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The deep ocean disposal of carbon dioxide
Energy Convers. Mgmt Vol. 33, No. 5-8, pp. 627-633, 1992
0196-8904/92 $5.00+0.00 Pergamon Press Ltd
Printed in Great Britain
THE DEEP OCEAN DISPOSAL OF CARBON DIOXIDE
T. R. S. WILSON Institute of Oceanographic Sciences Deacon Laboratory, Brook Road, Wormiey, Godalming, Surrey GU8 5UB, England.
ABSTRACT Once released to the environment, carbon dioxide equilibrates within the ocean-atmosphere pool. Because the mixing of surface seawater is relatively slow, release to the atmosphere causes a transient peak in atmosphere carbon dioxide concentration. If the release could be transferred to the ocean, this transient might be avoided. In addition, interaction with the large carbon pool contained within the ocean sediment would be accelerated. The resultant increase in ocean alkalinity would tend to reduce equilibrium PCO2 values. The dynamics of the processes involved are complex and not yet understood in detail. This paper reviews deep ocean carbonate chemistry and mixing, and estimates their relative importance for the prediction of atmospheric carbon dioxide concentrations resulting from deep ocean release. Predictions based on mathematical modelling suggest that disposal into the surface ocean ( < 1 km depth) would permit equilibration with the atmosphere within a few years to decades and would therefore offer little advantage. Disposal into ocean basins greater than 3km in depth would delay equilibration with the atmosphere for several hundred years, eliminating the atmospheric concentration transient. Resultant interaction with calcite-rich sediments could probably reduce the long-term ( > 2000 year) atmospheric enrichment by a significant amount (~ 50%) KEYWORDS Carbon dioxide; Ocean mixing; PCO2 : pH; CO2-hydrate, calcite, calcium carbonate, fossil fuel INTRODUCTION Since the discovery of fire, mankind has utilised the atmosphere to carry away the waste products of combustion. Modem industrial practices, base on the use of fossil fuel, may be altering atmospheric composition to an unacceptable degree. It is prudent to consider ahemative strategies of disposal. Marchetd (1978) suggested that sinking thermohaline currents might be used to carry waste carbon dioxide to abyssal depths in the ocean. Since disposal to the atmosphere eventually results in the equilibration of the disposed carbon dioxide within the oceanic pool of dissolved inorganic carbon, this strategy in effect short-circuits the slow equilibration and sinking process which would normally operate. As the atmospheric carbon reservoir (720 Gigatonnes; Clark, 1982) is much smaller than the deep ocean pool (37000 Gigatonnes), the perturbation of carbon dioxide concentration should be much reduced. BOX MODEL The circulation of the worlds oceans is characterised by sinking at high latitudes of surface water which has become denser by cooling, and the circulation of this ventilating water throughout the interconnected deep ocean basins. Upward mixing of this water eventually results in its return to the surface, predominantly at low latitudes, by upwelling currents originating at intermediate depths. The cycle is completed by surface transport polewards. LaTge natural fluxes of carbon dioxide to the atmosphere occur at low and intermediate latitudes, balanced in natural circumstances by fluxes from the atmosphere to the
627
628
WILSON: DEEPOCEAN DISPOSALOF CO2
ocean as it cools at higher latitudes. Thus, the carbon pools of the ocean and atmosphere are mixed on a timescale appropriate to this circulation. In order to illustrate this process, a box model of the ocean-atmosphere system (Figure 1) was used to compare the effects of differing disposal strategies. This model incorporates the advective processes which characterise the slow circulation of the deep oceans. Nutrient concentration, total inorganic carbon and alkalinity are modelled, and the calculated pCO2 of the surface boxes determines the model atmospheric carbon dioxide concenwation. The boxes are assumed to be completely mixed internally, while transfer between boxes is by advection and by the production, sinking and dissolution of organic and inorganic carbon particles. The rate at which these particles are produced is a function of surface nutrient levels, which in turn are controlled by circulation and by river run-off. Table 1 lists the parameters which were used for the model runs discussed in the present paper. These represent the current best estimates of water flux, and are similar to values previously used in recent box models (eg Ennever and McElroy, 1985: Toggweiler and Sarmiento, 1985: Wenk and Siegenthaler, 1985)
Aunosphere
Particulate sinking
I
Box 0 I I
(Surface, low to m e d i u m latitude)
'
Box 2 (Intermediate)
t
[
~4~ Box 1 (High
~ Water exchange
Box 3
~
latitude)
I
(Deep)
Fig 1. Schematic of box model. Solid lines indicate water exchanges, dashed lines particulate.
