The Carbon Cycle C02 And Carbonates

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Carbon is the key element in biological systems, and as carbon dioxide and carbonates, makes up an important aqueous acid-base system. Carbon dioxide is the beginning and the end product of biological processes, and its presence in the atmosphere influences climate (Chapter 3). The biogeochemical carbon cycle is of prime importance. The carbon cycle involves three reservoirs, which can act as sinks by absorbing carbon, or as sources by releasing it. They are the atmosphere, which contains about 7.5 x 1015 kg of carbon almost entirely as carbon dioxide; the terrestrial reservoir, which contains living organisms as well as organic material and inorganic carbonate in the soils and rocks; and the oceans, where most of the carbon exists as the bicarbonate ion, in principle in equilibrium with dissolved carbon dioxide and the carbonate ion, but also with a biological component.

Table 10-2 is one estimate of the relative distribution of carbon in nature. Most carbon is present as carbonate rock or as organic sediment material (including oil and coal), the greater portion of which is inaccessible. The material in the atmosphere and in water is more readily available for environmental and biological activities. The carbon is involved in biological and geological processes that can be represented by the cycle given in Figure 10-2.2 There is exchange among the carbon reservoirs, on the time scale of years for some components, centuries and even longer for others. As discussed in Chapter 2, although there have been significant variations in the carbon dioxide content of the atmosphere in the past (generally correlated with temperature), values over the past 10,000 years have been relatively stable near 280 ppm by volume. Over the last few decades, however, the CO2 content has increased markedly, beyond anything since 130,000 years ago and at an unprecedented rate. Most of this increase has been ascribed to anthropogenic sources.

2W. M. Post, T.-H. Peng, W. R. Emanuel, A. W. King, V. H. Dale and D. L. DeAngeles, Am. Sci., 78, 310 (1990), discuss the global carbon cycle.

TABLE 10-2

Approximate Distribution of Carbon in Nature Relative to Atmospheric CO2

Source Relative amount of carbon

TABLE 10-2

Approximate Distribution of Carbon in Nature Relative to Atmospheric CO2

Source Relative amount of carbon

CO2 in the atmosphere

1

Total dissolved inorganic carbon

49.5

As H2CO3

0.3

As HCO-

44

As CO2-

5

Dissolved organic carbon

1.3

Particulate organic carbon

0.04

Terrestrial plants

0.56-1.1

Carbon in soils

1.6-2.1

Estimate of recoverable fossil fuels

5.3

Organic sediments

13,000

Inorganic sediments (carbonates)

65,000

Source: Data from Post et al., Am. Sci., 78, 310 (1990), and other references.

Source: Data from Post et al., Am. Sci., 78, 310 (1990), and other references.

The data in Figure 10-2 are taken largely from Post et al., with values for some reservoirs as given in Climate Change 1995, published for the Intergovernmental Panel on Climate Change (IPPC).3 The fluxes given in the latter differ slightly from those given here, but are generally within the uncertainty range. There are in fact considerable uncertainties in some values, and fluxes in particular are known to fluctuate considerably with time.

Exchange of carbon dioxide between the atmosphere and the terrestrial reservoir involves absorption through photosynthesis and release through respiration and decay. Air-ocean exchange is of comparable magnitude, and involves release of C02 to the atmosphere at some times and places and removal through solution at others. Much deep water is supersaturated by surface standards and can be a source of atmospheric carbon dioxide if the deep water reaches the surface. For example, upwelling in the central Pacific typically makes this area a source rather than a sink for atmospheric C02. 0cean-atmosphere exchange is thus dependent on ocean circulation and surface-deep water mixing patterns. The response time, or the rate at which equilibrium can be achieved after a perturbation, is also important. This may be a few years for surface water (i.e., to depths < 100 m), but on the order of 1000 years for deeper ocean layers that make up well over 90% of the total.

3J. T. Houghton, L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell, eds., Climate Change 1995: The Science of Climate Change. Cambridge University Press, Cambridge, 1996. The standard reference work in the area of climate change, published for the Intergovernmental Panel on Climate Change.

