Dissolution from a liquid CO² lake disposed in the deep ocean

Abstract

The dissolution from a liquid CO 2 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 CO 2 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 CO 2 . 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 CO 2 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 CO 2 , suppresses the vertical mixing considerably and reduces the sensitivity to hydrate. The dissolution rate is found to be 0.1 m yr Ϫ1for realistic vertical profiles of longitudinal velocity (order of 5 cm s Ϫ1 ) and diffusivity. However, during conditions of a benthic storm (20 cm s Ϫ1 ), the dissolution rate reaches 1.6 m yr Ϫ1.Enhanced emission of greenhouse gases, particularly carbon dioxide (CO 2 ), to the atmosphere is widely accepted to affect the global climate system (Houghton et al. 1995). The atmospheric CO 2 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 CO 2 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 CO 2 . Marchetti (1977) was the first to propose ocean disposal to accelerate the natural ocean uptake of atmospheric CO 2 . He suggested that efficient long-term sequestration could be achieved through the Gibraltar Strait, where the outflow of dense water cascades to ϳ1,000 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 CO 2 and on quantifying the environmental impacts to marine systems (see, e.g., Handa and Ohsumi 1995). Different scenarios of CO 2 disposal in the ocean have been proposed at various depths and in different forms in relation to the phase properties of CO 2 . The phase diagram for the CO 2 -water system shows that when pressure is greater than ϳ4.5 MPa and the temperature is less than 9.85Њ...

Dynamics of a CO<SUB>2</SUB>-seawater interface in the deep ocean

“…We note that experimental results are available in the literature for dissolution rate of hydrate-covered droplets and CO 2 hydrate (Brewer et al, 2002;Rehder et al, 2004) at 1000 m depth or less. For a liquid-CO 2 lake at the deep seafloor, only model estimates are available (Haugan and Alendal, 2005;Fer and Haugan, 2003). The model estimates of change in CO 2 level range from less than 10 cm per year at low flow speeds to several meters per year with currents of about 20 cm/s or more.…”

Ocean abyssal carbon experiments at 0.7 and 4 KM depth

“…Companion experiments with CH 4 hydrates, and the observed response of CO 2 dissolution rate to time-varying currents, confirmed the assumption of diffusion-controlled dissolution governed by solubility. Since the solubility of CO 2 in seawater in the hydrate regime is lower at 4 km depth than at 1 km depth, dissolution is expected to be slower in our case than in that of [6], but still rapid compared to previously modelled release rates from large-scale storage on the seafloor [11].…”

Deep ocean experiments with fossil fuel carbon dioxide: Creation and sensing of a controlled plume at 4 km depth

“…For advection of water across very large-scale lakes of CO 2 on the ocean floor the increased time scale would move the system closer to equilibrium as plume reactions occur, and the strong density increase predicted and modeled (Fer and Haugan, 2003) would likely be observed.…”