atmosphere, evolution of

atmosphere, evolution of

 the development of Earth's (Earth) atmosphere across geologic time. The process by which the current atmosphere arose from earlier conditions is complex; however, evidence related to the evolution of Earth's atmosphere, though indirect, is abundant. Ancient sediments and rocks record past changes in atmospheric composition due to chemical reactions with Earth's crust and, in particular, to biochemical processes associated with life.

      Earth's original atmosphere was rich in methane, ammonia, water vapour, and the noble gas neon, but it lacked free oxygen. It is likely that hundreds of millions of years separated the first biological production of oxygen by unicellular organisms and its eventual accumulation in the atmosphere.

      The composition of the atmosphere encodes a great deal of information bearing on its origin. Furthermore, the nature and variations of the minor components reveal extensive interactions between the atmosphere, terrestrial environment, and biota.

      The development of the atmosphere and such interactions are discussed in this article, with particular attention given to the rise of biologically produced molecular oxygen, O2, as a major component of air. For modern atmospheric chemistry and physics, see atmosphere.

Concepts related to atmospheric development
      A complete reconstruction of the origin and development of the atmosphere would include details of its size and composition at all times during the 4.5 billion years since Earth's formation. This goal could not be achieved without knowledge of the pathways and rates of supply and consumption of all atmospheric constituents at all times. Information regarding these particular processes, however, is incomplete even for the present atmosphere, and there is almost no direct evidence regarding atmospheric constituents and their rates of supply and consumption in the past.

      The contrast with related fields of Earth's history is notable. Fossils and other structural and chemical details of ancient rocks provide information useful to evolutionary biologists and historical geologists, but ancient atmospheres, “mere vapours,” have not left such substantial remnants. These vapours are, however, the stuff of stars and the moving force of storms and erosion.

The atmosphere as part of the crust
      To the Earth scientist, the crust includes not only the top layer of solid material (soil and rocks to a depth of 6 to 70 km [4 to 44 miles], separated from the underlying mantle by differences in density and by susceptibility to surficial geologic processes) but also the hydrosphere (oceans, surface waters on land, and groundwater beneath the land surface) and the atmosphere. Interactions among these solid, liquid, and gaseous portions of the crust are so frequent and thorough that considering them separately introduces more complexities than it eliminates. As a result, a description of the history of the atmosphere must concern itself with all volatile components of the crust.

      Volatile compounds as well as elements important in present and past atmospheres or in interactions between the atmosphere, biosphere, and other portions of the crust include the following:
● Present major components: molecular nitrogen (N2) and molecular oxygen (O2)
● Noble gases (noble gas): helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe)
● Abundant variable components: water vapour (H2O) and carbon dioxide (CO2)
● Other components: molecular hydrogen (H2), methane (CH4), carbon monoxide (CO), ammonia (NH3), nitrous oxide (N2O), nitrogen dioxide (NO2), hydrogen sulfide (H2S), dimethyl sulfide [(CH3)2S], sulfur dioxide (sulfur oxide) (SO2), and hydrogen chloride (HCl).

      Some elements appear in multiple form—for example, carbon as carbon dioxide, methane, or dimethyl sulfide. It is useful to consider the occurrence of the elements before focusing on the more specific aspects of atmospheric chemistry (the forms in which the elements are present). One can speak of Earth's “inventory of volatiles,” recognizing that the components of the inventory may be reorganized from time to time, but also that it is always composed primarily of the compounds of hydrogen, carbon, nitrogen, and oxygen, along with the noble gases.

      A process that delivers a gas to the atmosphere is termed a source for the gas. Depending on the question under consideration, it can make sense to speak in terms of either an ultimate source—the process that delivered a component of the volatile inventory to Earth—or an immediate source—the process that sustains the abundance of a component of the present atmosphere. Any process that removes gas either chemically, as in the consumption of oxygen during the process of combustion, or physically, as in the loss of hydrogen to space at the top of the atmosphere, is called a sink.

      Throughout the history of the atmosphere, sources and sinks have often been simultaneously present. While one process consumes a particular component, another produces it, and the concentration of that component in the atmosphere will rise or fall depending on the relative strengths of the sources and sinks. If those strengths are balanced (or nearly so), the composition of the atmosphere will not change (or will change only very slowly, perhaps imperceptibly); however, the molecules of the gas in question are passing through the atmosphere and are not permanently resident. The rate of the resulting turnover of molecules in the atmosphere is expressed in terms of the residence time, the average time spent by a molecule in the atmosphere after it leaves a source and before it encounters a sink.

Processes affecting the composition of the early atmosphere

Ultimate sources
 The material from which the solar system formed is often described as a gas cloud or, at a later stage, a solar nebula. The cloud was rich in volatiles (termed primordial gases) and must have been the ultimate source of the atoms in the present atmosphere. What is of primary concern, however, is the sequence of events and processes by which the volatiles present in the initial gas cloud were transferred to Earth's inventory and the efficiency with which this was accomplished.

      The formation of the solar system began when one portion of the gas cloud became dense enough due to compression by some external force—a shock wave from the explosion of a nearby supernova, perhaps—to gravitationally attract the material around it. This material “fell” into the region of higher density, making it even denser and attracting other material from still further away. As gravitational collapse continued, the centre of the cloud became very dense and hot, because the kinetic energy of the incoming material was released as heat. Thermonuclear reactions (thermonuclear reaction) began at the core of the central object, the Sun.

