methanogenesis in the sulfate - reducing zone of surface sediments traversing the 1

10 We studied the concurrence of methanogenesis and sulfate reduction in surface sediments (0-25 cm 11 below seafloor) at six stations (70, 145, 253, 407, 990 and 1024 m) along the Peruvian margin (12°S). 12 This oceanographic region is characterized by high carbon export to the seafloor creating an extensive 13 oxygen minimum zone (OMZ) on the shelf, both factors that could favor surface methanogenesis. 14 Sediments sampled along the depth transect traversed areas of anoxic and oxic conditions in the 15 bottom-near water. Net methane production (batch incubations) and sulfate reduction (S-sulfate 16 radiotracer incubation) were determined in the upper 0-25 cmbsf of multiple cores from all stations, 17 while deep hydrogenotrophic methanogenesis (> 30 cmbsf, C-bicarbonate radiotracer incubation) 18 was determined in two gravity cores at selected sites (78 and 407 m). Furthermore, stimulation 19 (methanol addition) and inhibition (molybdate addition) experiments were carried out to investigate 20 the relationship between sulfate reduction and methanogenesis. 21 Highest rates of methanogenesis and sulfate reduction in the surface sediments, integrated over 0-25 22 cmbsf, were observed on the shelf (70-253 m, 0.06-0.1 mmol m d and 0.5-4.7 mmol m d, 23 respectively), while lowest rates were discovered at the deepest site (1024 m, 0.03 and 0.2 mmol m d 24 , respectively). The addition of methanol resulted in significantly higher surface methanogenesis 25 activity, suggesting that the process was mostly based on non-competitive substrates, i.e., substrates 26 not used by sulfate reducers. In the deeper sediment horizons, where competition was probably 27