ABSTRACT

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.

Acknowledgements

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.