FIGURE 10-2 The carbon cycle: Quantities in 1015 grams of carbon, fluxes in 1015 grams of carbon per year. DIC, and dissolved inorganic carbon; DOC, dissolved organic carbon; POC, particulate organic carbon. Data from Post et al., Am. Sci., 78, 310 (1990), and Houghton et al., eds., Climate Change 1995, Cambridge University Press, Cambridge, U.K., 1996.

FIGURE 10-2 The carbon cycle: Quantities in 1015 grams of carbon, fluxes in 1015 grams of carbon per year. DIC, and dissolved inorganic carbon; DOC, dissolved organic carbon; POC, particulate organic carbon. Data from Post et al., Am. Sci., 78, 310 (1990), and Houghton et al., eds., Climate Change 1995, Cambridge University Press, Cambridge, U.K., 1996.

Most of the ocean carbon is in the deep water, and some small fraction of this is transferred to sediments, where it is sequestered virtually permanently. About 3 x 1015 g of C is estimated to be in marine biota.

IPCC estimates of annual perturbations to the natural cycle averaged over the 1980-1989 decade, indicate that a total of 7.1 x 1015 g of C is released by anthropogenic sources; 5.5 x 1015g from fossil fuels and cement production, and 1.6 x 1015 g from changes in tropical land use, primarily destruction of forests. Of these emissions, the ocean takes up about 2 x 1015 g of C, Northern Hemisphere forest regrowth takes up another 0.5 x 1015 g, and other terrestrial sinks (increased plant growth from fertilization and other effects) 1.3 x 1015 g. About 3.3 x 1015 g of C is left in the atmosphere.

Take-up by the biosphere is dependent on the partial pressure of CO2, since many (but not all) plants increase their growth at higher CO2 pressures. This suggests that some buffering of atmospheric increases may come from the biosphere. The extent of this such action unknown, particularly in as much as other nutrients often are limiting. Experiments on some forest trees have shown that they respond to an increase in carbon dioxide with a period of rapid woody growth, but this ends relatively quickly. Arguments pointing out various advantages of a CO2 increase have been proposed—primarily reduced water transpiration and increased food production. Countering these possible benefits are the disadvantages that might come from associated effects such as climate change.

As mentioned, many uncertainties remain about the sizes of some reservoirs in the carbon cycle and about turnover rates. The estimates concerning terrestrial carbon are confused by uncertain land use data. In addition, there are indications that dissolved organic carbon in the oceans has been underestimated, and many of the transport rates are known very poorly. For example, the amount of carbon as CH4 trapped in clathrates as discussed later in this section is uncertain. These uncertainties are one of the factors that make greenhouse and other climate predictions difficult (Section 3.3).

From what has been discussed here and in Chapter 9, it is clear that the increase in atmospheric carbon dioxide the earth is now undergoing should be offset in part by the dissolution and neutralization reactions that it can undergo. Archer et al.4 have modeled the processes to estimate the fate of a sudden injection of CO2 into the atmosphere, considering the rates of the various processes. Their results suggest that the ocean should absorb 70-80% of such an input in 200-450 years. Reactions of seafloor CaCO3 should take up another 10-15% after 5500-6800 years. The same investigators estimate that 5-8% would then remain in the atmosphere, but that too would be taken up by reactions with other basic rocks in weathering reactions of the types discussed in Section 12.3.15 after 200,000 years. These time periods could change if climate changes alter ocean circulation and mixing patterns, or with changes in the biological activity that is largely responsible for mixing in the sediments that influence reaction with the solid carbonates, for example. At any rate, these times are long in human terms.

With growing concerns about the increase of atmospheric carbon dioxide, proposals have been made for sequestering some of this. Sequestration may involve removal of CO2 from the atmosphere after it has been released, or separating CO2 from an exhaust gas stream and dealing with it in concentrated form. Five proposals are summarized briefly.

4D. Archer, H. Kleshgi, and E. Maier-Reimer, Geophys. Res. Lett., 24, 405 (1997).

5Essentially, reactions with Ca and Mg silicates to give the carbonates.

