Hydrocarbons Associated with Fluid Venting Process in Monterey Bay, California

Cold Seep Hydrocarbon Gases

Chemosynthetic 'cold seep' communities occur at a1,000 m site interpreted as the surface expression of a mud volcano on Smooth Ridge near Monterey Canyon. In order to examine the gas composition of these seeps, sediment samples were collected within and nearby the clam fields using push cores from the Remotely Operated Vehicle (ROV) Ventana. Upon retrieval, the cores (maximum length, 30 cm) were subsampled for measurements of a variety of chemical parameters including compositions of hydrocarbon gases.

In a collection of 20 samples, methane concentrations range from 1.4 to 7,000 micromol/L; carbon isotopic compositions of the methane range from -71.0 to -86.6 per mil with two exceptions of -53.8 and -30.6 per mil. These results suggest that most of the methane is microbial in origin; the isotopically heavier methane may represent a thermogenic source or oxidation of the microbial methane. Minor concentrations of other hydrocarbon gases (ethane through butanes) accompany the methane. Colonies of clams mark the locations on the seafloor where gas apparently vents through small conduits.

The small amount of putative thermogenic methane which was found adjacent to the clam colonies implies that the expulsion of thermally generated hydrocarbons from depth is not the principal driving mechanism for flow in this region; however, microbial production of methane resulting in reduced fluid density could be a factor in facilitating fluid flow at these cold seeps.