SUMMARY

Anaerobic methane oxidation in Blake Ridge sediments overlying methane hydrates was characterized by three independent techniques. Pore-water methane concentration profiles from Hole 995B exhibit a region of upward concavity suggestive of methane consumption. Diagenetic modeling of the concentration profile indicates a 1.85-m-thick zone of AMO centered at 21.22 mbsf, with peak rate of 12.4 nM·d^-1. The thickness and locality of the model-predicted AMO zone are corroborated by a subsurface maximum in sulfate reduction rates. Depth-integrated rates of methane oxidation and sulfate reduction in the model-predicted AMO zone compare favorably, suggesting that nearly all of the sulfate reduction in the subsurface maximum is fueled by AMO. Methane stable carbon isotope chemistry provides further evidence of AMO. Methane ¹³C values are lightest at the point of peak model-predicted methane oxidation and become increasingly ¹³C enriched with decreasing sediment depth, consistent with kinetic isotope fractionation during methane oxidation. The isotopic evidence suggests the deepest occurrence of methane oxidation is about 60 cm shallower than that predicted by the model. The contribution of methane oxidation to the overall sulfate cycle in these sediments is difficult to assess; however, it is clear that the process represents an essentially quantitative sink for upwardly diffusing methane.