The Carbon Cycle

The methane-producing bacteria derive their energy from the oxidation of simple organic corn-pounds such as methanol and acetate, or from molecular hydrogen. Methane is the reduced product of their metabolism. These are among the most strictly anaerobic organisms. They are responsible for swamp gas (methane) production in the 

Carbon is a component of all organic substances, plants and animals. Dead sea creatures sink to deeper layers and decompose. Carbonate and hydrogencarbonate (COj and HCOj) ions are formed and are brought up to the surface by ocean currents. There they react with metal ions and form carbonate sediments. The carbon dioxide in the air compensates for the carbon losses as it is in equihbrium with carbon dioxide dissolved in the seawater and thus with its content of carbonate and hydrogen-carbonate ions. 

However, the equilibrium has been changed by large-scale burning of fossil fuels in modern society. Power production and automobile exhausts generate huge amounts of carbon dioxide, which are delivered to the atmosphere. A considerable disturbance to the global environment occurs. 

When we breathe out carbon dioxide, we return it to the air for further recycling. Also, when we die our carbon compounds becomes available for recycling. This means that we probably have in our bodies carbon atoms belonging to the dinosaurs, Einstein, our grandparents or even Elvis Presley - what a combination  

Artist s impression of carbon cycle. (Courtesy of NASA, http yearthobservatory.nasa.gov/ Library/CarbonCycle/carbon cycle4.html) 

C02 + H20 - 02 + carbohydrate Both plants and animals undergo respiration in which 

Additionally, when plants and animals die, the dead organisms decay and give off C02. 

According to some estimates [9], the present amount of C02 taken out of the atmosphere every year by plants is almost perfectly balanced by the amount of C02 put back into the atmosphere by respiration and decay. The C02 produced in this manner is part of a cycle in which new carbon does not enter the system, but rather keeps changing in form. In addition to the above, deforestation and the combustion of fossil fuels and traditional biomass release C02 into the atmosphere, and the oceans absorb C02 in various forms, thus sequestering it. The carbon cycle is therefore large and complex. 

It is not entirely straightforward to predict how it will behave. Nonetheless, there is merit in closing the C02 cycles as much as possible, as this is consistent with sustainability. 

Fundamentally, the carbon cycle is a series of linked chemical reactions, both biological and abiotic. Many are redox reactions. Although the principal source of energy that drives the global redox system is sunlight, humans have not only diverted naturally occurring sources of energy and carbon to their own use, but are also 

Diagram of the carbon cycle, showing movement of oxidized and reduced carbon species between the atmosphere, hydrosphere, biosphere, and geosphere. 

The carbon cycle is not complete—there are some sinks or areas where compounds accumulate, or are at least very slowly turned over. The approximate masses of carbon in the various atmospheric and terrestrial carbon pools and some of their approximate annual rates of conversion are given in Table 1.1. 

The most oxidized species, CO2, exists in the atmosphere as a gas whose concentration far exceeds that of other carbon-containing substances. In water, it takes part in a series of equilibrium reactions involving hydration, ionization, and precipitation  

CO2 CO CH4 7.3x10 2.27x10 3x10 5 Land to oceans Inorganic C Dissolved org. C 4x10 1 x10  

The most important component of the carbon cycle is the gas carbon dioxide (C02), and almost all of this section will be about this compound and the role it plays in the environment. 

Carbon is the fourth most abundant element in the universe. It is a key component of the atmosphere, sea, land, and all living things on Earth. In one form or another, carhon is recycled constantly between the oceans, land, air, and living things. The processes that move carhon atoms on Earth collectively make up the carbon cycle. 

Each part of the carbon cycle acts as a reservoir for carbon atoms a place where carbon enters, resides for some time, and then leaves. Each reservoir has its own characteristics. The amount of carbon present, the length of time carbon remains, the way carbon enters and exits, and the reactions and roles of carhon atoms vary for each reservoir. 

The biosphere is the part of Earth that contains living things. This includes all inhabitable parts of the atmosphere, the sea, and the land. Because all living things are made of carhon, all of the 

Photosynthesis is the process by which plants use energy from sunlight to convert carbon dioxide (CO2) and water (H2O) to carbohydrates. Both the carbon dioxide and water come from the environment. Photosynthesis is the main process that removes carbon dioxide from the atmosphere. Oxygen and water vapor are released into the atmosphere as by-products of the reactions of photosynthesis. 

Weathering is the process by which rock is broken down into smaller and smaller particles. It involves both mechanical and chemical breakdowns. The mechanical breakdown into smaller and smaller pieces occurs as a result of exposure to freeze-thaw cycles and to the action of wind and water. Chemical breakdown occurs as a result of exposure to air and water and other chemicals that may be dissolved in water, such as acids. Weathering by exposure to atmosphere results in some of the carbon dioxide being removed from the atmosphere along with the broken-down rock and eventually washed into the ocean. 