1. Increase forest growth, with the assumption that the wood produced will remain intact for some long period of time. Forests can be carbon dioxide sinks, but under natural circumstances will reach an equilibrium with CO2 released upon decay (or combustion) equal to that absorbed. Only wood that is prevented from decaying can be a long-term trap, but in the short term, significant CO2 might be removed from the atmosphere by increased forested areas.

2. Increase soil carbon through incorporation of vegetation. As with forest growth, equilibrium will eventually be reached.

3. Increase ocean biological uptake by plankton. As discussed later (Section 10.6.3), the growth of plankton can be significantly increased in some parts of the ocean by suitable fertilization. Some of the carbon taken up in their growth would be incorporated into marine sediments, but it is difficult to estimate how much, or to judge what other environmental consequences might result from the growth and decay of high concentrations of these organisms. Fertilization would have to be an on-going process over large areas of the ocean.

4. Injection of separated CO2 into locations where it would be isolated and stable. There are at least three possibilities.

(a) Exhausted petroleum fields. CO2 is already injected into oil fields to enhance recovery by dissolving in the oil and reducing its viscosity. Much of this CO2 is released, but a significant fraction remains. If no oil recovery is attempted, none would be released.

(b) Very deep saline aquifers, where the CO2 would be above critical pressure. About 600,000 metric tons removed from natural gas is being injected into an aquifer under the North Sea. It is estimated that this aquifer alone has the capacity to store 400 years' worth of CO2 production from all the European power stations.

(c) Deep sea locations, also under supercritical conditions, where the CO2 would remain an insoluble liquid or solid.6 Below 3000 m in the ocean, liquid CO2 is more dense than water and sinks; under these conditions, some of it also forms a solid hydrate, similar to methane hydrate discussed shortly. There seem to be few data on potential effects of slow release of CO2 at high local concentrations from such sinks.

5. Not sequestration, but recycling of carbon to reduce the rate of increase of CO2 in the atmosphere is another alternative. Use of renewable fuels (e.g., alcohol derived from plants as a motor fuel), is one such example. A second possibility is the use of bioengineered bacteria to convert CO2 to useful fuel such as methane. Nonbiologically, catalysts that could economically allow reduction of CO2 to more useful compounds could achieve the same result.

6P. G. Brewer, G. Friederich, E. P. Peltzer, and F. M. Orr Jr., Science, 284, 943 (1999).

Essentially all the transformations in the carbon cycle involve CO2 or, in aqueous media, carbonate or bicarbonate ions. However, incomplete combustion of reduced forms of carbon produces small amounts of carbon monoxide, especially from internal combustion engines, but much is also produced from oxidation of methane and other hydrocarbons, and some from natural microorganisms and vegetation.7 The natural level of CO in the atmosphere is uncertain, but is probably under 0.lppm (see Table 2-1). This can be exceeded locally, but the lifetime of CO in the atmosphere is short. A variety of chemical and photochemical reactions, but primarily reaction with the hydroxyl radical, convert CO to CO2, and soil organisms provide another sink.

The toxicity of carbon monoxide is well known and is the cause for the concern over high carbon monoxide concentrations that can develop in areas of heavy automobile traffic. The effects of chronic, sublethal CO levels on health are not fully understood. Higher concentrations such as can build up in a closed room through a faulty space heater, or in a closed car through a leaking exhaust system, can quickly be fatal. Carbon monoxide is toxic through attachment to the coordination site of the iron atom in hemoglobin (see Section 9.5.7).

Biological processes release more complex organic molecules to the atmosphere. Some of these can have significant environmental consequences, but in terms of the carbon budget they are not important. Other organic molecules may be dissolved in seawater (referred to as dissolved organic carbon, DOC). Undissolved organic materials (particulate organic carbon, POC) play a role in carbon transport processes between surface and deep waters as the particles sink. Biological pumping refers to the process in which photosynthesis in marine organisms in surface waters results in the production of biomass (and also inorganic carbonates such as foraminifera shells) that eventually falls to lower depths. Much of the organic material decays and is recycled, but some fraction may enter the sediments.

