Organoclastic sulfate reduction in the sulfate-methane transition of marine sediments

Sulfate and methane diffuse vertically along opposed concentration gradients into the sulfate-methane transition in subsurface marine sediments where anaerobic oxidation of methane with sulfate takes place. The stoichiometry of this process is 1:1, yet the calculated sulfate flux often exceeds the calculated methane flux. Our aim was to determine whether organoclastic sulfate reduction, fueled by the oxidation of buried organic matter, explains this excess sulfate flux into the sulfate-methane transition. We analyzed data for sulfate, methane and sulfate reduction from eight sites in Danish coastal waters. Fluxes of sulfate and methane were determined by diffusion-reaction modeling whereas sulfate reduction rates were determined by 35S radiotracer experiments at high depth resolution. The sulfate reduction data showed that the organic carbon mineralization rates followed a log-log linear depth trend throughout the sulfate zone. Extrapolation of this power law trend down into the sulfate-methane transition showed organic carbon oxidation corresponded to 14–59% of the total sulfate flux into the zone and in average explained 82% of the excess sulfate consumption above the 1:1 stoichiometry. A part of the excess sulfate was consumed directly by organoclastic sulfate reduction while a part drove a cryptic methane cycle by which the organic matter was degraded to methane and the methane concurrently oxidized to CO2 with sulfate. We extrapolated the same power law trend down through the methanogenic zone and found that this provided a good estimate of total methanogenesis in the sediment column when compared to the upward flux of methane.