Dissolution rates of pure methane hydrate
and carbon-dioxide hydrate in
undersaturated seawater at 1000-m depth

Gregor Rehder,1,* Stephen H. Kirby,2 William B. Durham,3 Laura A. Stern,2
Eward T. Peltzer,1 John Pinkston2 and Peter G. Brewer1

1: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, USA
2: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA
3: Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

*: Author to whom correspondence should be addressed, at GEOMAR Research Center,
Wischhofstr. 1-3, D-24148 Kiel, Germany (grehder@geomar.de).

Geochimica et Cosmochimica Acta (2004) 68: 285–292. doi:10.1016/j.gca.2003.07.001

Received: 2002 October 6.
Revision received: 2003 July 24.


ABSTRACT

To help constrain models involving the chemical stability and lifetime of gas clathrate hydrates exposed at the seafloor, dissolution rates of pure methane and carbon-dioxide hydrates were measured directly on the seafloor within the nominal pressure-temperature (P/T) range of the gas hydrate stability zone. Other natural boundary conditions included variable flow velocity and undersaturation of seawater with respect to the hydrate-forming species. Four cylindrical test specimens of pure, polycrystalline CH4 and CO2 hydrate were grown and fully compacted in the laboratory, then transferred by pressure vessel to the seafloor (1028 m depth), exposed to the deep ocean environment, and monitored for 27 hours using time-lapse and HDTV cameras. Video analysis showed diameter reductions at rates between 0.94 and 1.20 µm/s and between 0.09 and 0.106 µm/s for the CO2 and CH4 hydrates, respectively, corresponding to dissolution rates of 4.15 ± 0.5 mmol CO2/m²/s and 0.37 ± 0.03 mmol CH4/m²/s. The ratio of the dissolution rates fits a diffusive boundary layer model that incorporates relative gas solubilities appropriate to the field site, which implies that the kinetics of the dissolution of both hydrates is diffusion-controlled. The observed dissolution of several mm (CH4) or tens of mm (CO2) of hydrate from the sample surfaces per day has major implications for estimating the longevity of natural gas hydrate outcrops as well as for the possible roles of CO2 hydrates in marine carbon sequestration strategies.

© 2004 by Elsevier Ltd.


Acknowledgements

We thank the captain and the crew of R/V Point Lobos and the technicians and pilots of ROV Ventana for their skillful support during offshore operation. The help of Chris Rogers-Walz and Kyra Schlining in the video labs is gratefully acknowledged. We thank Susan Circone for her assistance in synthesizing the CO2 hydrate samples The detailed reviews of Dr. Izuo Aya (Maritime Research Institute, Osaka), Dr. Bjørn Kvamme (University of Bergen), and an unknown reviewer were highly appreciated and helped to improve the manuscript. This work was supported by a grant to MBARI from the David and Lucile Packard Foundation and funding from the U.S. Geological Survey Gas Hydrate Project and a contract from the Department of Energy under the Carbon Sequestration Program. Work performed by W. D. Durham under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract W-7405-ENG-48. This is publication no. GEOTECH-27 of the program GEOTECHNOLOGIEN of BMBF and DFG.


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