Deep ocean experiments with fossil fuel carbon dioxide: Creation and sensing of a controlled plume at 4 km depth

The rapidly rising levels of atmospheric and oceanic CO2 from the burning of fossil fuels has lead to well-established international concerns over dangerous anthropogenic interference with climate. Disposal of captured fossil fuel CO2 either underground, or in the deep ocean, has been suggested as one means of ameliorating this problem. While the basic thermoladynamic properties of both CO2 and seawater are well known, the problem of interaction of the two fluids in motion to create a plume of high CO2/low pH seawater has been modeled, but not tested. We describe here a novel experiment designed to initiate study of this problem. We constructed a small flume, which was deployed on the sea floor at 4 km depth by a remotely operated vehicle, and filled with liquid CO2. Seawater flow was forced across the surface by means of a controllable thruster. Obtaining quantitative data on the plume created proved to be challenging. We observed and sensed the interface and boundary layers, the formation of a solid hydrate, and the low pH/high CO2 plume created, with both pH and conductivity sensors placed downstream. Local disequilibrium in the CO2 system components was observed due to the finite hydration reaction rate, so that the pH sensors closest to the source only detected a fraction of the CO2 emitted. The free CO2 molecules were detected through the decrease in conductivity observed, and the disequilibrium was confirmed through trapping a sample in a flow cell and observing an unusually rapid drop in pH to an equilibrium value