DEVELOPMENT OF IN-SITU, REAL-TIME RAMAN ANALYSIS OF CLATHRATE HYDRATES ON THE SEA FLOOR

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.