Table 1. Model s u m m a r y
Box number: Fractional volume Fractional surface area Temperature
0 0.08 0.8 19
Flow field (Sverdrups) Flow from box: To box: 0 0 0 1 0 2 20 3 0
1 0.02 0.2 2
2 0.1 0 2
3 0.8 0 2
1 15 0 37 0
2 5 0 0 252
3 0 52 200 0
Notes: 1. The nutrient concentration, total carbon dioxide and alkalinity fields are modelled. 2. The rate of production of particulates in box 0 is controlled by the nutrient concentration. River nutrient flux 3.5 x 1010 moles/annum is balanced by burial. 3. pCO2 of boxes 0 and I is caloulated at each timestep, and controls the model atmosphere via empirically-determined transfer coefficients. 4. One Sverdrup = 106 m 3 sec-1
WILSON: DEEP OCEAN DISPOSALOF CO2
629
The model does not incorporate detailed plume behaviour, as it is intended to study medium to long-term effects. It is worth noting, however, that the injection of liquid carbon dioxide into the ocean at depths less than 2 km would be likely to produce a rapidly rising plume, as liquid carbon dioxide is significantly less dense than seawater at these depths. Rising velocities of several tens of centimetres per second have been calculated for such plumes (P.Killworth, personal communication.) Their detailed behaviour is very complex; solid carbon dioxide hydrates and dense carbon dioxide-saturated seawater would be formed and would tend to separate from the rising plume and sink. The rates and magnitudes of these side-processes are difficult to calculate. However, it is clear that the overall effect would be to mix the released carbon dioxide into the receiving water mass in a complex manner. In the present treatment, to set the scene, the conservative assumption has been made that this mixing is instantaneous and complete. Other papers presented at this symposium will discuss the likely influence of hydrate formation and the behaviour of dense, CO2-saturated seawater, which both tend to enhance the retention of released CO2 within the ocean. The model ocean was driven until steady-state was achieved, with a stable atmospheric carbon dioxide concentration corresponding to pre-industrial levels. This provided the time-zero configuration. Starting from this point, runs were then performed in which injections of 100 million tonnes per year of carbon dioxide were made. These injections continued at constant rate for 250 years, and then ceased. (100 million tonnes of carbon dioxide is approximately half the carbon dioxide produced from coal-fired electricity production per year in the UK). The model was then allowed to run until a new steady-state was reached. The point of injection was varied from run to run. The model timestep was 1 year Table 2 Variations studied: 1. No calcite solution. Release to:
atmosphere surface layer (< 200 metres) high latitude surface layer intermediate layer deep water (> I000 metres)
2. With calcite solution. Release to deep water (> 1000 metres)
Examples of these runs are shown in Figs. 2 to 4 below. Release into surface (box 0) produced atmospheric enrichment which was not significantly different from that produced by direct atmospheric release. Release to the high-latitude surface box produces an intermediate result with a diminished but still significant atmospheric enrichment (Fig. 2). Clearly, this result would be very sensitive to the point of release, since not all high-latitude water is actively sinking, and release directly to a sinking water mass might be more effective than is suggested by Fig. 2. However, the most effective isolation is obtained using release to the deep ocean (Fig. 3). These results agree broadly with those of Hoffert et al (1979), in showing that carbon dioxide released to the deep ocean would not contribute to the atmospheric transient peak. Shallow release of liquid carbon dioxide would result in a rapidly rising plume of fluid, and even if this did not break through into the surface layer, significant transport back to the atmosphere across the seasurface would occur quite rapidly. There are two possible exceptions to this generalisation, which are discussed below. Intermediate-depth disposal might be effective if confined to areas of sinking intermediate water, using a strategy similar to that suggested by Marchetti (1978). In general, however, the model suggests that release to the deep ocean is the most effective. An advantage of the present model is that the full carbonate system speciation can be calculated for all boxes. This provides not only PCO2 and pH, but also the ionic concentrations of carbonate and bicarbonate. The pH change induced in the model ocean is in fact very small. Even for much larger releases than that considered here, this remains true. This can be illustrated simply by consideration of a direct example. The mean depth of the Atlantic is rather greater than 3kin, and the volume of ocean water below this depth is perhaps 1.7 x 108 cubic kilometres. Since a cubic kilometer of seawater contains about 3 x 1010 grams of carbon, the deep Atlantic contains about 5 x 1018 grams of natural inorganic carbon, mainly as bicarbonate ion. This compares with an annual fossil fuel burn of 5 x 1015 grams of carbon, wordwide. Thus, if the entire world production of carbon dioxide derived from fossil fuel could be dispersed in the deep Atlantic, the net annual addition would be only about 0.1%. The consequent pH fall can be estimated. The system is buffered:
630
WILSON:
DEEP OCEAN DISPOSAL OF CO 2
(i)
CO2 + H20=H2CO3 = H + + HCO3" = 2H+ + CO3"
The addition of carbon dioxide causes a fall in pH which reduces the carbonate to bicarbonate ion concentration ratio. The fall in pH consequent upon the dispersion of the entire world annual production of fossil fuel derived carbon dioxide is about 0.005 pH units. The fall in carbonate ion concentration has the effect of raising the lysocline, the level below which significant calcite solution occurs at the sediment water interface. The consequent increased rate of calcite dissolution has the effect of further buffering the pH change and of increasing alkalinity. Both of these effects tend to reduce the pCO2 exhibited by the deep water when it eventually returns to the sea surface. The average time for this to occur is of the order of I000 years. In the final model run (Fig. 4) the solution of seabed calcite was assumed to increase at a rate proportional to the change in pH from its inital value. This is a fairly arbitrary assumption, since tittle is known about the response of deep ocean calcite to undersamrated conditions (Emerson and Archer, 1990; Wilson and Wallace, 1990). However, the release of carbon dioxide would undoubtedly cause the calcite lysocline to rise, so that sediments previously bathed in saturated seawater would become exposed to undersaturation. Hence, there is a direct link between the pH change of bottom water and the area of sediments newly exposed to undersamrated seawater in consequence. It will be seen that the long-term effect of this additional process is to reduce the atmospheric concentration of carbon dioxide still further, to about 50% of the level which is produced by ocean circulation acting alone.
