Rachel M. Dunk, Edward T. Peltzer, Peter M. Walz, Peter G. Brewer
Monterey Bay Aquarium Research Institute,
7700 Sandholdt Road, Moss Landing, California 95039-9644
Environmental Science and Technology (2005) 39: 9630-9636.
Received: 2005 June 20.
Revised: 2005 October 11.
Accepted: 2005 October 19.
We used a newly developed in situ laser Raman spectrometer (LRS) for detection of elevated levels of dissolved CO2 in seawater. The experiment was carried out at 500 m depth, 6°C, to examine new protocols for detection of CO2-enriched seawater emanating from a liquid CO2 source in the ocean, and to determine current detection limits under field conditions. A system of two interconnected 5 L chambers was built, with flow between them controlled by a valve and pump system, and this unit was mounted on an ROV. The first chamber was fitted with a pH electrode and the optical probe of the LRS. In the second chamber ~580 mL of liquid CO2 was introduced. Dissolution of CO2 across the CO2-seawater interface then occurred, the valves were opened, and a fixed volume of low-pH/CO2-enriched seawater was transferred to the first chamber for combined pH/Raman sensing, where we estimate a mean dissolution rate of ~0.5 µmol/cm²/s. This sequence was repeated, resulting in measurement of a progressively CO2 enriched seawater sample. The rapid in-growth of CO2 was readily detected as the Fermi dyad of the dissolved state with a detection limit of ~10 mM with spectral acquisition times of 150 sec. The detection of background levels of CO2 species in seawater (~2.2 mM, dominantly HCO3¯) will require an improvement in instrument sensitivity by a factor of 5-10, which could be obtained by the use of a liquid core waveguide.
© 2005 American Chemical Society.
We gratefully acknowledge the support of the David and Lucile Packard Foundation and of the U.S. Department of Energy, National Energy Technology Laboratory, under Contract De-FC26-00NT4092. We thank the captain and crew of the RV Point Lobos and the pilots of the ROV Ventana for their skilled assistance in making the fieldwork possible.