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 thermodynamic 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

Ocean abyssal carbon experiments at 0.7 and 4 km depth

Observations from small-scale (20 to 90 litres) CO2 experiments conducted off the coast of California at 684 m depth and at 3942 m depth are discussed. In both experiments, when the seawater velocity was sufficiently strong, parcels of liquid CO2 were torn off and transported away as discrete units by the turbulent water current. In the deep experiment, newly formed frazil hydrate was observed at the interface, occasionally including sediment particles. Hydrate furthermore collected and created a floating consolidated solid ("ice") in the downstream end of the trough, dissolving slowly from one day to the next. These observations have important implications for understanding and modelling of larger scale disposal at the seafloor. In particular, when CO2 is released by the interfacial instability mechanism driven by strong currents, the seawater density increase due to dissolution of CO2 may not have time to act and stabilize the water column before the discrete parcels of liquid phase CO2 are advected away from the disposal site. The floating solid that formed at the interface is hypothesized to consist of hydrate and additional trapped seawater. Its appearance was not expected in advance and its role in delaying dissolution can not be determined from the present experimental set-up. A capability for long-term seafloor perturbation experiments is deemed to be crucial both for direct ocean-storage research and for studying effects of invasion of anthropogenic CO2 from the atmosphere