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Evolutionary Aspects of Atmospheric Oxygen and Organisms

Daniel L. Gilbert

10.1002/cphy.cp040246

Source: Supplement 14: Handbook of Physiology, Environmental Physiology

Originally published: 1996

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 The Origin of Oxygen in the Universe
2 Oxygen Compounds in the Universe
3 Early History of the Earth
4 Production of the First Living Organisms
5 Energetics of the First Living Organisms
6 When did the Earth'S Biosphere Originate?
7 The Transition Stage Between a Reducing and an Oxidizing Atmosphere
8 Development of the Oxidizing Atmosphere
9 Chronology of Events in the Oxygen Atmosphere
10 Oxygen as a Biological Energy Store
11 Oxygen Compounds in the Atmosphere
12 Stability of the Oxygen Cycle
13 Dual Effects of Oxygen on the Biosphere
14 The Future Atmosphere on Earth
15 Summary and Conclusion
Figure 1. Figure 1.

Nuclear binding energies. These values were calculated from the following equation 314:where EBA = binding energy per nucleon; Z = number of protons; N = number of neutrons; Me = mass excess in MeV; H = mass excess in MeV for the proton; and n = mass excess in MeV for the neutron. The mass excess equals the atomic mass minus the integer atomic mass unit. The atomic mass of 12C is defined to equal 12 integer atomic mass units. H = 8.0714 MeV and n = 7.2890 MeV. For 16O, the atomic mass equals 15.99491466 and the atomic mass number equals 16. Thus the mass excess equals ‐0.00508534. The Einstein equation (energy equals mass times the square of the speed of light) enables one to calculate the equivalent energy for this mass excess. One MeV is the equivalent energy of 0.001073535 atomic mass units. Dividing the ‐0.00508534 mass units by 0.001073535 mass units/MeV gives ‐4.737 MeV for the mass excess of 16O 202. For 16O, Z = N = 8, so by substitution into this equation, the binding energy per nucleon equals 7.9762 MeV.

Values for the mass excesses were taken from McGervey 183. Only one isotope for each element was chosen. These isotopes were generally either the most common or the longest‐lived ones for the element. Note that about 99% of naturally occurring hydrogen, helium, carbon, nitrogen, and oxygen is in the form of the isotopes 1H, 4He, 12C, 14N, and 16O, respectively. For almost all the elements, it was not significant what isotope was used for representing the element. Note that the atomic number equals the number of protons in the nucleus and that the number of nucleons equals the number of protons plus neutrons. The dotted lines represent peaks in nuclear stability. The first peak on the left is the 4He peak, which points out that 4He is much more stable than 1H. This fact accounts for the energy released in reaction 1 (in the text). Similarly, note that 12C and 16O are also peaks, which accounts for the energy released in reactions 2 and 3. Note that 56Fe is the most stable nucleus of all the atoms; this is the reason why the big stars end up with an iron core. Fusion of atoms lighter than iron can result in a release of energy, whereas fission of atoms heavier than iron can also result in a release of energy. According to the shell theory of the nucleus, Mayer 182 has shown that nuclei that contain 2, 8, 20, 28, 50, 82, or 126 protons or neutrons are unusually stable. These nuclear shells are analogous to the atomic valence shells of electrons. Mayer's chart indicates that the only nuclei to contain double magic numbers are 4He, 16O, 40Ca, 48Ca, and 208Pb. Of these five nuclei, only 4He, 16O, and 40Ca have the same number of protons and neutrons. This figure illustrates the stability of 4He and 16O.
Figure 2. Figure 2.

Evolution of mantle. In Greenberg's model 111 of the evolution of a typical particle in a dust cloud, there is a change in the chemical composition over the comparatively short period of time lasting for about 1 · 106 years, or 1 Myr. Note that initial conditions of water, methane, and ammonia are reducing in nature. As the evolution of the mantle progresses, there is a change toward an oxidized mantle, consisting of carbon dioxide, carbon monoxide, and molecular nitrogen. Thus these dust particles become a pinpoint of oxidation in a reducing interstellar dust cloud.
Figure 3. Figure 3.