~
Atmospheric CO2 conch. (ppmv)
279.5
here
279.5
Release to box 1
278.5
!
1000
Time (yrs)
278.5 2000
Fig. 2 Release to atmosphere and to high latitudesurface box compared Atmospheric CO2 conch.
(ppmv) 279.5
279.5
sphere
/ 278.5
no carbonate solution assumed
)
Release
period
1000
Time (yrs)
250 yr
Fig 3 Release to atmosphere and to deep ocean (box 3) compared
278.5 2000
WILSON:
DEEP OCEAN DISPOSAL OF CO 2
631
Atmospheric C02 conch.
(ppmv) • 279.5
279.5 ere
carbonate solution assumed 278.5
|
0
1000
Time (yrs)
278.5 2000
Fig 4 As Fig. 3, but carbonate solution proportional to pH change added in box 3 The estimates presented here are in broad agreement with the results of a more sophisticated model of ocean-sediment interaction developed by Sundquist (1986). This predicts that the effects of calcite solution will eventually reduce long-term atmospheric enrichment to about 50% of the enrichment which would occur without this mechanism. Theae are considerable uncertainties in this estimate. It appears, for instance, that deep ocean carbonate solution rates are much slower in situ than would be estimated from laboratory measurements on recovered material (F~merson and Archer, 1990;, Wilson and Wallace, 1990). The reasons are unknown.
Even this is probably a conservative estimate of the efficiency of deep ocean disposal in reducing atmospheric contamination. The assumption of instantaneous equilibration ignores the possibilty of using release to restricted topographic basins (Whitmarsh et al, 1992). Further, the effects of clathrate formauon are ignored. Figure 5 is a phase diagram of the carbon dioxide/water system. It will be seen that the range of deep ocean temperatures and pressures coincides with the regton in which carbon dioxide (liquid) is m equilibrium with carbon dioxide (hydrate). It follows that the mixing of liquid carbon dioxide with sea water will tend to produce hydrate. This solid is apparently slightly denser than sea water (Sakai, 1990), and will thus sink. If free turbulant circulation of bulk sea water is maintained around the hydrate, it will tend to disassociate to produce seawater with dissolved carbon dioxide. If, however, the hydrate mass is protected from this interaction, it could be stable over long periods. Indeed, if the hydrate was formed within deep ocean sediments, it would be very effectively isolated from the rest of the ocean environment. Using the analogy of natural methane hydrates, it might be expected to remain buried indefinitely. These characteristics of the system suggest alternative approaches which might be used to improve the effectiveness of the simple approach adopted in the model study. If the carbon dioxide and seawater were pre-equilibrated at high pressure before release, a relatively dense solution of carbon dioxide in seawater could be produced. The pre-equllibration temperature would need to be above the temperature of hydrate stability (Fig. 5), to avoid the formation of hydrate which might block the equilibration equipment.. If released below the depth of bubble formation (dependent on equilibration pressure), this heavy solution would tend to sink. If in contact with the seafloor it would flow downslop¢, tending to turn right (in the Northern hemisphere) under the influence of the Coriolis force. In this manner, the release point could be within a few hundred metres of the ocean surface without incurring a high risk of rapid return transport to the atmosphere. If such a sinking solution was arranged to run into a topographic depression, the relatively low turbulance and the formation of hydrate at low temperatures and high pressure might well combine to delay return to the atmospheae for considerably longer than suggested by the model. Such delay is desirable not only because it defers any atmospheric effect, but because it allows more time for the reaction with carbonate sediments to increase ocean alkakinity and thus the eventual pCO2 increase However, it would be important that hydrate formation should not occur soon after release at a shallow depth. As figure 5 shows, hydrate dissociation will occur if the hydrate is washed continuously with clean seawater, as this will remove the CO2 (liquid) in equilibrium. If this occured above the main thermocline (say<1000m depth), the model presented here indicates that the carbon dioxide would return to the atmosphere relatively quickly.
632
WILSON: DEEP OCEAN DISPOSAL OF CO,
A second means to release close to the ocean surface with safety might be to release the carbon dioxide in solid form. To avoid excessive losses it would be desirable to rninimise the surface to volume ratio and to arrange for the highest practical sinking rate. Large blocks shaped to have a low drag would meet these requirements. There would be an additional benefit if these blocks could be made to penetrate some metres into the sediment, since the subsequent formation of hydrate within the sediment would tend to cement the sediment structure into an immobile mass. The close analogy of methane hydrate traps suggests that escape would then be very slow indeed.
