Stable sulfur isotopes indicate net sulfate reduction in near-surface sediments of the deep Arabian Sea

Depth profiles of the stable sulfur isotopic composition of dissolved sulfa te in near-surface sediments were measured at five stations of the deep Ara bian Sea between 1918 and 4426 m water depth (WAST, WAST-Top, CAST, SAST, N AST), sampled in April 1997. The results clearly indicate that net microbia l sulfate reduction took place in the sediments at stations WAST and NAST b elow about 12 cm depth. Sulfate reduction at WAST was more pronounced compa red to station NAST, most likely due to higher organic carbon content in tu rbiditic sediments. No net sulfate reduction took place within the upper 10 cm of the surface sediments at all stations, and no significant isotopic i ndication for sulfate reduction was found down to 30 cm bsf at station SAST . Results are in accordance with accumulation of reduced isotopically light sulfur species below about 6 cm bsf at station WAST. It is concluded that the sulfur isotopic composition of remaining sulfate is more sensitive to n et sulfate reduction than the [SO4]/[Cl] ratio. The sulfur isotopic composi tion of a vertical profile for dissolved sulfate through the water column a t station WAST was essentially constant (250-4047 m: delta(34)S = + 20.49 /- 008 parts per thousand, vs. V-CDT n = 8). A similar constancy (20-4565 m water depth delta(34)S = + 20.57 +/- 0.06 parts per thousand vs. V-CDT n = 15) was found for the station BIOTRANS in the northeastern Atlantic (47 de grees 11' 19 degrees 33W), indicating that the oxygen minimum zone in the A rabian Sea has no influence on the sulfur isotopic composition of dissolved sulfate. (C) 2000 Elsevier Science Ltd. All rights reserved.