Carbon isotopic values. (See Table 7 for explanation.) The reference used for measuring δ13C was the Peedee belemnite standard (PBS). The δ13C is a measure of the distribution of heavy carbon (13C) as compared to light carbon (12C) in a sample. Note that inorganic carbonate carbon has relatively more heavy carbon than organic carbon. This is due to the kinetic fixation of carbon by the biosphere. Most of the carbon fixation proceeds by the ribulose 1,5‐bisphosphate carboxylase (RuBP, RUBISCO) pathway to phosphoglycerate (PGA), a three‐carbon compound as in C3 plants. Of lesser importance is the phosphoenolpyruvate (PEP) pathway to produce oxaloacetate (OAA), a four‐carbon compound, as in C4 plants. Carbon fixed by the RUBISCO pathway has δ13C values between ‐20‰ and ‐30‰, whereas carbon fixed by the PEP pathway has δ13C values between ‐2‰ and ‐3‰ 248. Note that the geological samples are obtained at different depths (stratigraphic position), corresponding to the different ages when the samples were deposited. The ages are shown on top and they correspond to the Late Proterozoic, which lasted from about 550 Myr to 900 Myr. Individual data points are not shown, but they clearly showed dips in the δ13C for both carbonate and organic carbons. Dips are due to an increase of biological activity. If more carbon dioxide is involved in photosynthesis, then the organic carbon would be composed of more light carbon. If photosynthesis is increased, then the heavy carbon would be diluted by biological respiration in both the atmosphere and hydrosphere. This would explain the dips in both records. One consequence of this activity would be an increase in the oxygen concentrations in both the atmosphere and biosphere. Now, as a result of the increased oxygen, there would be an increase of oxygen toxicity. The increased oxygen would produce an evolutionary pressure to combat this poison by producing more natural antioxidant defenses. This would lead to the cyclic characteristics seen in both records. The figure was adapted from Holland 124.
Figure 4. Figure 4.

Stability of some carbon compounds in the presence of hydrogen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = 2a(n ‐ w).

The more thermodynamically stable a chemical is in the presence of hydrogen, the lower it is on this diagram. The free energies for Figures 4,5,7, and 8 were obtained from Latimer 164. The standard free energy for 1/6C6H12O6 is 5.8 kJ/mol, so the free energy for the reaction of 1/6C6H12O6 plus H2O yielding CO2 plus 2H2 is ‐5.8 kJ/mol. Similarly, the standard free energy for CH4 is ‐129.8 kJ/mol, so the free energy for the reaction of CO2 plus 4H2 yielding CH4 plus 2H2O is ‐129.8 kJ/mol. Summing these two reactions gives a value of ‐135.6 kJ/mol for the net reaction of 1/6C6H12O6 plus 2H2 yielding CH4 plus H2O.
Figure 5. Figure 5.

Stability of some nitrogen compounds in the presence of hydrogen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = 2(w ‐ n)/a.

For an explanation of the type of diagram, see Figure 4.
Figure 6. Figure 6.

Life‐energy profile in different atmospheres. A living cell is characterized by possessing more energy than its external environment. What keeps this energy difference? The cell membrane represented by A provides a barrier to the flow of energy out of the cell by means of active transport mechanisms. Gases, such as hydrogen and oxygen, represented by X2, can cross the membrane barrier by diffusion. X2 is taken up by the cell to produce biological energy, represented in this diagram by the X2 SINK. Some of the X2 in the X2 SINK can be used for anabolic synthesis as well. However, some of the X2 can destroy the cellular constituents. As a consequence, anti‐X2 defenses, represented by B, had to be developed by the cell to survive. Thus the first primitive cells had to deal with hydrogen as an energy source and hydrogen toxicity, whereas modern aerobic cells have to deal with oxygen as an energy source and oxygen toxicity.
Figure 7. Figure 7.

