Stable carbon isotopic evidence for methane oxidation in plumes above Hydrate Ridge, Cascadia Oregon Margin

Abstract

The transport and consumption of methane in the water column in the vicinity of the cold seeps of Hydrate Ridge on the Cascadia Oregon Margin were characterized using measurements of the stable carbon isotope composition of methane. The δ13C‐CH4 values measured in the water column ranged from approximately −65 to −16‰, PDB. The combination of measured methane concentration data and the stable carbon isotope values from the same depths support the hypothesis of biogenically produced methane which enters the water column from dissolving bubbles released from cold seepages, likely as a consequence of destabilized methane hydrate (δ13C‐CH4 = −65‰, PDB). Kinetic fractionation factors, α, associated with aerobic bacterial methane oxidation in the water column were calculated using a Rayleigh distillation equation applied to a subset of the data. Fractionation factors ranged from 1.002 to 1.013 (mean = 1.008) and were in the lower end of the range of those reported in the literature, a result likely due to the influence of temperature and mixing in plume waters. The fraction of methane remaining after oxidation calculated using the same Rayleigh model approach suggests that the aerobic oxidation of methane in the water column over Hydrate Ridge is nearly quantitative.