Organic residues added to the soil contain about 50% carbon which is eventually converted to carbon dioxide. Soil microorganisms are responsible for the evolution of about 95% of the gas which is then fixed mainly by green plants. A simplified representation of the carbon cycle is given in Fig. 4.8. 

Plant and animal remains, representing the main types of carbon compounds added to the soil, contain the carbon included in high-molecular weight compounds. In plants the main types of these are cellulose, hemi-cellulose and lignin, followed by smaller quantities of fats, waxes and oils, proteins and nucleic acids. In invertebrates and fungi chitin occurs, whereas in bacteria it is peptidoglycan (murein). 

Three general methods exist for the catabolism of organic compounds aerobic respiration anaerobic respiration and fermentation. 

On the surface of the soil, residues are attacked by bacteria, fungi and animals. In the soil the conditions for further decay are more favourable to microorganisms. The carbon in the residues is oxidized to carbon dioxide, incorporated in microbial cells and the reminder is incorporated into humus. Humus can be further decomposed by soil microflora. 

In waterlogged anaerobic soils, organic matter can be converted (via organic acids, carbon dioxide and hydrogen) to methane by methanogenic bacteria. A methane oxidizing flora at the soil surface oxidizes most of the methane to carbon dioxide. 

Emissions from fossil fuels and cement production 5.4 0.1 6.3 0.1 

Flux from land to atmosphere due to land-use change 1.7 (0.6 to 2.5) Not available 

Missing sink (possibly terrestrial vegetation) 1.9 (0.3 to 3.8) Not available 

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It has been detected spectroscopically in great abundance, especially in the hotter stars, and it is an important component in both the proton-proton reaction and the carbon cycle, which account for the energy of the sun and stars. 

One of the things that environmental scientists do IS to keep track of important elements in the biosphere—in what form do these ele ments normally occur to what are they transformed and how are they returned to their normal state Careful studies have given clear although compli cated pictures of the nitrogen cycle the sulfur cy cle and the phosphorus cycle for example The carbon cycle begins and ends with atmospheric carbon dioxide It can be represented in an abbrevi ated form as... 

Renewable carbon resources is a misnomer the earth s carbon is in a perpetual state of flux. Carbon is not consumed such that it is no longer available in any form. Reversible and irreversible chemical reactions occur in such a manner that the carbon cycle makes all forms of carbon, including fossil resources, renewable. It is simply a matter of time that makes one carbon from more renewable than another. If it is presumed that replacement does in fact occur, natural processes eventually will replenish depleted petroleum or natural gas deposits in several million years. Eixed carbon-containing materials that renew themselves often enough to make them continuously available in large quantities are needed to maintain and supplement energy suppHes biomass is a principal source of such carbon. 

Carbon. Most of the Earth s supply of carbon is stored in carbonate rocks in the Hthosphere. Normally the circulation rate for Hthospheric carbon is slow compared with that of carbon between the atmosphere and biosphere. The carbon cycle has received much attention in recent years as a result of research into the possible relation between increased atmospheric carbon dioxide concentration, most of which is produced by combustion of fossil fuel, and the "greenhouse effect," or global warming. Extensive research has been done on the rate at which carbon dioxide might be converted to cellulose and other photosyntheticaHy produced organic compounds by various forms of natural and cultivated plants. Estimates also have been made of the rate at which carbon dioxide is released to soil under optimum conditions by various kinds of plant cover, such as temperature-zone deciduous forests, cultivated farm crops, prairie grassland, and desert vegetation. 

The efficiency of the weathering of rocks in using carbonic acid produced in the carbon cycle is affected by various hydrologic, environmental, and cultural controls. The fact that the principal anion in fresh surface water worldwide almost always is bicarbonate attests to the overriding importance of this process. Exceptions are systems in which evaporite minerals are available for dissolution by groundwater or where human activities are major sources of sulfate or chloride inflow. 

Feedbacks within the Marine Segment of the Carbon Cycle... 

The harmful effects of air pollutants on human beings have been the major reason for efforts to understand and control their sources. During the past two decades, research on acidic deposition on water-based ecosystems has helped to reemphasize the importance of air pollutants in other receptors, such as soil-based ecosystems (1). When discussing the impact of air pollutants on ecosystems, the matter of scale becomes important. We will discuss three examples of elements which interact with air, water, and soil media on different geographic scales. These are the carbon cycle on a global scale, the sulfur cycle on a regional scale, and the fluoride cycle on a local scale. 