Stability of some carbon compounds in the presence of oxygen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = a(w + 2 ‐ 2n).

For an explanation of the type of diagram, see Figure 4.
Figure 8. Figure 8.

Stability of some nitrogen compounds in the presence of oxygen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = (2n + w)/a.

For an explanation of the type of diagram, see Figure 4.
Figure 9. Figure 9.

O2 exchanges in the atmosphere. Most of the values are derived from Gilbert 102. The value for the CO2 reservoir was calculated from Hall 117. For simplicity, we have rounded off the rates determined by Keeling and Shertz for the anthropogenic factors as well as the rate of CO2 entering the ocean 147.

The three most abundant oxygen species residing in the atmosphere are molecular oxygen or dioxygen, water vapor, and carbon dioxide. Other important oxygen species are ozone, carbon monoxide, hydrogen peroxide, nitrous oxide (N2O), sulfur oxides, the free radicals nitric oxide (NO) and nitrogen dioxide (NO2), the hydroperoxyl radical (HO2), and the hydroxyl radical (OH). Note that the bulk of the oxygen in the atmosphere is in the form of molecular oxygen or dioxygen. Photosynthetic production of oxygen is the predominant fast process, and the corresponding respiratory uptake rate of oxygen occurs at almost the same rate. Plants that produce oxygen also use oxygen. The plant respiration rate is 8,500 Emol/Myr, so the net rate of photosynthesis is 15,000 Emol/Myr minus 8,500 Emol/Myr, or 6,500 Emol/Myr. Thus the 37 Emol of dioxygen in the atmosphere is replaced by photosynthesis every 5,700 years.

Weathering and erosion processes expose reduced chemicals to the atmosphere and allow these chemicals to be oxidized by atmospheric oxygen. The release of reduced chemicals from volcanoes and midocean ridges due to tectonic activity also has the same effect as weathering on the atmospheric oxygen reservoir; therefore, this activity has been considered as part of weathering 17,300. However, decay and burial of organic chemicals removes carbon dioxide and increases oxygen 17. Mountain formation can bury organic carbon and lead to a release of oxygen 65. Planting trees also has the same effect as burial processes, while deforestation has the opposite effect. It has been suggested by many authors that planting trees can decrease the buildup of carbon dioxide produced by the burning of fossil fuels 69,132,201,256. Deforestation is occurring today at such a high rate that tree planting barely offsets its effect. Although these processes are of considerable importance to the small carbon dioxide reservoir, they are of little significance to the much larger oxygen reservoir.

Other gases, such as methane and carbon monoxide, also are oxidized by dioxygen. When carbohydrates are burned, one mole of oxygen is consumed for each mole of carbon dioxide released (see stage 6 in Table 9. Physiologists have used the respiratory exchange ratio or respiratory quotient (R) as the symbol for the ratio of carbon dioxide released to oxygen consumed by organisms 276,308. Similarly, we can use the atmospheric exchange of these two gases as also being represented by the symbol R. Thus when carbohydrates are burned, the value of the atmospheric exchange ratio, or atmospheric quotient R, is one. However, when hydrocarbons are burned, the atmospheric R is less than one, as illustrated in the following reaction:

In this reaction, the atmospheric R is 0.5. This results in more water being formed. The burning of fossil fuels reduces the atmospheric dioxygen at the rate of 670 Emol/Myr; 470 Emol/Myr of carbon dioxide are produced from this burning and 200 Emol/Myr of water are also produced. The atmospheric R due to anthropogenic sources is equal to 470 Emol/Myr divided by 670 Emol/Myr, or 0.7. About 250 Emol/Myr of carbon dioxide enter the ocean so that the net rate of increase in the atmospheric carbon dioxide reservoir is about 220 Emol/Myr.

