Dissolution from a liquid CO2 lake disposed in the deep ocean

The dissolution from a liquid CO2 lake source located at a flat ocean bottom at 3,000 m depth is investigated. Using the unsteady, two-dimensional advection–diffusion equation, temporal and spatial distribution of CO2 dissolved from the source of 500 m length and of unit span is sought in a domain of 20 km horizontal and 200 m vertical extent. Different cases were run with uniform longitudinal speed and constant horizontal and vertical diffusion coefficients and with vertical profiles of velocity and diffusivity derived from turbulent boundary layer theory. Each case was run with and without a hydrate film at the interface between the seawater and the liquid CO2. The properties of the hydrate film are modeled using a capillary permeation model. The computations show that the presence of a hydrate layer retards the dissolution rate by a factor of 2.7 when the density effects due to the increase of CO2 concentration as a result of the dissolution are neglected. However, the strong, stable stratification above the hydrate layer, as a consequence of the increase in density of seawater enriched by CO2, suppresses the vertical mixing considerably and reduces the sensitivity to hydrate. The dissolution rate is found to be 0.1 m yr21 for realistic vertical profiles of longitudinal velocity (order of 5 cm s21) and diffusivity. However, during conditions of a benthic storm (20 cm s21), the dissolution rate reaches 1.6 m yr21. Enhanced emission of greenhouse gases, particularly carbon dioxide (CO2), to the atmosphere is widely accepted to affect the global climate system (Houghton et al. 1995). The atmospheric CO2 content at present is about 25% higher than preindustrial levels. Over the past two decades, multidisciplinary research has been intensified with a focus to stabilize the CO2 level in the atmosphere. One of the potential options to mitigate atmospheric levels is to capture it from fossil fuel combustors and purposefully dispose of and sequester it elsewhere (e.g., in ocean, deep saline aquifers, depleted gas and oil wells, coal beds, etc.). The ocean appears to be a preferable option because it is the largest potential sink for anthropogenic CO2. Marchetti (1977) was the first to propose ocean disposal to accelerate the natural ocean uptake of atmospheric CO2. He suggested that efficient long-term sequestration could be achieved through the Gibraltar Strait, where the outflow of dense water cascades to ;1,000 m depth and, in consequence, spreads out in the North Atlantic. The research on ocean disposal options has mostly focused on predicting the behavior and the dissolution time scale of the released CO2 and on quantifying the environmental impacts to marine systems (see, e.g., Handa and Ohsumi 1995). Different scenarios of CO2 disposal in the ocean have been proposed at various depths and in different forms in relation to the phase properties of CO2. The phase diagram for the CO2–water system shows that when pressure is greater than ;4.5 MPa and the temperature is less than 9.858C, clathrate-hydrate crystal (hereafter hydrate) develops. Density profiles of liquid CO2, CO2-saturated seawater, and seawater are shown in Fig. 1, together with the approximate 1 Corresponding author ([email protected]).