In Situ Raman analyses of natural gas and gas hydrates at Hydrate, Ridge, Oregon
Edward T Peltzer III1, Sheri N White1, Rachel, M. Dunk1, Peter G Brewer1, Alana D. Sherman1, Kristen Schmidt1, Keith C. Hester2, and E. Dendy Sloan2
1MBARI, 7700 Sandholdt Rd., Moss Landing, CA 95039
2Colorado School of Mines, Golden, CO 08401
During a July 2004 cruise to Hydrate Ridge, Oregon, MBARI’s sea-going laser Raman spectrometer was used to obtain in situ Raman spectra of natural gas hydrates and natural gas venting from the seafloor. This was the first in situ analysis of gas hydrates on the seafloor. The hydrate spectra were compared to laboratory analyses performed at the Center for Hydrate Research, Colorado School of Mines. The natural gas spectra were compared to MBARI gas chromatography (GC) analyses of gas samples collected at the same site.
DORISS (Deep Ocean Raman In Situ Spectrometer) is a laboratory model laser Raman spectrometer from Kaiser Optical Systems, Inc modified at MBARI for deployment in the deep ocean. It has been successfully deployed to depths as great as 3600 m. Different sampling optics provide flexibility in adapting the instrument to a particular target of interest. An immersion optic was used to analyze natural gas venting from the seafloor at South Hydrate Ridge (~780 m depth). An open-bottomed cube was placed over the vent to collect the gas.The immersion optic penetrated the side of the cube as did a small heater used to dissociate any hydrate formed during sample collection. To analyze solid hydrates at both South and North Hydrate Ridge (~590 m depth), chunks of hydrate were excavated from the seafloor and collected in a glass cylinder with a mesh top. A stand-off optic was used to analyze the hydrate inside the cylinder. Due to the partial opacity of the hydrate and the small focal volume of the sampling optic, a precision underwater positioner (PUP) was used to focus the laser spot onto the hydrate. PUP is a stand-alone system with three degrees-of-freedom, capable of moving the DORISS probe head with a precision of 0.1 mm.
In situ Raman analyses of the gas indicate that it is primarily methane. This is verified by GC analyses of samples collected from the same site. Other minor constituents (such as CO2 and higher hydrocarbons) are present but may be in concentrations too low to be detected by the current DORISS instrument. In situ analyses of the hydrates show them to be structure I hydrates with methane as the primary guest molecule; the data compare well to laboratory data.