What will be the final outcome of these anthropogenic factors on these atmospheric reservoirs? The carbon in the Earth's fossil‐fuel reservoir is about 0.9 Emol 102. If we assume that the burning of carbon from this reservoir is the principal anthropogenic factor leading to the increase of atmospheric carbon dixoide and that the rate of this increase is kept constant, then we can answer this question. The fossil‐fuel reservoir will be depleted in roughly 2,000 years (0.9 Emol divided by 0.47 Emol/thousand years). The increase of the atmospheric carbon dioxide reservoir will then equal 2,000 years times 0.22 Emol/thousand years, or 0.44 Emol; this reservoir will then equal 0.0619 Emol plus 0.44 Emol, or roughly 0.5 Emol. Dividing the 0.5 Emol by 0.0619 Emol gives a value of 8.1; this means that the present carbon dioxide reservoir will have increased by 810%! The corresponding decrease of the atmospheric oxygen will then equal 2,000 years times 0.67 Emol/thousand years, or 1.34 Emol; this reservoir will then equal 37 Emol minus 1.34 Emol, or roughly 35.7 Emol. Dividing the 35.7 Emol by 37 Emol gives a value of 0.965; this means that the present oxygen reservoir will have decreased by only 3.5%. Hence, the atmospheric carbon dioxide is easier to change than the relatively stable oxygen reservoir.

Walker 298 assumed that atmospheric oxygen was in a steady state, but he emphasized that this was just an assumption. Kump 161 has pointed out that due to the lack of precise knowledge about cycling rates it is not possible to state if the atmospheric dioxygen reservoir is decreasing, increasing, or remaining the same.
Figure 10. Figure 10.

Slow processes in the oxygen cycle. Photosynthesis is the predominant biological process for producing oxygen; this process is reversed by respiration (see Fig. 9. The mantle releases gases in volcanoes and solid materials in midocean rifts caused by plate tectonics. Iron is released in the ferrous state in these rifts. X represents reduced chemicals such as iron 300, sulfur 17,299,301, and other gases, such as methane and carbon monoxide 300. Reduction of the oxidized X (XO2) is accomplished by biological activity. Hydrogen is released from volcanoes and is removed from the Earth by photodissociation of water in the upper atmosphere at about equal rates 300.


Figure 1.

Nuclear binding energies. These values were calculated from the following equation 314:where EBA = binding energy per nucleon; Z = number of protons; N = number of neutrons; Me = mass excess in MeV; H = mass excess in MeV for the proton; and n = mass excess in MeV for the neutron. The mass excess equals the atomic mass minus the integer atomic mass unit. The atomic mass of 12C is defined to equal 12 integer atomic mass units. H = 8.0714 MeV and n = 7.2890 MeV. For 16O, the atomic mass equals 15.99491466 and the atomic mass number equals 16. Thus the mass excess equals ‐0.00508534. The Einstein equation (energy equals mass times the square of the speed of light) enables one to calculate the equivalent energy for this mass excess. One MeV is the equivalent energy of 0.001073535 atomic mass units. Dividing the ‐0.00508534 mass units by 0.001073535 mass units/MeV gives ‐4.737 MeV for the mass excess of 16O 202. For 16O, Z = N = 8, so by substitution into this equation, the binding energy per nucleon equals 7.9762 MeV.