2000
Ocean depth (metres)
CO2 hydrate + CO2 liquid
| | | |
1000
•
-
Water+ CO2 liquid
| |
|
Critical point
| I W
,mO
... i
Hydrate + CO2 (gas) ~ -Ice + C02 (gas) f
I"o'
lb
ab 4b
Temperature (deg C) Fig. 5
Phase diagram of carbon dioxide/water system
CONCLUSIONS 1. The release of carbon dioxide into the atmosphere produces a transient peak in atmospheric carbon dioxide concentration. This is because the rate of passage of carbon dioxide into the deep ocean reservoir is slow. 2. Release of carbon dioxide directly into the deep ocean avoids this transient. Instead, the atmospheric concentration rises over several hundred years to a value determined by the ratio of the quantity of carbon released to the size of the total atmosphere/ocean carbon reservoir. 3. The above conclusions assume that no interaction takes place with other carbon reservoirs. In fact, deep ocean carbonate sediments will dissolve as ocean pH falls. The effect of this cannot be accurately quantified at present, but is likely to be very significant in reducing the final atmospheric carbon dioxide concentration still further. 4. There are a number of strategies which might be adopted to delay or prevent the return of released carbon dioxide. These depend on delaying or preventing the equilibration of the released material with the natural carbon reservoirs, and are based on physical isolation, sequestering in carbon dioxide hydrate masses, or some combination of these.
WILSON: DEEPOCEAN DISPOSALOF CO,
633
This presentation is necessarily speculative. Quite apart from the complex engineering and economic questions which are not yet answered, there are a number of important areas of environmental uncertainty which will need considerable further research to clarify. Among these is the response of the calcite of deep sediments to undersaturation, the effect of pH variation on deep ocean life, the behaviour of complex multi-phase plumes, and the kinetics of hydrate formation and dissolution under deep-ocean conditions. There is, however, one point which is already clear: the carbon dioxide we are releasing now will find its way into the deep oceans. Whatever release method is used, this remains true. Hence, the impact of this material on the deep ocean environment should be a subject of careful study whatever method of carbon dioxide release is adopted
ACKNOWLEDGEMENTS Dissussions with several colleagues have contributed to this presentation. Contract support by the Coal Research Establishment, British Coal (CRE/CON 1169), is gratefully acknowledged. This paper is Institute of Oceanographic Sciences Deacon Laboratory contribution number 92006.
REFERENCES Clark W.C. (ed) (1982).Carbon Dioxide Review 1982. 469pp.Clarendon Press, Oxford. OUP, New York. Emerson, S.R. and Archer, D. (1990). Calcium Carbonate preservation in the ocean. Phil. Trans. Roy. Soc. Lond. A331.29-40 Ennever, F.K. and McElroy, M.B. (1985). Changes in atmospheric CO2: factors regulating the glacial to interglacial transition, in The Carbon cycle and atmospheric C02: natural variations Archean to present. (E.T. Sundquist & W.S. Broecker, eds) AGU Geophysical Monograph 32. 154-162 Hoffert, M.J., Wey, Y-C, Callegari, A.J., and Broecker, W.S. (1979) Atmospheric response to deep-sea injection of fossil-fuel carbon dioxide. Climatic Change, 2. 53-68 Marchetti, C. (1977) On geoengineering the CO2 problem. Climatic Change, i , 59-68. Sakai, H., Gamo, T., Kim, E-S., Tsutsumi, M., Tanaka, T., Ishibashi, J., Wakita, H., Yamano, M. & Oomori, T. 1990 Venting of carbon dioxide-rich fluid and hydrate formation in mid-Okinawa Trough Back-arc Basin. Science, 248(4959). 1093-1096. Sunquist, E.T. (1986) Geologic analogs: their value and limitations in carbon dioxide research, in The Changing Carbon Cycle, a Global Analysis. (J.R.Trabalka. and D.E Reichle, eds.) New York, SpringerVerlag. 371-402 Toggweiler J.R. and Sarmiento, J.L. (1985). Glacial to interglacial changes in atmospheric carbon dioxide: the critical role of ocean surface water in high latitudes, in The Carbon cycle and atmospheric C02: natural variations Archean to present. (E.T. Sundquist & W.S. Broecker, eds) AGU Geophysical Monograph 32._163-184 Wenk T and Siegenthaler U. (1985). The high-latitude ocean as a control of atmospheric CO2 inThe Carbon cycle and atmospheric C02: natural variations Archean to present. (E.T. Sundquist & W.S. Broeeker, eds) AGU Geophysical Monograph 32.185-194 Whitmarsh R.B., Hunter, P.M. and Kenyon, N.H. (1992). Topographic implications of ocean disposal of carbon dioxide. Report prepared for British Coal under contract CRE./CON 1169 by IOSDL 42pp + charts. Wilson, T.R.S. and Wallace, H.(1990). The rate of dissolution of calcium carbonate from the surface of deep-ocean turbidite sediments. Phil. Trans. Roy. Soc. Lond. A331 .41-49.
