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Oxygen in the contemporary atmosphere
Atmospheric E~vir~~~t
OXYGEN
Pergamon Press 1972. Vol. 6, pp. 575-578. Printed in Great Britain.
IN THE CONTEMPORARY
ATMOSPHERE
JAMESE. LOVELOCK Universityof Reading,Departmentof AppliedPhysicalSciences,Reading
and JAMES P. LODGE, JR. National Center for Atmospheric
Research, Boulder, Colorado
(First received 12 January 1972 and in final form 21 February 1972) Abstract-The contemporary oxygen content of the Earth’s atmosphere is explained either as a result of the abiological process of water vapour photolysis in the upper atmosphere or by the action of green plants and algae at the surface which produce a net increment of oxygen by the burial of carbon in sediments. This paper attempts to bridge the gap between these apparently contradictory views by recalling that a substantial proportion of water vapour in the upper atmosphere comes from the oxidation of biologically generated methane. It is further suggested that the oxygen ~ncentration is maintained at an optium by and for the Biosphere. IT IS usual
to explain the present composition of the Earth’s atmosphere as the end result of the working of abiological processes, such as upper atmosphere photochemistry on the gases and precursors of gases released from Earth’s interior. A few, most notably, HUTCHINSON(1954), BERKNERand MARSHALL(1965) and CLOUD(1968) have assigned some role to the biosphere in modifying the present atmospheric composition away from a wholly inorganic distribution of gases; their conclusions have been challenged by BRINKMAN(1969) who argues that oxygen in the present atmosphere is principally the product of inorganic processes. These and other arguments concerning oxygen in the atmosphere are discussed by VAN VALEN(1971). HITCHCOCKand LOVELOCK(1967) proposed that the presence of life on Earth could be inferred from an examination of the chemical composition of the atmosphere. In particular the simultaneous presence of oxygen and methane at their current levels is impossible on any conceivable abiologiczl steady state process. Later LOVELOCKand GIFFEN (1969) developed this notion further to include nitrogen as a life indicator and predicted that if Mars were lifeless then nitrogen would be absent from its atmosphere. This prediction was found to be correct by the ultra violet spectroscopic observations from the 1969 Mariner space craft, BARTH et al. (1969). The arguments which were used to prove the presence of life on Earth from a mere knowledge of the atmospheric composition can also be used to prove, given the fact that life on Earth is abundant, that the atmosphere is itself almost wholly a biological contrivance. The purpose of this note is to re-examine the state of oxidation of the contemporary atmosphere from this biological viewpoint. Almost all past models of the Earth’s atmosphere have assumed that abiological processes would in the long run tend to an oxidizing atmosphere. The loss of hydrogen, following upon the upper atmospheric dissociation of hydrogen compounds, the only conceivable process which could lead to a net increase in oxygen concentration, is in these models assumed to exceed the gain of hydrogen from the solar wind and from the processes at the surface and in the interior of the Earth. This conclusion 575
JAMESE.
576
LOVELOCK and JAMESP. LODOE,JR.
