THE FLUX OF METHANE FROM HYDRATES AND SEEPS TO THE NE PACIFIC OCEAN OFF OREGON

As part of the TECFLUX project, measurements of methane (CH₄) concentrations and its stable carbon isotope signature, d ¹³C-CH₄, were carried out on over 130 hydrocasts to constrain the spatial and temporal distributions, the fluxes, and the fate of CH₄ in the water column above the accretionary wedge off the Oregon coast (1998-2000). Hydrate Ridge and other units of the Cascadia Accretionary Prism, are known for fluid and gas venting driven by tectonic compression, which leads to gas hydrate deposits near the seafloor and free methane gas release to the water column.

Venting of free methane gas with light, biogenic isotopic signatures d ¹³C-CH₄ was found at the northern summit (590m) and southern summit (790 m) of Hydrate Ridge, producing thin layers of CH₄ maxima as high as 4400 nmol/L several tens of meters above the ocean floor. The methane distribution below 480m is primarily controlled by fluctuating gas discharge at the summit vent sites and by the highly variable currents in this coastal transition zone. The methane concentration and isotopes in this region can largely be modeled by mixing of three endmembers: the isotopically light biogenic source at Hydrate Ridge (—66‰), a shallower heavy source which dominates towards the slope (—28‰), and a low-methane background component. The shallow source appears to be derived from thermogenic gas seeps on the shelf/slope such as Heceta Bank to the southeast of Hydrate Ridge (see Embley et al., this session). Given the wide range in source compositions and the complex hydrography in the study area, it was not possible to resolve significant in situ oxidation of methane derived from Hydrate Ridge.

The methane flux from the working area (1230 km²) was estimated by combining the 'clearance time' of the area, calculated from observed mean water transports, with the total integrated excess methane. The resulting CH₄ flux for hydrate-related sources from Southern and Northern Hydrate Ridge are on the order of 3*10³ mol/h each, which is equivalent to the flux estimated by direct submersible observations of gas expulsion [Torres et al., this session]. The total flux from the working area, dominated by the shallower thermogenic sources, was approximately ten times larger than the flux from Northern Hydrate Ridge.