21 August 2000

Recent results of deep-sea carbon dioxide 
sequestration experiments

Editor's note: MBARI researchers are expanding our knowledge about the dynamics of carbon dioxide sequestration in the deep ocean using novel field experiments. Ocean chemists Peter Brewer, Gernot Friederich, Ed Peltzer, and Gregor Rehder are presenting the results of some of their recent studies this week at the national American Chemical Society meeting in Washington, D.C. Dr. Peltzer, lead author on the ACS paper, describes the experiment and results below.

As part of our continuing effort to apply MBARI's advanced remotely operated vehicle (ROV) technology to studies of ocean carbon dioxide (CO2) sequestration, we conducted several experiments using the ROV to measure the rise rate of CO2 droplets in the upper ocean. These were the first attempts to investigate the fate of a rising plume of CO2 in the ocean and to test the veracity of several ocean CO2 release models. One advantage to this approach is that our CO2 releases are very small and have minimal impact on the environment, yet yield information that can be scaled-up to large experiments. We also have developed new technology that can be scaled-up as well to larger releases.

In this series of experiments, we carried several liters of liquid CO2 down on the ROV, but instead of releasing it into a beaker on the sea-floor, we let it go in small bursts in the midwater at about 800m depth. In order to constrain the motion of the CO2, we released it inside a clear "box" which was open to the ocean at the top and bottom. This served to keep it protected from lateral movements, but allow us to measure its upward velocity. (It also made the release a little easier to track for the pilots.) By maintaining the droplets in the center of the field of view, we could track the rise rate of the bubbles by simply looking at the record of ROV depth vs time. 

The high-resolution HDTV camera on the ROV also allowed us to image the droplets with enough clarity that we were able to measure droplet diameters to better than a millimeter precision. By studying the decrease in droplet diameter as a function of time we could calculate the dissolution rate of liquid CO2 droplets in real-world conditions (salinity, temperature and pressure) for the first time.

In addition to the technological achievement of chasing gram quantities of liquid CO2 with a 3.5 ton vehicle and imaging the droplets to millimeter precison, these measurements (and especially the bubble lifetimes, mass dissolution rates and rise rates) are sure to become integral to models of CO2 plume dynamics and solution phenomenon in seawater and the natural environment.

—Ed Peltzer

For more information about this research, see the greenhouses gases website.