B. BOLtN, The carbon cycle. Scientific American, September 1970, reprinted in Chemisiry in the Environment, pp. 53-61, W. H. Freeman, San Francisco, 1973. 

Like all matter, carbon can neither be created nor destroyed it can just be moved from one place to another. The carbon cycle depicts the various places where carbon can be found. Carbon occurs in the atmosphere, in the ocean, in plants and animals, and in fossil fuels. Carbon can be moved from the atmosphere into either producers (through the process of photosynthesis) or the ocean (through the process of diffusion). Some producers will become fossil fuels, and some will be eaten by either consumers or decomposers. The carbon is returned to the atmosphere when consumers respire, when fossil fuels are burned, and when plants are burned in a fire. The amount of carbon in the atmosphere can be changed by increasing or decreasing rates of photosynthesis, use of fossil fuels, and number of fires. 

With high-hardness waters, the carbonate-cycle form of precipitation treatment is often preferred to the phosphate-cycle because it forms a less bulky and less dense sludge. The disadvantages of hard-water carbonate-cycle precipitation treatments include ... 

In summary, the carbonate-cycle program provides preferred precipitation and coagulation reactions to prevent hard scale from forming. Key functions are ... 

Phosphate-cycle programs, which first became available at the turn of the twentieth century and were researched and formalized in the 1920s, have gradually replaced the carbonate-cycle programs. Formulation developments include combined phosphate-carbonate cycle programs, which incorporate the best of both basic programs while minimizing the problems associated with carbonate breakdown. 

Alternatives to fossil fuels, such as hydrogen, are explored in Box 6.2 and Section 14.3. Coal, which is mostly carbon, can be converted into fuels with a lower proportion of carbon. Its conversion into methane, CH4, for instance, would reduce C02 emissions per unit of energy. We can also work with nature by accelerating the uptake of carbon by the natural processes of the carbon cycle. For example, one proposed solution is to pump C02 exhaust deep into the ocean, where it would dissolve to form carbonic acid and bicarbonate ions. Carbon dioxide can also be removed from power plant exhaust gases by passing the exhaust through an aqueous slurry of calcium silicate to produce harmless solid products ... 

Feedbacks may be affected directly by atmospheric CO2, as in the case of possible CO2 fertilization of terrestrial production, or indirectly through the effects of atmospheric CO2 on climate. Furthermore, feedbacks between the carbon cycle and other anthropogenically altered biogeochemical cycles (e.g., nitrogen, phosphorus, and sulfur) may affect atmospheric CO2. If the creation or alteration of feedbacks have strong effects on the magnitudes of carbon cycle fluxes, then projections, made without consideration of these feedbacks and their potential for changing carbon cycle processes, will produce incorrect estimates of future concentrations of atmospheric CO2. 

Species Replacement and Feedbacks Influencing the Carbon Cycle. 

Olson, J. S. In The Carbon Cycle and Atmospheric CO2 Natural Variations Archean to Present Sundquist, E. T. Broecker, W. S., Eds. Geophysical Monograph 32 American Geophysical Union Washington DC, 1985 pp. 377-396. 

The models also assume a steady-state condition which suggests that the carbon cycle is structured, stable, and balanced and will remain so indefinitely. This mechanistic view of biogeochemistry allows for little variation even though it is known that fluctuations and variation occur seasonally. The concentration of... 

Schimel, D. S. (1995). Terrestrial ecosystems and the carbon cycle. Glob. Change Biol. 1, 77-91. 

As an application of the turnover time concept, let us consider the model of the carbon cycle shown in Fig. 4-3. This diagram is different from the one used in the chapter on the carbon cycle (Chapter 11), because it serves our purposes better for this chapter. The values given for fhe various fluxes and burdens are very similar to the corresponding figure in Chapter 11 (Fig. 11-1). 

Fig. 4-3 Principal reservoirs and fluxes in the carbon cycle. Units are 10 g (Pg) C (burdens) and PgC/yr (fluxes). (From Bolin (1986) with permission from John Wiley and Sons.)...

An important example of non-linearity in a biogeochemical cycle is the exchange of carbon dioxide between the ocean surface water and the atmosphere and between the atmosphere and the terrestrial system. To illustrate some effects of these non-linearities, let us consider the simplified model of the carbon cycle shown in Fig. 4-12. Ms represents the sum of all forms of dissolved carbon (CO2, H2CO3, HCOi" and... 

Assuming that the carbon cycle of Fig. 4-12 will remain a closed system over several thousands of years, we can ask how the equilibrium distribution within the system would change after the introduction of a certain amount of fossil carbon. Table 4-2 contains the answer for two different assumptions about the total input. The first 1000 Pg corresponds to the total input from fossil fuel up to about the year 2000 the second (6000 Pg) is roughly equal to the now... 

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