0196-8904/92 $5.00+0.00 Pergamon Press Ltd
Printed in Great Britain
THE DEEP OCEAN DISPOSAL OF CARBON DIOXIDE
T. R. S. WILSON Institute of Oceanographic Sciences Deacon Laboratory, Brook Road, Wormiey, Godalming, Surrey GU8 5UB, England.
ABSTRACT Once released to the environment, carbon dioxide equilibrates within the ocean-atmosphere pool. Because the mixing of surface seawater is relatively slow, release to the atmosphere causes a transient peak in atmosphere carbon dioxide concentration. If the release could be transferred to the ocean, this transient might be avoided. In addition, interaction with the large carbon pool contained within the ocean sediment would be accelerated. The resultant increase in ocean alkalinity would tend to reduce equilibrium PCO2 values. The dynamics of the processes involved are complex and not yet understood in detail. This paper reviews deep ocean carbonate chemistry and mixing, and estimates their relative importance for the prediction of atmospheric carbon dioxide concentrations resulting from deep ocean release. Predictions based on mathematical modelling suggest that disposal into the surface ocean ( < 1 km depth) would permit equilibration with the atmosphere within a few years to decades and would therefore offer little advantage. Disposal into ocean basins greater than 3km in depth would delay equilibration with the atmosphere for several hundred years, eliminating the atmospheric concentration transient. Resultant interaction with calcite-rich sediments could probably reduce the long-term ( > 2000 year) atmospheric enrichment by a significant amount (~ 50%) KEYWORDS Carbon dioxide; Ocean mixing; PCO2 : pH; CO2-hydrate, calcite, calcium carbonate, fossil fuel INTRODUCTION Since the discovery of fire, mankind has utilised the atmosphere to carry away the waste products of combustion. Modem industrial practices, base on the use of fossil fuel, may be altering atmospheric composition to an unacceptable degree. It is prudent to consider ahemative strategies of disposal. Marchetd (1978) suggested that sinking thermohaline currents might be used to carry waste carbon dioxide to abyssal depths in the ocean. Since disposal to the atmosphere eventually results in the equilibration of the disposed carbon dioxide within the oceanic pool of dissolved inorganic carbon, this strategy in effect short-circuits the slow equilibration and sinking process which would normally operate. As the atmospheric carbon reservoir (720 Gigatonnes; Clark, 1982) is much smaller than the deep ocean pool (37000 Gigatonnes), the perturbation of carbon dioxide concentration should be much reduced. BOX MODEL The circulation of the worlds oceans is characterised by sinking at high latitudes of surface water which has become denser by cooling, and the circulation of this ventilating water throughout the interconnected deep ocean basins. Upward mixing of this water eventually results in its return to the surface, predominantly at low latitudes, by upwelling currents originating at intermediate depths. The cycle is completed by surface transport polewards. LaTge natural fluxes of carbon dioxide to the atmosphere occur at low and intermediate latitudes, balanced in natural circumstances by fluxes from the atmosphere to the
627
628
WILSON: DEEPOCEAN DISPOSALOF CO2
ocean as it cools at higher latitudes. Thus, the carbon pools of the ocean and atmosphere are mixed on a timescale appropriate to this circulation. In order to illustrate this process, a box model of the ocean-atmosphere system (Figure 1) was used to compare the effects of differing disposal strategies. This model incorporates the advective processes which characterise the slow circulation of the deep oceans. Nutrient concentration, total inorganic carbon and alkalinity are modelled, and the calculated pCO2 of the surface boxes determines the model atmospheric carbon dioxide concenwation. The boxes are assumed to be completely mixed internally, while transfer between boxes is by advection and by the production, sinking and dissolution of organic and inorganic carbon particles. The rate at which these particles are produced is a function of surface nutrient levels, which in turn are controlled by circulation and by river run-off. Table 1 lists the parameters which were used for the model runs discussed in the present paper. These represent the current best estimates of water flux, and are similar to values previously used in recent box models (eg Ennever and McElroy, 1985: Toggweiler and Sarmiento, 1985: Wenk and Siegenthaler, 1985)
Aunosphere
Particulate sinking
I
Box 0 I I
(Surface, low to m e d i u m latitude)
'
Box 2 (Intermediate)
t
[
~4~ Box 1 (High
~ Water exchange
Box 3
~
latitude)
I
(Deep)
Fig 1. Schematic of box model. Solid lines indicate water exchanges, dashed lines particulate.
