[en] Carrier transported liquid CO₂ at 55 ^oC is denser than ambient seawater at mid-ocean depths. We have investigated whether this property effectively enables sinking of injected CO₂ from mid-depth to the ocean floor, >3500 m depth, where CO₂ is gravitationally stable as a lake on the dented sea floor. In order to obtain basic data for the realization of this idea, the National Maritime Research Institute, and the Monterey Bay Aquarium Research Institute, conducted three joint in situ experiments of CO₂ sending method for the ocean storage (COSMOS), to release cold CO₂ at the mid-ocean depths. The experiments were carried out in Monterey Bay from October 1999 to February 2002 using remotely operated vehicle (ROV) techniques to effect the controlled release and subsequent imaging. From the data obtained, it was clear that a cold CO₂ mass, released as a large unit, was apt to be broken up into small droplets by a Taylor type interface instability. Even for a unit of sufficient heat capacity for formation of a significant ice layer, break up into droplets due to liquid instabilities occurred in a short time. However, in experiments with a CO₂ slurry mass (a mixture of dry ice and liquid CO₂) of 8 cm size we observed that the released material could keep its shape and sink even further until the covering ice layer melted. The behavior of the CO₂ slurry mass strongly suggests that this technique offers the potential for effective transfer of released CO₂ from mid-depth to the ocean floor, and our experiments provide numerical constraints on the required design goals for this. (author)

[en] Since 2002, researchers from Japan, the United States and Norway have been collaborating on the Ocean Abyssal Carbon Experiment (OACE) project. This paper presented information on the three year project. It presented a discussion of the high-pressure laboratory work, instrument development and theoretical and numerical modelling associated with field experiments. The purpose of the project is to conduct groundbreaking and challenging experiments to determine of the fate of carbon dioxide (CO₂) disposed onto the ocean floor. Several observations from small-scale CO₂ experiments at different depths conducted off the coast of California were presented. In the 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. Newly formed frazil hydrate was observed at the interface, occasionally including sediment particles in the deep experiment. In addition, hydrate collected and created a floating consolidated solid consisting of ice in the downstream end of the trough, dissolving slowly from one day to the next. It was concluded that these observations have significant implications for understanding and modelling of larger scale anthropogenic CO₂ disposal at the seafloor. 15 refs., 3 figs