Values for the mass excesses were taken from McGervey 183. Only one isotope for each element was chosen. These isotopes were generally either the most common or the longest‐lived ones for the element. Note that about 99% of naturally occurring hydrogen, helium, carbon, nitrogen, and oxygen is in the form of the isotopes 1H, 4He, 12C, 14N, and 16O, respectively. For almost all the elements, it was not significant what isotope was used for representing the element. Note that the atomic number equals the number of protons in the nucleus and that the number of nucleons equals the number of protons plus neutrons. The dotted lines represent peaks in nuclear stability. The first peak on the left is the 4He peak, which points out that 4He is much more stable than 1H. This fact accounts for the energy released in reaction 1 (in the text). Similarly, note that 12C and 16O are also peaks, which accounts for the energy released in reactions 2 and 3. Note that 56Fe is the most stable nucleus of all the atoms; this is the reason why the big stars end up with an iron core. Fusion of atoms lighter than iron can result in a release of energy, whereas fission of atoms heavier than iron can also result in a release of energy. According to the shell theory of the nucleus, Mayer 182 has shown that nuclei that contain 2, 8, 20, 28, 50, 82, or 126 protons or neutrons are unusually stable. These nuclear shells are analogous to the atomic valence shells of electrons. Mayer's chart indicates that the only nuclei to contain double magic numbers are 4He, 16O, 40Ca, 48Ca, and 208Pb. Of these five nuclei, only 4He, 16O, and 40Ca have the same number of protons and neutrons. This figure illustrates the stability of 4He and 16O.



Figure 2.

Evolution of mantle. In Greenberg's model 111 of the evolution of a typical particle in a dust cloud, there is a change in the chemical composition over the comparatively short period of time lasting for about 1 · 106 years, or 1 Myr. Note that initial conditions of water, methane, and ammonia are reducing in nature. As the evolution of the mantle progresses, there is a change toward an oxidized mantle, consisting of carbon dioxide, carbon monoxide, and molecular nitrogen. Thus these dust particles become a pinpoint of oxidation in a reducing interstellar dust cloud.



Figure 3.

Carbon isotopic values. (See Table 7 for explanation.) The reference used for measuring δ13C was the Peedee belemnite standard (PBS). The δ13C is a measure of the distribution of heavy carbon (13C) as compared to light carbon (12C) in a sample. Note that inorganic carbonate carbon has relatively more heavy carbon than organic carbon. This is due to the kinetic fixation of carbon by the biosphere. Most of the carbon fixation proceeds by the ribulose 1,5‐bisphosphate carboxylase (RuBP, RUBISCO) pathway to phosphoglycerate (PGA), a three‐carbon compound as in C3 plants. Of lesser importance is the phosphoenolpyruvate (PEP) pathway to produce oxaloacetate (OAA), a four‐carbon compound, as in C4 plants. Carbon fixed by the RUBISCO pathway has δ13C values between ‐20‰ and ‐30‰, whereas carbon fixed by the PEP pathway has δ13C values between ‐2‰ and ‐3‰ 248. Note that the geological samples are obtained at different depths (stratigraphic position), corresponding to the different ages when the samples were deposited. The ages are shown on top and they correspond to the Late Proterozoic, which lasted from about 550 Myr to 900 Myr. Individual data points are not shown, but they clearly showed dips in the δ13C for both carbonate and organic carbons. Dips are due to an increase of biological activity. If more carbon dioxide is involved in photosynthesis, then the organic carbon would be composed of more light carbon. If photosynthesis is increased, then the heavy carbon would be diluted by biological respiration in both the atmosphere and hydrosphere. This would explain the dips in both records. One consequence of this activity would be an increase in the oxygen concentrations in both the atmosphere and biosphere. Now, as a result of the increased oxygen, there would be an increase of oxygen toxicity. The increased oxygen would produce an evolutionary pressure to combat this poison by producing more natural antioxidant defenses. This would lead to the cyclic characteristics seen in both records. The figure was adapted from Holland 124.



Figure 4.

Stability of some carbon compounds in the presence of hydrogen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = 2a(n ‐ w).

The more thermodynamically stable a chemical is in the presence of hydrogen, the lower it is on this diagram. The free energies for Figures 4,5,7, and 8 were obtained from Latimer 164. The standard free energy for 1/6C6H12O6 is 5.8 kJ/mol, so the free energy for the reaction of 1/6C6H12O6 plus H2O yielding CO2 plus 2H2 is ‐5.8 kJ/mol. Similarly, the standard free energy for CH4 is ‐129.8 kJ/mol, so the free energy for the reaction of CO2 plus 4H2 yielding CH4 plus 2H2O is ‐129.8 kJ/mol. Summing these two reactions gives a value of ‐135.6 kJ/mol for the net reaction of 1/6C6H12O6 plus 2H2 yielding CH4 plus H2O.



