First Expeditionary Deployments of the Deep Ocean Raman In Situ Spectrometer

MBARI's in situ laser Raman spectrometer (DORISS -- Deep Ocean Raman In Situ Spectrometer) was deployed at gas vents in Guaymas Basin, Gulf of California in April 2003. The first in situ Raman spectra of natural gas venting from the seafloor were obtained. These spectra will be compared to GC analyses of samples collected at the same vent and Raman analyses of pure methane deployed as a standard for at-sea testing. Laser Raman spectroscopy is a powerful geochemical technique for analyzing the chemical composition and molecular structure of solids, liquids, and gases. The prototype deep-ocean Raman instrument is based on an off-the-shelf laboratory model from Kaiser Optical Systems, Inc. Engineering test deployments conducted over the past 1.5 years have proved that Raman spectra of high quality can be obtained in the deep ocean. We have acquired spectra from solid, liquid and gaseous standards deployed as deep as 3600 m in the ocean. We are currently in the next phase of development which consists of using DORISS for exploratory measurements of natural targets in the deep ocean. During an expedition to the Gulf of California on the R/V Western Flyer in April 2003, DORISS was deployed successfully by the ROV Tiburon at natural gas vents on the seafloor ( ∼1700 m depth). The laser probe head was focussed into a small volume of gas trapped in a sampling funnel. Spectra were collected of gas samples at the vent site and at depths above the hydrate stability zone. Bands for methane dominate the spectra. Attempts to collect spectra of natural gas hydrates at the vent were difficult due to the stringent positioning requirements of the probe head when analyzing solid, opaque targets. This problem has been addressed by the development of a precision underwater positioner that provides a stable platform for analyses and movement of the DORISS probe head with a precision of 0.1 mm.