Development of a Laser Raman Spectrometer for Deep-Ocean Science

We have extensively modified and successfully used a laser Raman spectrometer (DORISS, Deep-Ocean Raman In Situ Spectrometer) for geochemical studies in the deep ocean. The initial instrument from Kaiser Optical, was separated into 3 components: an optical head, a laser-power supply-telemetry unit, and the spectrometer. These components were modified to fit into custom designed pressure housings, and connected by deep-sea cables and optical penetrators designed to minimize signal loss. The instrument ensemble has been field deployed on remotely operated vehicles (ROVs) for a variety of experiments and observations, with successful operation at 1.6°C, 3600m depth. Power supply, instrument control, and signal telemetry are provided through the ROV tether, which contains copper conductors and single mode optical fibers. The optical head is deployable by the ROV robotic arm for sample interrogation; the remaining components are fixed within the vehicle tool-sled. Challenges of system calibration at depth, temperature and pressure artifacts, and system control through over 4 km of cable were successfully overcome. We present exemplary spectra obtained in situ of gas, liquid, and solid specimens, and of the ubiquitous signal of sea water itself. Future challenges include weight and size reduction, and advances in precise beam positioning on mineral targets on the sea floor. INTRODUCTION Geochemical studies in the deep ocean have traditionally relied upon sample recovery by bottles, cores, and dredges deployed from surface ships, or collected by manned submersibles and remotely operated vehicles (ROVs), to provide specimens for ship or shore based analysis. And while this remains the principal technique, there is a compelling case to be made for advances in in situ detection and analysis. This has been greatly aided by the development and increasing use of deep-ocean submersibles and ROVs. These vehicles now provide sophisticated carrying platforms with the power, telemetry and data handling, and precise manipulation capabilities (Brewer et al., 1999) to support advanced geochemical measurement systems (Kleinberg et al., 2003). In this paper we describe the adaptation of a laser Raman spectrometer (LRS) for in situ measurements in the deep ocean, and its successful deployment from ROVs operated by the Monterey Bay Aquarium Research Institute (MBARI). In principle the system we describe here could be adapted for use on a variety of ROVs and research submersibles available worldwide. We present exemplary spectra of sea water itself, and of selected geochemical targets examined in a carefully constructed set of controlled field experiments during the development phase, and we briefly describe some important calibration protocols. The qualification of these spectra though companion laboratory studies, and detailed discussion of the spectra obtained, will be reported by Pasteris et al. (in preparation). A unique advantage of Raman spectroscopy for the field geochemist (Pasteris, 1998) lies in its ability to measure solid, liquid, and gas phases, thereby greatly extending the range of possible targets. Sensors typically used (Varney, 2000) for oceanic geochemical