Table 1. Model s u m m a r y
Box number: Fractional volume Fractional surface area Temperature
0 0.08 0.8 19
Flow field (Sverdrups) Flow from box: To box: 0 0 0 1 0 2 20 3 0
1 0.02 0.2 2
2 0.1 0 2
3 0.8 0 2
1 15 0 37 0
2 5 0 0 252
3 0 52 200 0
Notes: 1. The nutrient concentration, total carbon dioxide and alkalinity fields are modelled. 2. The rate of production of particulates in box 0 is controlled by the nutrient concentration. River nutrient flux 3.5 x 1010 moles/annum is balanced by burial. 3. pCO2 of boxes 0 and I is caloulated at each timestep, and controls the model atmosphere via empirically-determined transfer coefficients. 4. One Sverdrup = 106 m 3 sec-1
WILSON: DEEP OCEAN DISPOSALOF CO2
629
The model does not incorporate detailed plume behaviour, as it is intended to study medium to long-term effects. It is worth noting, however, that the injection of liquid carbon dioxide into the ocean at depths less than 2 km would be likely to produce a rapidly rising plume, as liquid carbon dioxide is significantly less dense than seawater at these depths. Rising velocities of several tens of centimetres per second have been calculated for such plumes (P.Killworth, personal communication.) Their detailed behaviour is very complex; solid carbon dioxide hydrates and dense carbon dioxide-saturated seawater would be formed and would tend to separate from the rising plume and sink. The rates and magnitudes of these side-processes are difficult to calculate. However, it is clear that the overall effect would be to mix the released carbon dioxide into the receiving water mass in a complex manner. In the present treatment, to set the scene, the conservative assumption has been made that this mixing is instantaneous and complete. Other papers presented at this symposium will discuss the likely influence of hydrate formation and the behaviour of dense, CO2-saturated seawater, which both tend to enhance the retention of released CO2 within the ocean. The model ocean was driven until steady-state was achieved, with a stable atmospheric carbon dioxide concentration corresponding to pre-industrial levels. This provided the time-zero configuration. Starting from this point, runs were then performed in which injections of 100 million tonnes per year of carbon dioxide were made. These injections continued at constant rate for 250 years, and then ceased. (100 million tonnes of carbon dioxide is approximately half the carbon dioxide produced from coal-fired electricity production per year in the UK). The model was then allowed to run until a new steady-state was reached. The point of injection was varied from run to run. The model timestep was 1 year Table 2 Variations studied: 1. No calcite solution. Release to:
atmosphere surface layer (< 200 metres) high latitude surface layer intermediate layer deep water (> I000 metres)
2. With calcite solution. Release to deep water (> 1000 metres)
Examples of these runs are shown in Figs. 2 to 4 below. Release into surface (box 0) produced atmospheric enrichment which was not significantly different from that produced by direct atmospheric release. Release to the high-latitude surface box produces an intermediate result with a diminished but still significant atmospheric enrichment (Fig. 2). Clearly, this result would be very sensitive to the point of release, since not all high-latitude water is actively sinking, and release directly to a sinking water mass might be more effective than is suggested by Fig. 2. However, the most effective isolation is obtained using release to the deep ocean (Fig. 3). These results agree broadly with those of Hoffert et al (1979), in showing that carbon dioxide released to the deep ocean would not contribute to the atmospheric transient peak. Shallow release of liquid carbon dioxide would result in a rapidly rising plume of fluid, and even if this did not break through into the surface layer, significant transport back to the atmosphere across the seasurface would occur quite rapidly. There are two possible exceptions to this generalisation, which are discussed below. Intermediate-depth disposal might be effective if confined to areas of sinking intermediate water, using a strategy similar to that suggested by Marchetti (1978). In general, however, the model suggests that release to the deep ocean is the most effective. An advantage of the present model is that the full carbonate system speciation can be calculated for all boxes. This provides not only PCO2 and pH, but also the ionic concentrations of carbonate and bicarbonate. The pH change induced in the model ocean is in fact very small. Even for much larger releases than that considered here, this remains true. This can be illustrated simply by consideration of a direct example. The mean depth of the Atlantic is rather greater than 3kin, and the volume of ocean water below this depth is perhaps 1.7 x 108 cubic kilometres. Since a cubic kilometer of seawater contains about 3 x 1010 grams of carbon, the deep Atlantic contains about 5 x 1018 grams of natural inorganic carbon, mainly as bicarbonate ion. This compares with an annual fossil fuel burn of 5 x 1015 grams of carbon, wordwide. Thus, if the entire world production of carbon dioxide derived from fossil fuel could be dispersed in the deep Atlantic, the net annual addition would be only about 0.1%. The consequent pH fall can be estimated. The system is buffered:
630
WILSON:
DEEP OCEAN DISPOSAL OF CO 2
(i)
CO2 + H20=H2CO3 = H + + HCO3" = 2H+ + CO3"
The addition of carbon dioxide causes a fall in pH which reduces the carbonate to bicarbonate ion concentration ratio. The fall in pH consequent upon the dispersion of the entire world annual production of fossil fuel derived carbon dioxide is about 0.005 pH units. The fall in carbonate ion concentration has the effect of raising the lysocline, the level below which significant calcite solution occurs at the sediment water interface. The consequent increased rate of calcite dissolution has the effect of further buffering the pH change and of increasing alkalinity. Both of these effects tend to reduce the pCO2 exhibited by the deep water when it eventually returns to the sea surface. The average time for this to occur is of the order of I000 years. In the final model run (Fig. 4) the solution of seabed calcite was assumed to increase at a rate proportional to the change in pH from its inital value. This is a fairly arbitrary assumption, since tittle is known about the response of deep ocean calcite to undersamrated conditions (Emerson and Archer, 1990; Wilson and Wallace, 1990). However, the release of carbon dioxide would undoubtedly cause the calcite lysocline to rise, so that sediments previously bathed in saturated seawater would become exposed to undersaturation. Hence, there is a direct link between the pH change of bottom water and the area of sediments newly exposed to undersamrated seawater in consequence. It will be seen that the long-term effect of this additional process is to reduce the atmospheric concentration of carbon dioxide still further, to about 50% of the level which is produced by ocean circulation acting alone.