receives its strongest support from the fact that there is no process other than the expulsion of hydrogen by which in the long run the Earth can be oxidized. The rate of exchange of hydrogen between the Earth’s and the Solar atmosphere is unknown; also uncertain is the rate of hydrogen production equivalent to the reducing tendency of tectonic processes, aIthough HOLLAND(1964) has estimated that the reducing equivalent of volcanic gaseous emissions requires 2 x lo*’ g of oxygen over the past IO9 yr. This is comparable with the quantity of oxygen B~nkman proposes to have come from photolysis, but neither estimate is accurate enough to determine if abiological oxygen is in credit or debit balance. We do know, however, that if Mars and Venus are at the equilibrium redox potentiai appropriate to their stations in the solar system, then the interpolated oxidation state of the Earth at eq~librium would be about pE 8 which is very different from its current pE of 13. Should Mars and Venus be shown to be normal in this respect then the Earth has an anomalous redox potential. In this context the redox potential is a measure of the reducing or oxidizing tendency of a chemical system. This is ~onvenie~tIy expressed by the 1og~ithmic pE scale; a statement of -log&) where (e) is the electron concentration. This is analogous to the pH scale, -log&H+) for acidity and basicity. If we assume that this anomaly is a consequence of the presence of life it encourages a closer look for processes which could provide a farge increment of oxygen and then sustain its presence against the reducing tendencies of the internal and external bounds of the atmosphere. RKJBEY (1951) proposed that the release of a large quantity of oxygen could have accompanied the burying of carbon and sulphur #mounds, residues of earlier biospheres. Although no certain estimate of the quantity of oxygen equivalent to this burial has been made, there are undoubtedly substantial quantities of reduced materiaf of biological origin in the Earth’s crust. However, even if it could be shown that all the current quantity of oxygen has been released by this process it would still be necessary to account for the maintenance of a constant oxygen ~on~ntration in the face of con~nuing removal by exposure of reduced materials by tectonic processes. The Rubey mechanism is more in the nature of a loan of redox capital which eventually must be repaid through the recycling of buried materials; it is now in the course of repayment considerably augmented by man’s use of fossil fuels. Some ad~tional TABLE 1. SOURCESOF STRATOSPHERIC WATERVAPOKYR
By CH4
oxidation
(3) (1) Direct transfer from troposphere 7-o
(2) 34
-85 8.2
Cold trap temperature (“c) -75 -80 -70 3.5
1.54
0.71
-45 O-35
Units metric tons sT2. (1) NEWELL,1971. (2) SINGER,1971, (3) Calculated from the CH4: Hz0 mixing ratio using (1) is correct and taking the water vapour wncen~ations as those of the saturated vapour pressures over ice at the stated tern~~t~.
Oxygen in the Contemporary Atmosphere
577
biological process seems necessary to sustain the oxidizing atmosphere and perhaps also to maintain its composition constant. One may be found in the proposal of BATES and NICOLIX(1950) that the hydrogen compounds of the upper atmosphere, including water vapour, come not only from the upward movement of water vapour but also from the upward movement of methane, Transfer from the troposphere to the stratosphere occurs almost entirely at the tropical tropopause and at a rate equivalent to 2 x 10’4gyr-” water vapour (NEWELL, 1971). The temperature at the coldest level in this region IS close to -SO%, at which the saturated water vapour pressure expressed as a concentration by volume is 5 x 10m7 and is to be compared with the methane concentration of 15 x 10m6 at the same level, EHALT (1967). There will be no segregation of molecular species in the vapour phase during the transfer across the tropopause, therefore methane and water vapour will move upwards in the proportions of their mixing ratio. If all of the methane so transferred is oxidized to water vapour in the stratosphere then six times as much water would have come from methane oxidation as from direct transfer of water. This is because the oxidation of one molecule of methane gives two molecules of water vapour. SINGER (1971) has challenged Newell’s estimate of the quantity of water vapour transferred to the stratosphere and has suggested that extra water is transferred upwards by thunderstorm activity. Singer also calculates the quantity of water formed by methane oxidation from the decrement of methane concentration with altitude in the stratosphere, Recent measurements of stratospheric methane, EHALTet ai. (1972), find a rapid decrease in concentration with height. This implies that the proportion of water coming from methane oxidation may be greater than Singer calculated and would bring the two estimates closer together. Both Singer and Newell’s calculations are based on meagre information but their results agree to the extent that between 40 and 80 per cent of the water vapour in the stratosphere comes directly from methane oxidation. There is ample proof that almost all of the atmospheric methane has a biological origin ROBINSON and ROBBINS(197 1). It follows that the production of oxygen in the upper atmosphere by water vapour photolysis is sustained by the production of methane at the Earth’s surface. This we propose is the second biological process for oxygen production. In addition to methane other hydrogen compounds more volatile than water are made in the biosphere; for example, molecular hydrogen and ammonia. In all it would seem that the passive inorganic upward transport of water vapour, as such, is by no means the only process for the upward expulsion of hydrogen compounds. ROBINS and ROHNSON {1971) estimated that the total biologica production of methane might be as high as 2 x lo9 tons yr’. This would imply that as much as 8 per cent of the total energy of photosy~~esis in the biosphere went to the production of methane. Mature biospheres are likely to be parsimonious over waste and it is difficuh to escape the conclusion that the methane production has an important purpose such as the maintenance of the planetary redox potential. It is important also that the current oxygen concentration is close to the maximum consistent with the survival of the current biosphere. At present grass and forest fires consume 2 x lo9 tons of carbon per annum. The probabiIity of fires doubles, approximately, for each I per cent increase in oxygen concentration in the region of 21 per cent. A 25 per cent oxygen atmosphere would be incompatible with standing vegetation even in rain forests. It is possible that grass and forest fires are the regulator of oxygen concentration although this suggestion seems an insult to the subtlety of biological A.%6/8---F,
578
JAMESE. L~VELIXK and JAMESP. LOWE, JR.