Figure 5.

Stability of some nitrogen compounds in the presence of hydrogen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = 2(w ‐ n)/a.

For an explanation of the type of diagram, see Figure 4.



Figure 6.

Life‐energy profile in different atmospheres. A living cell is characterized by possessing more energy than its external environment. What keeps this energy difference? The cell membrane represented by A provides a barrier to the flow of energy out of the cell by means of active transport mechanisms. Gases, such as hydrogen and oxygen, represented by X2, can cross the membrane barrier by diffusion. X2 is taken up by the cell to produce biological energy, represented in this diagram by the X2 SINK. Some of the X2 in the X2 SINK can be used for anabolic synthesis as well. However, some of the X2 can destroy the cellular constituents. As a consequence, anti‐X2 defenses, represented by B, had to be developed by the cell to survive. Thus the first primitive cells had to deal with hydrogen as an energy source and hydrogen toxicity, whereas modern aerobic cells have to deal with oxygen as an energy source and oxygen toxicity.



Figure 7.

Stability of some carbon compounds in the presence of oxygen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = a(w + 2 ‐ 2n).

For an explanation of the type of diagram, see Figure 4.



Figure 8.

Stability of some nitrogen compounds in the presence of oxygen. The ordinate represents the decrease in free energy for the various compounds, according to the following equation 96:where b = (2n + w)/a.

For an explanation of the type of diagram, see Figure 4.



Figure 9.

O2 exchanges in the atmosphere. Most of the values are derived from Gilbert 102. The value for the CO2 reservoir was calculated from Hall 117. For simplicity, we have rounded off the rates determined by Keeling and Shertz for the anthropogenic factors as well as the rate of CO2 entering the ocean 147.

The three most abundant oxygen species residing in the atmosphere are molecular oxygen or dioxygen, water vapor, and carbon dioxide. Other important oxygen species are ozone, carbon monoxide, hydrogen peroxide, nitrous oxide (N2O), sulfur oxides, the free radicals nitric oxide (NO) and nitrogen dioxide (NO2), the hydroperoxyl radical (HO2), and the hydroxyl radical (OH). Note that the bulk of the oxygen in the atmosphere is in the form of molecular oxygen or dioxygen. Photosynthetic production of oxygen is the predominant fast process, and the corresponding respiratory uptake rate of oxygen occurs at almost the same rate. Plants that produce oxygen also use oxygen. The plant respiration rate is 8,500 Emol/Myr, so the net rate of photosynthesis is 15,000 Emol/Myr minus 8,500 Emol/Myr, or 6,500 Emol/Myr. Thus the 37 Emol of dioxygen in the atmosphere is replaced by photosynthesis every 5,700 years.

Weathering and erosion processes expose reduced chemicals to the atmosphere and allow these chemicals to be oxidized by atmospheric oxygen. The release of reduced chemicals from volcanoes and midocean ridges due to tectonic activity also has the same effect as weathering on the atmospheric oxygen reservoir; therefore, this activity has been considered as part of weathering 17,300. However, decay and burial of organic chemicals removes carbon dioxide and increases oxygen 17. Mountain formation can bury organic carbon and lead to a release of oxygen 65. Planting trees also has the same effect as burial processes, while deforestation has the opposite effect. It has been suggested by many authors that planting trees can decrease the buildup of carbon dioxide produced by the burning of fossil fuels 69,132,201,256. Deforestation is occurring today at such a high rate that tree planting barely offsets its effect. Although these processes are of considerable importance to the small carbon dioxide reservoir, they are of little significance to the much larger oxygen reservoir.

