TORRES, M. E., COAS, Oregon State University; R. W. COLLIER, COAS, Oregon State University; M. A. DeANGELIS, Humboldt State University; J. McMANUS, COAS, Oregon State University, G. REHDER, Geomar, Germany; and E. SUESS, Geomar, Germany

Abstract: Isotopic Evidence for Methane Oxidation of Hydrate Methane in Water Samples Overlying Hydrate Ridge, Cascadia

Hydrate Ridge, in the Cascadia margin, is covered by extensive deposits of methane hydrate. Decomposition of this hydrate at the northern end of the Ridge results in a methane plume hundreds of meters high and several kilometres wide, with methane concentrations as high as 50,000 nl/l. The magnitude of this methane injection is comparable to or larger than that over oceanic spreading centers. The methane profiles suggest the presence of other methane seeps at shallower water depths on the margin.

A single methane profile shows near surface concentrations that are significantly higher than saturation. Comparison of the methane concentrations observed over Hydrate Ridge with methane levels in other highly productive areas suggests the possibility that methane originating at the seafloor -- either on Hydrate Ridge or at other localities on the shelf and slope -- may reach the ocean surface.

Methane oxidation produces isotopic anomalies on the d¹³C composition of the dissolved inorganic carbon pool; a result that has significant implications in paleo-oceanographic reconstructions. Based on mass balance calculations we estimate that 0.5% of the total dissolved CO2 in the bottom water (approximately 11 µmol/l) must result from methane oxidation. This estimate implies that a large fraction of the methane discharged at the seafloor is oxidized to inorganic carbon. However, methane oxidation rates measured in the methane plume, do not show levels above ambient deep-water values; suggesting that methane oxidation is occurring at highly localized sites at the seafloor.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California