contrivance. It is also possible that methane production is the regulator as well as the source of oxygen. These views provide a bridge between the abiological theories of oxygen production such as proposed by BRINKMAN (1967) and those of BERKNER and MARSHALL (1965) who saw a prime role for the biosphere. Where methane of biological origin is the carrier of hydrogen from the Earth’s surface to provide water vapour in the regions above the cold trap both of these past theories become partially true. REFERENCES BATESD. R. and NICOLETM. (1950) The photochemistry of atmospheric water. J. Geuphys. Res. 55, 301-327. BARTHC. A., FASTICC. W., HORD C. W., PEARCEJ. B., KELLYK. K., STEWARTA. I., THOMASG. E., ANDERSONG. P. and RAPER 0. F. (1969) Mariner 6, ultra-violet spectrum of Mars upper atmosphere. Science 165,1004-1007. BERKNERL. V. and MARSHALLL. C. (1965) On the origin of oxygen concentration in the Earth’s atmosphere. J. Atrrws. Sci. 22,225-261. BRINKMANR. T. (1969) Dissociation of water vapour and the evolution of oxygen. J. Geophys. Res. 74, 5355-5386. CLOUD P. E. (1968) Atmospheric and hydrospheric evolution of the primitive Earth. Science 160, 729-736. EHALTD. H. (1967) Methane in the atmosphere. A.P.C.A. J. 17,518-519. EHALT D. H., HEIDT L. E. and MARTELLE. A. (1972) The concentration of atmospheric methane between 44 and 62 kilometres. J. Geophys. Res. Oceans and Atmospheres. (In Press). HITCHCOCKD. R. and LOVELOCKJ. E. (1967) Life detection by atmospheric analysis. Icarus 7, 149 159. HOLLANDH. D. (1964) The Origin and Evolution of Atmospheres and Oceans. (Edited by BRANCAZIO P. J. and CAMERONA. G. W.) 86-91. Wiley, New York. HIJTCHIN~~NG. E. (1954) The biochemistry of the terrestrial atmosphere. In The Earth as a Plant (Edited by KULPER G. P.) 371-433. Chicago Univ. Press. LOVELOCKJ. E. and GIFFEN G. E. (1969) Planetary atmospheres: compositional and other changes associated with life. In Advanced Space Experiments, 1968. (Edited by TIFFANY0. L. and ZAITZEFFE.), 179-194. Am. Astronautical Sot. Washington, D.C. NEWELLR. E. (1971) The global circulation of atmospheric pollutants. Sci. Am. 224,32-42. ROBINSONE and ROBBINSR. C. (1971) Global Effects of Environmental Pollution (Edited by SINGER F. S.). p 50. Springer-Verlag, New York. SINGER S. F. (1971) Stratospheric water vapour increase due to human activities. Nature 223,543-545. VAN VALENL. (1971) The history and stability of atmospheric oxygen. Science 171,439-443.