The cyanide ion CN~, occurring as hydrogen cyanide (hydrocyanic acid, HCN), and salts such as KCN, is also unimportant in the overall carbon cycle but has local significance. The cyanide ion is a very strong ligand for many metal ions, often forming soluble complexes with them. It does have some natural sources (e.g., apricot pits), but it is more important in industrial applications such as its use in mining. Several major releases of cyanide-containing water to rivers have taken place with disastrous consequences on wildlife and are discussed further in Section 12.2.6. The toxic cyanide ion is quickly oxidized to less toxic products, mostly cyanate, CNO~, so the direct effects from it are short term. Indirect effects from the heavy metals that

Global atmospheric CO is estimated to amount to between 1400 and 3700 x 1012 g/yr, with less than 25% each from biomass and fossil fuel burning and about half from oxidation of other hydrocarbons.

FIGURE 10-3 The structure of methane hydrate.

accompany such releases may be longer lasting. Deliberate use of cyanide compounds to incapacitate fish by some third world fishermen is another cause of environmental damage.

Methane is a small but potentially important component of the atmosphere because it is a greenhouse gas, and its concentration in the atmosphere, although small, has been increasing (Section 2.2). Approximately one-third of the total methane emissions are anthropogenic, with the decreasing order of importance being rice production, cattle growing, waste systems, natural gas losses, burning of biomass, and coal mining. Natural sources are dominated by release from anaerobic processes in swamps, sediments, and other anoxic locations.

One potentially large reservoir of methane exists in the form of gas hydrates in ocean sediments and permafrost.8 The open structure of ice was described in Section 9.1. In the presence of small molecules such as methane, water can crystallize in a structure of linked pentagons containing cavities in which the molecule is trapped. Such combinations are called clathrates, or cage compounds. The solid, which resembles ordinary ice, can form at a few degrees above 0°C if the pressure is high enough. This structure of solid water, based on hydrogen bonding as in normal ice, owes its stability to the weak van der Waals interactions of the guest molecule with the water molecules in the cage and is not stable if the cavities are empty. The structure of a cavity is shown in Figure 10-3. The overall structure is based on 46 water molecules that link to form six such cavities with pentagonal faces, and two slightly larger cavities having two hexagonal faces; the overall stoichiometry is 8CH4 • 46H2O (almost 1:6) if all the cavities are filled, which they may not be—these systems are nonstoichiometric. Other small molecules may form clathrates with the same structure (e.g., ethane, H2S, CO2), while other clathrate structures exist with larger gas molecules.

Temperature (°C)

FIGURE 10-4 Ocean depth-temperature stability range of methane hydrate.

Temperature (°C)

FIGURE 10-4 Ocean depth-temperature stability range of methane hydrate.

Figure 10-4 shows the pressures and temperatures at which methane hydrate is stable. (Pressure increases by about 1 atm for each increase in depth of 10 m). In ocean sediments at depths of 1000 ft (300 m) or more, or in permafrost below about 400 ft (120 m), conditions may be suitable for methane clathrate formation from biologically generated methane, and deposits have been found in such regions. Methane hydrates have also been observed in natural gas pipelines, clogging and sometimes damaging them. The amount of gas hydrate deposits in nature is unknown, but estimates of the methane in clathrates or trapped below layers of them range up to twice the carbon in all the oil, coal, and ordinary natural gas deposits on earth. As yet, plans for tapping this methane as an energy source are highly preliminary, although at least one Siberian gas field, Messoyakha, apparently has produced natural gas at least partially from such a source since the 1970s.

Recognition of the existence of methane hydrate reservoirs has led to expressions of concern over possible release of methane if global warming allows the decomposition of some hydrates as permafrost warms, or as warmer ocean water gets into the depths through changes in circulation patterns. Because of the strong greenhouse property of methane, as discussed earlier, this could have a reinforcing effect. Interestingly, there are some indications that such releases may have occurred in past glacial periods when sea levels fell enough to release the pressure sufficiently. This might have been responsible for warming periods which occurred then. At the present time, knowledge of these systems is inadequate to make definite predictions.

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