Other gases, such as methane and carbon monoxide, also are oxidized by dioxygen. When carbohydrates are burned, one mole of oxygen is consumed for each mole of carbon dioxide released (see stage 6 in Table 9. Physiologists have used the respiratory exchange ratio or respiratory quotient (R) as the symbol for the ratio of carbon dioxide released to oxygen consumed by organisms 276,308. Similarly, we can use the atmospheric exchange of these two gases as also being represented by the symbol R. Thus when carbohydrates are burned, the value of the atmospheric exchange ratio, or atmospheric quotient R, is one. However, when hydrocarbons are burned, the atmospheric R is less than one, as illustrated in the following reaction:

In this reaction, the atmospheric R is 0.5. This results in more water being formed. The burning of fossil fuels reduces the atmospheric dioxygen at the rate of 670 Emol/Myr; 470 Emol/Myr of carbon dioxide are produced from this burning and 200 Emol/Myr of water are also produced. The atmospheric R due to anthropogenic sources is equal to 470 Emol/Myr divided by 670 Emol/Myr, or 0.7. About 250 Emol/Myr of carbon dioxide enter the ocean so that the net rate of increase in the atmospheric carbon dioxide reservoir is about 220 Emol/Myr.

What will be the final outcome of these anthropogenic factors on these atmospheric reservoirs? The carbon in the Earth's fossil‐fuel reservoir is about 0.9 Emol 102. If we assume that the burning of carbon from this reservoir is the principal anthropogenic factor leading to the increase of atmospheric carbon dixoide and that the rate of this increase is kept constant, then we can answer this question. The fossil‐fuel reservoir will be depleted in roughly 2,000 years (0.9 Emol divided by 0.47 Emol/thousand years). The increase of the atmospheric carbon dioxide reservoir will then equal 2,000 years times 0.22 Emol/thousand years, or 0.44 Emol; this reservoir will then equal 0.0619 Emol plus 0.44 Emol, or roughly 0.5 Emol. Dividing the 0.5 Emol by 0.0619 Emol gives a value of 8.1; this means that the present carbon dioxide reservoir will have increased by 810%! The corresponding decrease of the atmospheric oxygen will then equal 2,000 years times 0.67 Emol/thousand years, or 1.34 Emol; this reservoir will then equal 37 Emol minus 1.34 Emol, or roughly 35.7 Emol. Dividing the 35.7 Emol by 37 Emol gives a value of 0.965; this means that the present oxygen reservoir will have decreased by only 3.5%. Hence, the atmospheric carbon dioxide is easier to change than the relatively stable oxygen reservoir.

Walker 298 assumed that atmospheric oxygen was in a steady state, but he emphasized that this was just an assumption. Kump 161 has pointed out that due to the lack of precise knowledge about cycling rates it is not possible to state if the atmospheric dioxygen reservoir is decreasing, increasing, or remaining the same.



Figure 10.

Slow processes in the oxygen cycle. Photosynthesis is the predominant biological process for producing oxygen; this process is reversed by respiration (see Fig. 9. The mantle releases gases in volcanoes and solid materials in midocean rifts caused by plate tectonics. Iron is released in the ferrous state in these rifts. X represents reduced chemicals such as iron 300, sulfur 17,299,301, and other gases, such as methane and carbon monoxide 300. Reduction of the oxidized X (XO2) is accomplished by biological activity. Hydrogen is released from volcanoes and is removed from the Earth by photodissociation of water in the upper atmosphere at about equal rates 300.

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Article Title: Evolutionary Aspects of Atmospheric Oxygen and Organisms
 

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Daniel L. Gilbert. Evolutionary Aspects of Atmospheric Oxygen and Organisms. Compr Physiol 2011, Supplement 14: Handbook of Physiology, Environmental Physiology: 1059-1094. First published in print 1996. doi: 10.1002/cphy.cp040246