OXYGEN
Pergamon Press 1972. Vol. 6, pp. 575-578. Printed in Great Britain.
IN THE CONTEMPORARY
ATMOSPHERE
JAMESE. LOVELOCK Universityof Reading,Departmentof AppliedPhysicalSciences,Reading
and JAMES P. LODGE, JR. National Center for Atmospheric
Research, Boulder, Colorado
(First received 12 January 1972 and in final form 21 February 1972) Abstract-The contemporary oxygen content of the Earth’s atmosphere is explained either as a result of the abiological process of water vapour photolysis in the upper atmosphere or by the action of green plants and algae at the surface which produce a net increment of oxygen by the burial of carbon in sediments. This paper attempts to bridge the gap between these apparently contradictory views by recalling that a substantial proportion of water vapour in the upper atmosphere comes from the oxidation of biologically generated methane. It is further suggested that the oxygen ~ncentration is maintained at an optium by and for the Biosphere. IT IS usual
to explain the present composition of the Earth’s atmosphere as the end result of the working of abiological processes, such as upper atmosphere photochemistry on the gases and precursors of gases released from Earth’s interior. A few, most notably, HUTCHINSON(1954), BERKNERand MARSHALL(1965) and CLOUD(1968) have assigned some role to the biosphere in modifying the present atmospheric composition away from a wholly inorganic distribution of gases; their conclusions have been challenged by BRINKMAN(1969) who argues that oxygen in the present atmosphere is principally the product of inorganic processes. These and other arguments concerning oxygen in the atmosphere are discussed by VAN VALEN(1971). HITCHCOCKand LOVELOCK(1967) proposed that the presence of life on Earth could be inferred from an examination of the chemical composition of the atmosphere. In particular the simultaneous presence of oxygen and methane at their current levels is impossible on any conceivable abiologiczl steady state process. Later LOVELOCKand GIFFEN (1969) developed this notion further to include nitrogen as a life indicator and predicted that if Mars were lifeless then nitrogen would be absent from its atmosphere. This prediction was found to be correct by the ultra violet spectroscopic observations from the 1969 Mariner space craft, BARTH et al. (1969). The arguments which were used to prove the presence of life on Earth from a mere knowledge of the atmospheric composition can also be used to prove, given the fact that life on Earth is abundant, that the atmosphere is itself almost wholly a biological contrivance. The purpose of this note is to re-examine the state of oxidation of the contemporary atmosphere from this biological viewpoint. Almost all past models of the Earth’s atmosphere have assumed that abiological processes would in the long run tend to an oxidizing atmosphere. The loss of hydrogen, following upon the upper atmospheric dissociation of hydrogen compounds, the only conceivable process which could lead to a net increase in oxygen concentration, is in these models assumed to exceed the gain of hydrogen from the solar wind and from the processes at the surface and in the interior of the Earth. This conclusion 575
JAMESE.
576
LOVELOCK and JAMESP. LODOE,JR.
