THE FATE OF CO2 SEQUESTERED IN THE DEEP OCEAN

Based on published phase diagrams, laboratory and in situ observations we predict that liquid CO2 released from a diffuser below 500 m depth in the ocean will form solid hydrate particles of composition CO2.6H2O with bulk density 1.1 gcm-3. Being heavier than seawater the hydrate particles will sink to the ocean bottom. The sedimentation rate w depends on the particle diameter, drag coefficient, and the difference between particle and seawater bulk density. For particles in the size range 10(-5)-10(-3) m, w=5x10(-6)-0.02 ms-1. The dispersion of the particles on the ocean bottom also depends on the distance between the diffuser and the ocean bottom and the turbulent characteristics of the water column. For the above particle size range and a fall distance of 500 m, the median radius of the mound varies from 1 km for large particles to more than 500 km for small, slowly falling particles. For a 500 MW(el) coal fired power plant with a discharge rate of 170 kgs-1 of CO2, these parameters correspond to mass depositions of 4x10(-3)-1x10(3) kgm-2yr-1 at the plume centerline, or mound thicknesses that grow at rates of 4x10(-6)-1 myr-1. Hydrate particles may (1) interfere with the feeding habits of swimming creatures as they descend and (2) bury benthic organisms as they deposit to the ocean floor.