Where has all the methane gone?

Tori Hoehler, Ph.D.
University of North Carolina at Chapel Hill

Thursday, May 28, 1998
3:30 p.m.—Pacific Forum

Gas-hydrate-bearing and other anoxic sediments around the world contain an enormous reservoir of methane. Release of this gas from the sediments could represent a significant term in the atmospheric methane budget, and hence an important contribution to the greenhouse effect and to the oxidizing capacity of the atmosphere. However, in marine sediments, upwardly diffusing methane is typically consumed before reaching the sediment-water interface, eliminating this potentially important term from the global methane budget. While the occurrence of this "anaerobic-methane oxidation" (AMO) is documented in a range of environments by a wealth of geochemical evidence, a mechanism-level understanding of the process remains elusive.

In this study, AMO was characterized in sediments overlying the Blake Ridge gas hydrate field through a combination of radio-tracer, stable isotope, and modeling techniques. The process was found to occur over a larger depth range and at orders-of-magnitude slower rates than has typically been observed, but still represents a quantitative methane sink. The mechanism of AMO was studied in a near-shore, organic-rich sediment where the process exhibits a seasonality that appears to relate to changes in temperature and sulfate availability. Variations in these parameters affect H₂ concentrations, which in turn form a thermodynamic basis for controlling the occurrence of methane oxidation. Laboratory experiments are consistent with the hypothesis that methanogenic bacteria oxidize methane and generate H₂ in the process. Efficient scavenging of this H₂ by sulfate-reducing bacteria makes the reaction thermodynamically favorable and allows net methane oxidation to take place.

Next: The physical oceanography of Georges Bank