On the fate of past gas: What happens to methane released from a bacterially mediated gas hydrate capacitor?

Abstract

Gas hydrates and associated free gas in marine sediment may constitute a large capacitor of CH 4 in the global carbon cycle with outputs controlled by water temperature at intermediate depths of the ocean. The best support for this concept previously has come from stable isotope records of benthic foraminifera that show pronounced negative δ 13 C excursions during certain brief intervals of deep to intermediate water warming. New work by Hinrichs [2001] demonstrates that such isotope excursions in the Santa Barbara Basin coincide with high accumulations of diplopterol, a biomarker for aerobic CH 4 oxidation by bacteria. This is the first direct evidence for enhanced CH 4 concentrations in the deep ocean during or immediately after bottom water warming and injection of 12 C‐rich carbon. However, the formation of biomarkers indicates that a fraction of CH 4 released during warming is oxidized to biomass and ΣCO 2 in the water column. In contrast to most literature, the primary consequences of CH 4 release from dissociated gas hydrate may be dissolved O 2 depletion and carbonate dissolution rather than atmospheric warming.