International Conference on Gas Hydrates (ICGH) (6th : 2008)

DETECTION OF METHANE SOURCES ALONG THE CALIFORNIA CONTINENTAL MARGIN USING WATER COLUMN ANOMALIES Ussler III, William; Paull, Charles K.

Abstract

Water column methane measurements have been used to understand both the global distribution of methane in the oceans and the local flux of methane from geologic sources on the continental margins, including methane vents and gas-hydrate-bearing sites. We have measured methane concentrations in 1607 water samples collected along the central California continental margin. Methane supersaturation of the surface mixed layer (0-50 msbsl) is widespread and above a well-defined subsurface particle maximum (~50 mbsl) that generally corresponds with the pycnocline. Local production of methane appears to be occurring in the surface mixed layer above the particle maximum and may not be particle-associated. Methane concentrations in water column CTD cast profiles and ROV-collected bottom waters obtained in Partington, Hueneme, Santa Monica, and Redondo submarine canyons increase towards the seafloor and are distinctly higher (up to 186 nM) compared to open-slope and shelf waters at similar depths. These values are in excess of measured surface water methane concentrations and could not be generated by mixing with surface water. Elevated methane concentrations in these submarine canyons and persistent mid-water methane anomalies in Ascension and Ano Nuevo Canyons could result from restricted circulation and/or proximity to gas vents, seafloor exposure of methane gas hydrates, recently-eroded methane-rich sediment, submarine discharge of methane-rich groundwater, or particle-associated methane production. On the Santa Barbara shelf water column methane profiles near known gas vents also increase in concentration with increasing depth. Thus, elevated bottom water methane concentrations observed in submarine canyons may not be diagnostic of proximity to methane vents and may be caused by other processes.

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