The Characteristics, Behavior and Fate of a Stream of Liquid CO2 Released Into the Ocean

With the recent discovery of three sites venting a stream of volcanically derived liquid CO2 from the seafloor, questions arise as to the chemical characteristics, physical behavior, and ultimate fate and impact of the vented flow of this fluid. We now have a great deal of information, and compelling images, of liquid CO2 behavior in the deep-sea, derived from small-scale experiments carried out to investigate possibilities of ocean sequestration of fossil fuel CO2. The critical point of CO2 occurs at 31.3°C and 738.9 dbar. Thus volcanically derived CO2 will begin its transit to the seafloor as a supercritical fluid and will acquire chemical signatures consistent with this as it progresses through the pore space. As it approaches the cooler sediments and the seafloor, it condenses to the highly immiscible liquid phase. CO2 is a very low viscosity, highly compressible, non-polar fluid, with a remarkable ability to dissolve other non-polar species. Thus the magmatic gases, He and H2, will tend to be strongly enriched in the CO2 phase. Equilibrium calculations may be carried out on this process with considerable accuracy, but the extent to which the plume reaches equilibrium with the surrounding pore fluids during transit is unknown. On venting to the seafloor, hydrodynamic instabilities quickly result in CO2 droplet formation with droplet diameters on a cm scale. At depths above ~2700m, liquid CO2 is less dense than seawater. Furthermore, liquid CO2 readily forms a Structure I hydrate, the phase boundary is well known, and in Pacific Ocean waters typically occurs at ~400m depth. Thus at all three vent sites discovered to date, (JADE hydrothermal site, Okinawa Trough 1335 to 1550m; Champagne vent site NW Eifuku, 1650m; Vailulu'u seamount, 940m) a rising plume of CO2 droplets is formed within the hydrate stability zone, and a thin hydrate skin forms on the ascending droplets. As the hydrate coated droplets rise through the water column, they dissolve at a rate of ~3 μmol/cm2/sec, lowering local pH. Typically, 90% of the droplet mass dissolves on a length scale of about 200m. Liquid CO2 exhibits a very strong acoustic contrast with respect to seawater, so that a cloud of liquid CO2 droplets can be readily detected by sonar. Two important questions regarding these natural CO2 vents come to mind: firstly, are these vents a common feature of seamounts; and secondly, what impact have they had on the development of the benthic communities? Although the presence of thermal and sulfidic anomalies may confound the purely CO2 associated signal, answers to these questions will be important, especially with regards to how well these natural environments provide an analogue for processes associated with (i) the potential leakage of CO2 from sub-seafloor geologic sequestration sites (e.g. Sleipner in the North Sea); (ii) direct oceanic CO2 sequestration, and; (iii) the impacts of a high CO2-low pH ocean.