Raman spectroscopic measurements of synthetic gas hydrates
in the ocean

K.C. Hester,a S.N. White,b E.T. Peltzer,c P.G. Brewerc and E.D. Sloana
a: Center for Hydrate Research, Colorado School of Mines, Golden, CO 80401, USA
b: Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
c: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA

Marine Chemistry (2006) 98: 304314.

Received: 2005 January 14.
Revised: 2005 August 10.
Accepted: 2005 September 12.


ABSTRACT

A Raman spectrometer extensively modified for deep ocean use was used to measure synthetic hydrates formed in an ocean environment. This was the first time hydrates formed in the ocean have been measured in situ using Raman spectroscopy. Gas hydrates were formed in situ in the Monterey Bay by pressurizing a Pyrex cell with various gas mixtures. Raman spectra were obtained for sI methane hydrate and sII methane+ethane hydrate. Gas occlusion resulting from rapid gas growth of methane hydrate was measured immediately after formation. The Raman shift for methane free gas was coincident with that of methane in the small 5¹² hydrate cage. The methane Raman peak widths were used to discriminate between methane in the free gas and hydrate phase. Methane+ethane sII hydrate was formed for 43 days on the seafloor. In this case, gas occlusion was not measured when the gas hydrates were allowed to form over an extended time period. Equivalent Raman spectra were obtained for the in situ and laboratory-formed sII methane+ethane hydrates, under similar p, T, and x conditions. With the Raman spectrometer operating in the ocean, seawater contributes to the Raman spectra obtained. Both the Raman bands for the sulfate ion and water were used to qualitatively determine the distribution of water phases measured (hydrate, seawater) in the Raman spectra.

© 2005 Elsevier B.V. All rights reserved.


Acknowledgements

This work was supported through NURP grant UAF03-0098. DORISS and PUP development was funded by a grant to MBARI from the David and Lucile Packard Foundation. Many thanks to the captains and crews of the R/V Point Lobos and R/V Western Flyer and pilots of the ROV Ventana (Knute Brekke, D.J. Osborne, Mark Talkovic and T. Craig Dawe) and ROV Tiburon (Buck Reynolds, Buzz Scott, Dave French, Paul Tucker, and Brian Schaefer); without their efforts this work would not have been possible. We gratefully acknowledge the technical assistance provided by Alana Sherman, Mark Brown, Peter Walz, John Ferreira, and Randy Prickett.


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