Laser Raman Spectroscopy Used to Study
the Ocean at 3600m Depth

P.G. Brewer,¹ J. Pasteris,² G. Malby,¹ E.T. Peltzer,¹ S. White,¹
J. Freeman,² B. Wopenka,² M. Brown¹ and D. Cline¹

¹: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 USA
²: Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, St. Louis, MO 63132-4899

Eos, Transactions, American Geophysical Union (2002) 83(42): 469-470.

Received: 2002 September 30.
Published: 2002 October 15.


ABSTRACT

Making geochemical measurements in the deep ocean is fundamentally difficult. For this reason, century-old technologies using water bottles and cores for sample recovery still provide the basic tools. With the development of research submersibles and remotely operated vehicles (ROVs), however, new opportunities for sophisticated sampling and analysis have arisen. A laser Raman spectrometer has been deployed for the first time in situ to study deep ocean science. Many laboratory results of laser-excited Raman spectroscopy are applicable to deep ocean research. The identification and characterization of minerals, the speciation of dissolved complexes and gases, chemical-structural characterization of gas hydrates [Sum et al., 1997], and sulfur speciation in filamentous benthic bacteria [Pasteris et al., 2001] provide important examples. The latter two represent especially important ocean geochemical targets that are best studied in situ. The instrument we adapted is a Kaiser Optical Systems spectrometer with a transmissive holographic grating and a Charge-Coupled Device (CCD) array detector. The exciting laser is a 532-nm, frequency-doubled Nd:YAG laser yielding ~30 mW at the probe head. The system, which was placed on the ROV Tiburon, was deployed on four successful dives to a maximum of 3610 m depth. The spectrometer and laser housings remained on the ROV tool sled, while the probe head – which was connected by fiber-optic cable – was manipulated by the robotic arm to bring the laser into focus on the sample. Raman spectra of sea water were obtained throughout the water column to characterize both instrument performance and the ocean background medium. Spectra were taken of CO2 injected in situ into an inverted glass beaker at 200 m depth and changes observed during transit to 664 m. Results show the band shifts associated with changes in the density of CO2, including the spectrally obvious gas-liquid phase transformation. The growth of CO2 clathrate hydrate, the formation of secondary solid and dissolved species, the formation of CO2–saturated boundary layers, and the dissolution of sea floor minerals due to the introduction of CO2 are future targets of Raman investigation.

© 2002 by the American Geophysical Union.


Acknowledgements

We thank the pilots of the ROV Tiburon and the crew of the RV Western Flyer. We acknowledge the support of the David and Lucile Packard Foundation, and the U.S. DoE Ocean Carbon Sequestration Program.


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