Gas Hydrates in Convergent Margins: Formation, Occurrence, Geochemistry, and Global Significance

Abstract

Methane is the dominant gas (>99%) occupying marine gas hydrate cages, except for a high CO 2 (23%) occurrence in the Middle America Trench and a high H 2 S occurrence (∼10%) at the Cascadia Margin. Methane C isotope ratios indicate that most is biogenically derived; the amount of admixed thermogenic methane is as yet undetermined. Convergent margins provide a highly favorable environment for methane hydrate formation. The physical-chemical constraints on methane hydrate stability, together with the distinct tectonic, sedimentologic, and hydrologic characteristics of convergent margins, are responsible (1) for higher rates and amounts of formation of both disseminated and massive methane hydrate. Estimates of pore volume occupancies range from a few percent to >25%; (2) for increased methane hydrate concentrations at and near the rather strong amplitude BSRs, that are often discontinuous and patchy, and along fluid conduits, resulting in a spatially irregular distribution throughout the stability field; and (3) for a more widespread occurrence of near seafloor methane hydrate, intimately associated with massive authigenic carbonates, indicating intense upward migration of methane-rich fluids. The BSR may occur above or below the base of the equilibrium stability field. Occurrence above the predicted BSR depth suggests limited supply of methane, and occurrence below it indicates ample supply of methane that causes transient time-intervals of overpressure. Based on global areal distribution, tectonics, sedimentology, hydrology, and organic matter content and type, thus potential of methane yield, it is estimated that, exclusive of shelf and permafrost occurrences, about two-thirds (60-65%) of the vast marine gas hydrate reservoir is situated in convergent margins. This tectonic environment is, therefore, of particular importance for considerations of the role of methane hydrate in the oceanic C cycle, and of the potential responses of methane hydrate to environmental stressers. The methane hydrates near the seafloor and at the BSR are most vulnerable to dissociation upon environmental stresses. Catastrophic events of methane release from hydrate, for example by major submarine landslides, may profoundly influence seawater chemistry, i.e. cause oceanic anoxia and somewhat reduce the ocean's capacity of CO 2 incorporation. The non-oxidized methane will be released to the atmosphere and enhance global warming.