~
Atmospheric CO2 conch. (ppmv)
279.5
here
279.5
Release to box 1
278.5
!
1000
Time (yrs)
278.5 2000
Fig. 2 Release to atmosphere and to high latitudesurface box compared Atmospheric CO2 conch.
(ppmv) 279.5
279.5
sphere
/ 278.5
no carbonate solution assumed
)
Release
period
1000
Time (yrs)
250 yr
Fig 3 Release to atmosphere and to deep ocean (box 3) compared
278.5 2000
WILSON:
DEEP OCEAN DISPOSAL OF CO 2
631
Atmospheric C02 conch.
(ppmv) • 279.5
279.5 ere
carbonate solution assumed 278.5
|
0
1000
Time (yrs)
278.5 2000
Fig 4 As Fig. 3, but carbonate solution proportional to pH change added in box 3 The estimates presented here are in broad agreement with the results of a more sophisticated model of ocean-sediment interaction developed by Sundquist (1986). This predicts that the effects of calcite solution will eventually reduce long-term atmospheric enrichment to about 50% of the enrichment which would occur without this mechanism. Theae are considerable uncertainties in this estimate. It appears, for instance, that deep ocean carbonate solution rates are much slower in situ than would be estimated from laboratory measurements on recovered material (F~merson and Archer, 1990;, Wilson and Wallace, 1990). The reasons are unknown.
Even this is probably a conservative estimate of the efficiency of deep ocean disposal in reducing atmospheric contamination. The assumption of instantaneous equilibration ignores the possibilty of using release to restricted topographic basins (Whitmarsh et al, 1992). Further, the effects of clathrate formauon are ignored. Figure 5 is a phase diagram of the carbon dioxide/water system. It will be seen that the range of deep ocean temperatures and pressures coincides with the regton in which carbon dioxide (liquid) is m equilibrium with carbon dioxide (hydrate). It follows that the mixing of liquid carbon dioxide with sea water will tend to produce hydrate. This solid is apparently slightly denser than sea water (Sakai, 1990), and will thus sink. If free turbulant circulation of bulk sea water is maintained around the hydrate, it will tend to disassociate to produce seawater with dissolved carbon dioxide. If, however, the hydrate mass is protected from this interaction, it could be stable over long periods. Indeed, if the hydrate was formed within deep ocean sediments, it would be very effectively isolated from the rest of the ocean environment. Using the analogy of natural methane hydrates, it might be expected to remain buried indefinitely. These characteristics of the system suggest alternative approaches which might be used to improve the effectiveness of the simple approach adopted in the model study. If the carbon dioxide and seawater were pre-equilibrated at high pressure before release, a relatively dense solution of carbon dioxide in seawater could be produced. The pre-equllibration temperature would need to be above the temperature of hydrate stability (Fig. 5), to avoid the formation of hydrate which might block the equilibration equipment.. If released below the depth of bubble formation (dependent on equilibration pressure), this heavy solution would tend to sink. If in contact with the seafloor it would flow downslop¢, tending to turn right (in the Northern hemisphere) under the influence of the Coriolis force. In this manner, the release point could be within a few hundred metres of the ocean surface without incurring a high risk of rapid return transport to the atmosphere. If such a sinking solution was arranged to run into a topographic depression, the relatively low turbulance and the formation of hydrate at low temperatures and high pressure might well combine to delay return to the atmospheae for considerably longer than suggested by the model. Such delay is desirable not only because it defers any atmospheric effect, but because it allows more time for the reaction with carbonate sediments to increase ocean alkakinity and thus the eventual pCO2 increase However, it would be important that hydrate formation should not occur soon after release at a shallow depth. As figure 5 shows, hydrate dissociation will occur if the hydrate is washed continuously with clean seawater, as this will remove the CO2 (liquid) in equilibrium. If this occured above the main thermocline (say<1000m depth), the model presented here indicates that the carbon dioxide would return to the atmosphere relatively quickly.
632
WILSON: DEEP OCEAN DISPOSAL OF CO,
A second means to release close to the ocean surface with safety might be to release the carbon dioxide in solid form. To avoid excessive losses it would be desirable to rninimise the surface to volume ratio and to arrange for the highest practical sinking rate. Large blocks shaped to have a low drag would meet these requirements. There would be an additional benefit if these blocks could be made to penetrate some metres into the sediment, since the subsequent formation of hydrate within the sediment would tend to cement the sediment structure into an immobile mass. The close analogy of methane hydrate traps suggests that escape would then be very slow indeed.
