 50-85% of sulfate reduction in Amazon Fan sediments takes place by methane oxidation. Estimated rates of methane oxidation range from 0.3-1.0 x 10(-5) moles/l/yr. The reasons for the high percentage of sulfate reduction taking place by methane oxidation are thought to be the large decrease in fan sedimentation rates in the early Holocene and the terrestrial source of organic matter in fan sediments. Copyright (C) 1998 Elsevier Science Ltd.