receives its strongest support from the fact that there is no process other than the expulsion of hydrogen by which in the long run the Earth can be oxidized. The rate of exchange of hydrogen between the Earth’s and the Solar atmosphere is unknown; also uncertain is the rate of hydrogen production equivalent to the reducing tendency of tectonic processes, aIthough HOLLAND(1964) has estimated that the reducing equivalent of volcanic gaseous emissions requires 2 x lo*’ g of oxygen over the past IO9 yr. This is comparable with the quantity of oxygen B~nkman proposes to have come from photolysis, but neither estimate is accurate enough to determine if abiological oxygen is in credit or debit balance. We do know, however, that if Mars and Venus are at the equilibrium redox potentiai appropriate to their stations in the solar system, then the interpolated oxidation state of the Earth at eq~librium would be about pE 8 which is very different from its current pE of 13. Should Mars and Venus be shown to be normal in this respect then the Earth has an anomalous redox potential. In this context the redox potential is a measure of the reducing or oxidizing tendency of a chemical system. This is ~onvenie~tIy expressed by the 1og~ithmic pE scale; a statement of -log&) where (e) is the electron concentration. This is analogous to the pH scale, -log&H+) for acidity and basicity. If we assume that this anomaly is a consequence of the presence of life it encourages a closer look for processes which could provide a farge increment of oxygen and then sustain its presence against the reducing tendencies of the internal and external bounds of the atmosphere. RKJBEY (1951) proposed that the release of a large quantity of oxygen could have accompanied the burying of carbon and sulphur #mounds, residues of earlier biospheres. Although no certain estimate of the quantity of oxygen equivalent to this burial has been made, there are undoubtedly substantial quantities of reduced materiaf of biological origin in the Earth’s crust. However, even if it could be shown that all the current quantity of oxygen has been released by this process it would still be necessary to account for the maintenance of a constant oxygen ~on~ntration in the face of con~nuing removal by exposure of reduced materials by tectonic processes. The Rubey mechanism is more in the nature of a loan of redox capital which eventually must be repaid through the recycling of buried materials; it is now in the course of repayment considerably augmented by man’s use of fossil fuels. Some ad~tional TABLE 1. SOURCESOF STRATOSPHERIC WATERVAPOKYR
By CH4
oxidation
(3) (1) Direct transfer from troposphere 7-o
(2) 34
-85 8.2
Cold trap temperature (“c) -75 -80 -70 3.5
1.54
0.71
-45 O-35
Units metric tons sT2. (1) NEWELL,1971. (2) SINGER,1971, (3) Calculated from the CH4: Hz0 mixing ratio using (1) is correct and taking the water vapour wncen~ations as those of the saturated vapour pressures over ice at the stated tern~~t~.
Oxygen in the Contemporary Atmosphere
577
biological process seems necessary to sustain the oxidizing atmosphere and perhaps also to maintain its composition constant. One may be found in the proposal of BATES and NICOLIX(1950) that the hydrogen compounds of the upper atmosphere, including water vapour, come not only from the upward movement of water vapour but also from the upward movement of methane, Transfer from the troposphere to the stratosphere occurs almost entirely at the tropical tropopause and at a rate equivalent to 2 x 10’4gyr-” water vapour (NEWELL, 1971). The temperature at the coldest level in this region IS close to -SO%, at which the saturated water vapour pressure expressed as a concentration by volume is 5 x 10m7 and is to be compared with the methane concentration of 15 x 10m6 at the same level, EHALT (1967). There will be no segregation of molecular species in the vapour phase during the transfer across the tropopause, therefore methane and water vapour will move upwards in the proportions of their mixing ratio. If all of the methane so transferred is oxidized to water vapour in the stratosphere then six times as much water would have come from methane oxidation as from direct transfer of water. This is because the oxidation of one molecule of methane gives two molecules of water vapour. SINGER (1971) has challenged Newell’s estimate of the quantity of water vapour transferred to the stratosphere and has suggested that extra water is transferred upwards by thunderstorm activity. Singer also calculates the quantity of water formed by methane oxidation from the decrement of methane concentration with altitude in the stratosphere, Recent measurements of stratospheric methane, EHALTet ai. (1972), find a rapid decrease in concentration with height. This implies that the proportion of water coming from methane oxidation may be greater than Singer calculated and would bring the two estimates closer together. Both Singer and Newell’s calculations are based on meagre information but their results agree to the extent that between 40 and 80 per cent of the water vapour in the stratosphere comes directly from methane oxidation. There is ample proof that almost all of the atmospheric methane has a biological origin ROBINSON and ROBBINS(197 1). It follows that the production of oxygen in the upper atmosphere by water vapour photolysis is sustained by the production of methane at the Earth’s surface. This we propose is the second biological process for oxygen production. In addition to methane other hydrogen compounds more volatile than water are made in the biosphere; for example, molecular hydrogen and ammonia. In all it would seem that the passive inorganic upward transport of water vapour, as such, is by no means the only process for the upward expulsion of hydrogen compounds. ROBINS and ROHNSON {1971) estimated that the total biologica production of methane might be as high as 2 x lo9 tons yr’. This would imply that as much as 8 per cent of the total energy of photosy~~esis in the biosphere went to the production of methane. Mature biospheres are likely to be parsimonious over waste and it is difficuh to escape the conclusion that the methane production has an important purpose such as the maintenance of the planetary redox potential. It is important also that the current oxygen concentration is close to the maximum consistent with the survival of the current biosphere. At present grass and forest fires consume 2 x lo9 tons of carbon per annum. The probabiIity of fires doubles, approximately, for each I per cent increase in oxygen concentration in the region of 21 per cent. A 25 per cent oxygen atmosphere would be incompatible with standing vegetation even in rain forests. It is possible that grass and forest fires are the regulator of oxygen concentration although this suggestion seems an insult to the subtlety of biological A.%6/8---F,
578
JAMESE. L~VELIXK and JAMESP. LOWE, JR.
