Development of In-Situ, Real-Time Raman Analysis of Clathrate Hydrates on the Sea Floor
Jill D Pasteris1, Peter G Brewer2, Sheri N White2, Edward T Peltzer III2, Brigitte Wopenka1, John J. Freeman1
1Dept. of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130
2MBARI, 7700 Sandholdt Rd., Moss Landing, CA 95039
We report on the successful development of a Raman spectrometer system that has been deployed by the Monterey Bay Aquarium Research Institute (MBARI) at ocean depths as great as 3600m. Raman spectroscopy is a technique that is well suited for measurements in aqueous environments, and we have used it to identify and characterize solid, liquid, and gas phases in the deep ocean. MBARI's Deep-Ocean Raman In-Situ Spectrometer (DORISS) system is encased in three separate pressure-resistant housings, connected fiber optically, and deployed by an ROV. There are two interchangeable optical configurations of the probe head, one with a several-millimeter and the other with a 10-cm working distance between sample and lens. The latter stand-off distance permits one to analyze a reaction or interaction without disturbing it. The shorter working-distance optic is a direct immersion probe. A major goal of the first several deployments of DORISS was to develop methods to spectroscopically monitor the formation of synthetic CO2 and natural CH4-dominated hydrates. We successfully have recorded the Raman spectra of liquid and gaseous CO2 delivered to the sea floor in Monterey Bay and CH4-rich fluids from gas-seeps in the Gulf of California. In both cases, the density-dependent shift in the Raman bands was recorded as a function of fluid pressure/ocean depth. Our Raman spectra of sea water show prominent sulfate and OH bands; the ratio of those bands might be used to monitor salt exclusion (brine formation) during hydrate formation and sea-water freshening during hydrate dissociation. Although CO2 and CH4 hydrates were detected visually via HDTV in several of the sea-floor experiments, we have not yet obtained a Raman spectrum of the hydrate. Our laboratory simulations indicate that the relatively small depth of focus of the probe optics, coupled with intense light-scattering from the fine-grained hydrate, severely limits the acquisition of the hydrate spectrum from thin films and microcrystals. In order to more precisely control the lens-to-sample distance and thereby acquire better spectra of translucent to opaque materials, MBARI is designing a motorized stage that can position/focus the probe head to within 0.1 mm.