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2000 2100 2200 2300 2400 Dissolved Inorganic carbon (ijmol kg-1)

In either case, Total Alkalinity does not change. The combined reactions lower both ocean pH, and carbonate ion concentration. For current ocean composition, CO2 that is added to sea water is partitioned primarily into HCO3- with the net reaction resulting in the generation of H+ and thus decreasing pH and making sea water more acidic; adding CO2 thereby decreases the concentration of CO32-. Total Alkalinity is increased when, for example, alkaline minerals such as CaCO3 are dissolved in sea water through the reaction,

2000 2100 2200 2300 2400 Dissolved Inorganic carbon (ijmol kg-1)

Figure 6.5 Composition diagram for ocean surface waters at 15°C (adapted from Baes, 1982). The white lines denote compositions with the same value of pCO2 (in ppm); the black lines denote compositions with the same pH. The tan shaded region is undersaturated and the green shaded region is supersaturated with respect to calcite at atmospheric pressure (calcite solubility increases with depth). Surface water and average ocean compositions are also indicated. Adding CO2 increases Dissolved Inorganic Carbon (DIC) without changing Total Alkalinity (TAlk); dissolving CaCO3 increases both DIC and TAlk, with 2 moles of TAlk added for eac3h mole of DIC added.

Figure 6.6 Observed variation in open oceanpH for the 1990s (shown on the total hydrogen scale; data from Key et al., 2004). In this figure the oceans are separated into separate panels. The three panels are on the same scale and coloured by latitude band to illustrate the large north-south changes in the pH of intermediate waters. Pre-industrial surface values would have been about 0.1 pH units greater than in the 1990s.

Figure 6.6 Observed variation in open oceanpH for the 1990s (shown on the total hydrogen scale; data from Key et al., 2004). In this figure the oceans are separated into separate panels. The three panels are on the same scale and coloured by latitude band to illustrate the large north-south changes in the pH of intermediate waters. Pre-industrial surface values would have been about 0.1 pH units greater than in the 1990s.

Figure 6.7 Natural variation in total dissolved inorganic carbon concentration at 3000 m depth (data from Key et al., 2004). Ocean carbon concentrations increase roughly 10% as deep ocean waters transit from the North Atlantic to the North Pacific due to the oxidation of organic carbon in the deep ocean.

Figure 6.7 Natural variation in total dissolved inorganic carbon concentration at 3000 m depth (data from Key et al., 2004). Ocean carbon concentrations increase roughly 10% as deep ocean waters transit from the North Atlantic to the North Pacific due to the oxidation of organic carbon in the deep ocean.

would inject the CO2 below the thermocline1 for more effective storage.

Depending on the details of the release and local sea floor topography, the CO2 stream could be engineered to dissolve in the ocean or sink to form a lake on the sea floor. CO2, dissolved in sea water at high concentrations can form a dense plume or sinking current along an inclined sea floor. If release is at a great enough depth, CO2 liquid will sink and could accumulate on the sea floor as a pool containing a mixture of liquid and hydrate. In the short-term, fixed or towed pipes appear to be the most viable methods for oceanic CO2 release, relying on technology that is already largely commercially available.

Authorities' initial decision. After the public hearing procedure and subsequent decision by the Authority to confirm their initial permit, Brende said, 'The possible future use of the sea as storage for CO2 is controversial. ... Such a deposit could be in defiance of international marine laws and the ministry therefore had to reject the application.' The Norwegian Environment ministry subsequently announced that the project would not go ahead (Giles, 2002).

Several smaller scale scientific experiments (less than 100 litres of CO2) have however been executed (Brewer et al., 1999, Brewer et al., 2005) and the necessary permits have also been issued for experiments within a marine sanctuary.

6.2.1.2 Status of development

To date, injection of CO2 into sea water has only been investigated in the laboratory, in small-scale in-situ experiments, and in models. Larger-scale in-situ experiments have not yet been carried out.

An international consortium involving engineers, oceanographers and ecologists from 15 institutions in the United States, Norway, Japan and Canada proposed an in-situ experiment to help evaluate the feasibility of ocean carbon storage as a means of mitigating atmospheric increases. This was to be a collaborative study of the physical, chemical, and biological changes associated with direct injection of CO2 into the ocean (Adams et al., 2002). The proposed CO2 Ocean Sequestration Field Experiment was to inject less than 60 tonnes of pure liquid carbon dioxide (CO2) into the deep ocean near Keahole Point on the Kona coast of the Island of Hawaii. This would have been the largest intentional CO2 release into the ocean water column. The test was to have taken place in water about 800 m deep, over a period of about two weeks during the summer of 2001. Total project cost was to have been roughly US$ 5 million. A small steel pipeline, about 4 cm in diameter, was to have been deployed from a ship down to the injection depth, with a short section of pipeline resting on the sea floor to facilitate data collection. The liquid CO2 was to have been dispersed through a nozzle, with CO2 droplets briefly ascending from the injection point while dissolving into the sea water. However, the project met with opposition from environmental organizations and was never able to acquire all of the necessary permits within the prescribed budget and schedule (de Figueiredo, 2002).