contrivance. It is also possible that methane production is the regulator as well as the source of oxygen. These views provide a bridge between the abiological theories of oxygen production such as proposed by BRINKMAN (1967) and those of BERKNER and MARSHALL (1965) who saw a prime role for the biosphere. Where methane of biological origin is the carrier of hydrogen from the Earth’s surface to provide water vapour in the regions above the cold trap both of these past theories become partially true. REFERENCES BATESD. R. and NICOLETM. (1950) The photochemistry of atmospheric water. J. Geuphys. Res. 55, 301-327. BARTHC. A., FASTICC. W., HORD C. W., PEARCEJ. B., KELLYK. K., STEWARTA. I., THOMASG. E., ANDERSONG. P. and RAPER 0. F. (1969) Mariner 6, ultra-violet spectrum of Mars upper atmosphere. Science 165,1004-1007. BERKNERL. V. and MARSHALLL. C. (1965) On the origin of oxygen concentration in the Earth’s atmosphere. J. Atrrws. Sci. 22,225-261. BRINKMANR. T. (1969) Dissociation of water vapour and the evolution of oxygen. J. Geophys. Res. 74, 5355-5386. CLOUD P. E. (1968) Atmospheric and hydrospheric evolution of the primitive Earth. Science 160, 729-736. EHALTD. H. (1967) Methane in the atmosphere. A.P.C.A. J. 17,518-519. EHALT D. H., HEIDT L. E. and MARTELLE. A. (1972) The concentration of atmospheric methane between 44 and 62 kilometres. J. Geophys. Res. Oceans and Atmospheres. (In Press). HITCHCOCKD. R. and LOVELOCKJ. E. (1967) Life detection by atmospheric analysis. Icarus 7, 149 159. HOLLANDH. D. (1964) The Origin and Evolution of Atmospheres and Oceans. (Edited by BRANCAZIO P. J. and CAMERONA. G. W.) 86-91. Wiley, New York. HIJTCHIN~~NG. E. (1954) The biochemistry of the terrestrial atmosphere. In The Earth as a Plant (Edited by KULPER G. P.) 371-433. Chicago Univ. Press. LOVELOCKJ. E. and GIFFEN G. E. (1969) Planetary atmospheres: compositional and other changes associated with life. In Advanced Space Experiments, 1968. (Edited by TIFFANY0. L. and ZAITZEFFE.), 179-194. Am. Astronautical Sot. Washington, D.C. NEWELLR. E. (1971) The global circulation of atmospheric pollutants. Sci. Am. 224,32-42. ROBINSONE and ROBBINSR. C. (1971) Global Effects of Environmental Pollution (Edited by SINGER F. S.). p 50. Springer-Verlag, New York. SINGER S. F. (1971) Stratospheric water vapour increase due to human activities. Nature 223,543-545. VAN VALENL. (1971) The history and stability of atmospheric oxygen. Science 171,439-443.