Determination of gas bubble fractionation rates in the deep ocean
by laser Raman spectroscopy

S.N. White,+ P.G. Brewer & E.T. Peltzer
Monterey Bay Aquarium Research Institute Moss Landing, CA 95039-9644, USA.
+: Now at Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.

Marine Chemistry (2006) 99: 12-23.

Received: 2004 June 8.
Revised: 2004 October 21.
Accepted: 2004 October 29.
Published: 2006 January 18.


ABSTRACT

A new deep-sea laser Raman spectrometer (DORISS—Deep Ocean Raman In Situ Spectrometer) is used to observe the preferential dissolution of CO2 into seawater from a 50%–50% CO2–N2 gas mixture in a set of experiments that test a proposed method of CO2 sequestration in the deep ocean. In a first set of experiments performed at 300 m depth, an open-bottomed 1000 cm³ cube was used to contain the gas mixture; and in a second set of experiments a 2.5 cm³ funnel was used to hold a bubble of the gas mixture in front of the sampling optic. By observing the changing ratios of the CO2 and N2 Raman bands we were able to determine the gas flux and the mass transfer coefficient at 300 m depth and compare them to theoretical calculations for air–sea gas exchange. Although each experiment had a different configuration, comparable results were obtained. As expected, the ratio of CO2 to N2 drops off at an exponential rate as CO2 is preferentially dissolved in seawater. In fitting the data with theoretical gas flux calculations, the boundary layer thickness was determined to be ~42 µm for the gas cube, and ~165 µm for the gas funnel reflecting different boundary layer turbulence. The mass transfer coefficients for CO2 are k(L) = 2.82 × 10e-5 m/s for the gas cube experiment, and k(L) = 7.98 × 10e-6 m/s for the gas funnel experiment.

© 2005 Elsevier B.V. All rights reserved.


Acknowledgements

We acknowledge the help and support of the pilots of the ROV Ventana, and the captain and crew of the RV Point Lobos for their skilled assistance with the field work. We acknowledge the valuable assistance of J. Pasteris, B. Wopenka, and J. Freeman in the development of the DORISS system; the engineering support of Mark Brown, Danelle Cline and George Malby; and technical assistance from Peter Walz and Randy Prickett. Funding was provided by a grant to MBARI from the David and Lucile Packard Foundation, and by the U.S. Dept. of Energy Ocean Carbon Sequestration Program (Grants No. DE-FC26-00NT40929 and DEFC03-01ER6305). This paper has benefited from the reviews of Dr. Gregor Rehder and Prof. Gordon Taylor.


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