2000
Ocean depth (metres)
CO2 hydrate + CO2 liquid
| | | |
1000
•
-
Water+ CO2 liquid
| |
|
Critical point
| I W
,mO
... i
Hydrate + CO2 (gas) ~ -Ice + C02 (gas) f
I"o'
lb
ab 4b
Temperature (deg C) Fig. 5
Phase diagram of carbon dioxide/water system
CONCLUSIONS 1. The release of carbon dioxide into the atmosphere produces a transient peak in atmospheric carbon dioxide concentration. This is because the rate of passage of carbon dioxide into the deep ocean reservoir is slow. 2. Release of carbon dioxide directly into the deep ocean avoids this transient. Instead, the atmospheric concentration rises over several hundred years to a value determined by the ratio of the quantity of carbon released to the size of the total atmosphere/ocean carbon reservoir. 3. The above conclusions assume that no interaction takes place with other carbon reservoirs. In fact, deep ocean carbonate sediments will dissolve as ocean pH falls. The effect of this cannot be accurately quantified at present, but is likely to be very significant in reducing the final atmospheric carbon dioxide concentration still further. 4. There are a number of strategies which might be adopted to delay or prevent the return of released carbon dioxide. These depend on delaying or preventing the equilibration of the released material with the natural carbon reservoirs, and are based on physical isolation, sequestering in carbon dioxide hydrate masses, or some combination of these.
WILSON: DEEPOCEAN DISPOSALOF CO,
633
This presentation is necessarily speculative. Quite apart from the complex engineering and economic questions which are not yet answered, there are a number of important areas of environmental uncertainty which will need considerable further research to clarify. Among these is the response of the calcite of deep sediments to undersaturation, the effect of pH variation on deep ocean life, the behaviour of complex multi-phase plumes, and the kinetics of hydrate formation and dissolution under deep-ocean conditions. There is, however, one point which is already clear: the carbon dioxide we are releasing now will find its way into the deep oceans. Whatever release method is used, this remains true. Hence, the impact of this material on the deep ocean environment should be a subject of careful study whatever method of carbon dioxide release is adopted
ACKNOWLEDGEMENTS Dissussions with several colleagues have contributed to this presentation. Contract support by the Coal Research Establishment, British Coal (CRE/CON 1169), is gratefully acknowledged. This paper is Institute of Oceanographic Sciences Deacon Laboratory contribution number 92006.
REFERENCES Clark W.C. (ed) (1982).Carbon Dioxide Review 1982. 469pp.Clarendon Press, Oxford. OUP, New York. Emerson, S.R. and Archer, D. (1990). Calcium Carbonate preservation in the ocean. Phil. Trans. Roy. Soc. Lond. A331.29-40 Ennever, F.K. and McElroy, M.B. (1985). Changes in atmospheric CO2: factors regulating the glacial to interglacial transition, in The Carbon cycle and atmospheric C02: natural variations Archean to present. (E.T. Sundquist & W.S. Broecker, eds) AGU Geophysical Monograph 32. 154-162 Hoffert, M.J., Wey, Y-C, Callegari, A.J., and Broecker, W.S. (1979) Atmospheric response to deep-sea injection of fossil-fuel carbon dioxide. Climatic Change, 2. 53-68 Marchetti, C. (1977) On geoengineering the CO2 problem. Climatic Change, i , 59-68. Sakai, H., Gamo, T., Kim, E-S., Tsutsumi, M., Tanaka, T., Ishibashi, J., Wakita, H., Yamano, M. & Oomori, T. 1990 Venting of carbon dioxide-rich fluid and hydrate formation in mid-Okinawa Trough Back-arc Basin. Science, 248(4959). 1093-1096. Sunquist, E.T. (1986) Geologic analogs: their value and limitations in carbon dioxide research, in The Changing Carbon Cycle, a Global Analysis. (J.R.Trabalka. and D.E Reichle, eds.) New York, SpringerVerlag. 371-402 Toggweiler J.R. and Sarmiento, J.L. (1985). Glacial to interglacial changes in atmospheric carbon dioxide: the critical role of ocean surface water in high latitudes, in The Carbon cycle and atmospheric C02: natural variations Archean to present. (E.T. Sundquist & W.S. Broecker, eds) AGU Geophysical Monograph 32._163-184 Wenk T and Siegenthaler U. (1985). The high-latitude ocean as a control of atmospheric CO2 inThe Carbon cycle and atmospheric C02: natural variations Archean to present. (E.T. Sundquist & W.S. Broeeker, eds) AGU Geophysical Monograph 32.185-194 Whitmarsh R.B., Hunter, P.M. and Kenyon, N.H. (1992). Topographic implications of ocean disposal of carbon dioxide. Report prepared for British Coal under contract CRE./CON 1169 by IOSDL 42pp + charts. Wilson, T.R.S. and Wallace, H.(1990). The rate of dissolution of calcium carbonate from the surface of deep-ocean turbidite sediments. Phil. Trans. Roy. Soc. Lond. A331 .41-49.