Following this experience, the group developed a plan to release 5.4 tonnes of liquefied CO2 at a depth of 800 metres off the coast of Norway, and monitor its dispersion in the Norwegian Sea. The Norwegian Pollution Control Authority granted a permit for the experiment. The Conservative Party environment minister in Norway's coalition government, B0rge Brende, decided to review the Norwegian Pollution Control

1 The thermocline is the layer of the ocean between about 100 and 1000 m depth that is stably stratified by large temperature and density gradients, thus inhibiting vertical mixing. Vertical mixing rates in the thermocline can be about 1000 times less than those in the deep sea. This zone of slow mixing would act as a barrier to slow degassing of CO2 released in the deep ocean to the atmosphere.

6.2.1.3 Basic behaviour of CO2 released in different forms The near-field behaviour of CO2 released into the ocean depends on the physical properties of CO2 (Box 6.2) and the method for CO2 release. Dissolved CO2 increases the density of sea water (e.g., Bradshaw, 1973; Song, et al., 2005) and this affects transport and mixing. The near field may be defined as that region in which it is important to take effects of CO2-induced density changes on the fluid dynamics of the ocean into consideration. The size of this region depends on the scale and design of CO2 release (Section 6.2.1.4).

CO2 plume dynamics depend on the way in which CO2 is released into the ocean water column. CO2 can be initially in the form of a gas, liquid, solid or solid hydrate. All of these forms of CO2 would dissolve in sea water, given enough time (Box 6.1). The dissolution rate of CO2 in sea water is quite variable and depends on the form (gas, liquid, solid, or hydrate), the depth and temperature of disposal, and the local water velocities. Higher flow rates increase the dissolution rate.

Gas. CO2 could potentially be released as a gas above roughly 500 m depth (Figure 6.8). Below this depth, pressures are too great for CO2 to exist as a gas. The gas bubbles would be less dense than the surrounding sea water so tend to rise towards the surface, dissolving at a radial speed of about 0.1 cm hr-1 (0.26 to 1.1 ^mol cm-2 s-1; Teng et al., 1996). In waters colder than about 9°C, a CO2 hydrate film could form on the bubble wall. CO2 diffusers could produce gaseous CO2 bubbles that are small enough to dissolve completely before reaching the surface.

Liquid. Below roughly 500 m depth, CO2 can exist in the ocean as a liquid. Above roughly 2500 m depth CO2 is less dense than sea water, so liquid CO2 released shallower than 2500 m would tend to rise towards the surface. Because most ocean water in this depth range is colder than 9°C, CO2 hydrate would tend to form on the droplet wall. Under these conditions, the radius of the droplet would diminish at a speed of about 0.5 cm hr-1 (= 3 ^mol cm-2 s-1; Brewer et al., 2002). Under these conditions a 0.9 cm diameter droplet would rise about 400 m in an hour before dissolving completely; 90% of its mass would be lost in the first 200 m (Brewer et al., 2002). Thus, CO2 diffusers could be designed to produce droplets that will dissolve within roughly 100 m of the depth of release. If the droplet reached approximately 500 m depth, it would become a gas bubble.

CO2 is more compressible than sea water; below roughly

Box 6.2 Physical properties of CO..

The properties of CO2 in sea water affect its fate upon release to the deep-sea environment. The conditions under which CO2 can exist in a gas, liquid, solid hydrate, or aqueous phase in sea water are given in Figure 6.8 (see Annex I).

At typical pressures and temperatures that exist in the ocean, pure CO. would be a gas above approximately 500 m and a liquid below that depth. Between about 500 and 2700 m depth, liquid CO2 is lighter than sea water. Deeper than 3000 m, CO. is denser than sea water. The buoyancy of CO2 released into the ocean determines whether released CO. rises or falls in the ocean column (Figure 6.9). In the gas phase, CO2 is lighter than sea water and rises. In the liquid phase CO. is a highly compressible fluid compared to sea water. A fully formed crystalline CO. hydrate is denser than sea water and will form a sinking mass (Aya et al., .003); hydrate formation can thus aid ocean CO. storage by more rapid transport to depth, and by slowing dissolution. It may also create a nuisance by impeding flow in pipelines or at injectors.

The formation of a solid CO. hydrate (Sloan, 1998) is a dynamic process (Figure 6.10; Brewer et al., 1998, 1999, .000) and the nature of hydrate nucleation in such systems is imperfectly understood. Exposed to an excess of sea water, CO. will eventually dissolve forming an aqueous phase with density higher than surrounding sea water. Release of dense or buoyant CO. - in a gas, liquid, hydrate or aqueous phase - would entrain surrounding sea water and form plumes that sink, or rise, until dispersed.

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