Capture and retention of primordial gases
       Abundances of elementsFar from the central point, the material in the gas cloud tended to settle to an extensive equatorial plane around the Sun. As the material in this disk cooled, chunks of rock grew and accreted to form the planets. The planets are much less massive than the Sun, but if they grew large enough and if the gases around them were cool enough, they could accumulate an atmosphere from the volatile components of the gas cloud. A partial inventory of that cosmo-chemical stockpile, the starting point for atmospheric development, is shown in the column for the solar system in the table (Abundances of elements). This direct capture is the first of three source mechanisms that can be described.

      A planetary (planet) atmosphere accumulated in this way would consist of primordial gases, but the relative abundances of the individual components would differ from those in the gas cloud if the gravitational field of the new planet were strong enough to hold some, but not all, of the gases around it. It is convenient to express the strength of a gravitational field in terms of escape velocity, the speed at which any particle (a molecule or spacecraft) must be traveling in order to overcome the force of gravity. For Earth, this velocity is 11.3 km (7.0 miles) per second, and it follows that, once the solid material had accumulated, gas molecules passing Earth at lower speeds would have been captured and accumulated to form an atmosphere.

      The speed at which a gas molecule moves is proportional to (T/M)1/2, where T is absolute temperature in kelvins (kelvin) (K) and M is molecular mass. The uppermost layers of the present atmosphere are still very hot and might have been much hotter early in Earth's history. At temperatures below 2,000 K, however, molecules of any compound with a molecular weight greater than about 10 will have an average velocity of less than 11.3 km per second (7.0 miles per second). On this basis, it has long been thought that Earth's earliest atmosphere must have been a mixture of the primordial gases with molecular weights greater than 10. hydrogen and helium, with molecular weights of 2 and 4, should have been able to escape. Because hydrogen is the most abundant element in the solar system, it is thought that the most abundant forms of the other volatile elements were their compounds with hydrogen. If so, methane, ammonia, and water vapour, together with the noble gas neon, would have been the most abundant volatiles with molecular weights greater than 10 and, thus, the major constituents of Earth's primordial atmosphere. The atmospheres of the four giant outer planets ( Jupiter, Saturn, Uranus, and Neptune) are rich in such components, as well as in molecular hydrogen and, presumably, helium, which those more massive and colder bodies were apparently able to retain.

Outgassing of the solid planet
 The release of gases during volcanic eruptions is one example of outgassing; releases at submarine hydrothermal vents are another. Although the gas in modern volcanic emanations (volcano) commonly derives from rocks that have picked up volatiles at Earth's surface and then have been buried to depths at which high temperatures remobilize the volatile material, a very different situation must have prevailed at the earliest stages of Earth's history.

      The planet accreted from solid particles that formed as the primordial gas cloud cooled. Long before the volatile components of the cloud began to condense to form massive solid phases (that is, long before water vapour condensed to form ice), their molecules would have coated the surfaces of the solid particles of rocky material that were forming. As these solid particles continued to grow, a portion of the volatiles coating their surfaces would have been trapped and carried thereafter by the particles. If the solids were not remelted by impact as they collected to form the planet, the volatiles they carried would have been incorporated in the solid planet. In this way, even without collecting an enveloping gaseous atmosphere, a newly formed planet could include—as material occluded in its constituent grains—a substantial inventory of volatiles.

      At some point in its early history, Earth became so hot that much of the iron dispersed among the solid particles melted, became mobile, and collected to form the core. Related events led to the formation of rocky layers that were the precursors of Earth's present-day mantle and crust. As part of this process of differentiation, volatiles present in the particles would have been released through outgassing. The outgassing must have occurred on a colossal scale if the accreting particles had retained their volatiles right up to the time of differentiation.

      An atmosphere created by retention of these outgassing products would derive ultimately from nebular gases. Its chemical composition, however, would be expected to differ in two principal respects from that of an atmosphere formed by the capture of primordial gases: (1) whereas the captured atmosphere would contain all gases that were moving slowly enough (that is, that were sufficiently cold and/or of sufficient molecular weight) so that it was possible for the planet to retain them gravitationally, the outgassed atmosphere would contain only those gases “sticky” enough to have been significantly retained in the rocky particles from which the planet formed; and (2) methane and ammonia, two presumed components of a captured atmosphere, would probably not be stable under the conditions involved in outgassing. Thus, the noble gases, which would be poorly held by particles, would be of low abundance relative to gases derived from chemically active elements. Further, the principal forms of carbon and nitrogen in an outgassed atmosphere would be carbon monoxide or carbon dioxide together with molecular nitrogen.

      A compromise between the extremes of direct capture and outgassing proposes that Earth's inventory of volatiles was delivered to the planet late in its accretionary history—possibly after differentiation was nearly complete—by impact of a “last-minute” crop of solid bodies that were very strongly enriched in volatile materials (these were the last substances to condense as the solar nebula cooled). Such bodies might have had compositions similar to those of comets (comet) that still can be observed in the solar system. These last-minute condensates may have coated the planet as a surface veneer that yielded gases only when heated during differentiation, or they may have released their volatiles on impact.

      Because such bodies would have been relatively small, they would not have been able to retain primordial gases by means of a substantial gravitational field. Their complement of volatiles, retained by cold trapping in ices and on particle surfaces, would be expected to resemble the “sticky” (that is, polar (chemical bonding) and reactive) gases occluded by solid particles at earlier stages of cooling of the gas cloud but possibly lost during earlier higher temperature phases of Earth's accretion.

      The dominant pathways by which gases are removed from the present atmosphere are discussed below in the section Biogeochemical cycles (atmosphere, evolution of). Apart from those processes, three other sinks merit attention and are described here.

Photochemical reactions (photochemical reaction)
      Sunlight can provide the energy required to drive chemical reactions that consume some gases. Due to a rapid and efficient photochemical (photochemical reaction) consumption of methane (CH4) and ammonia (NH3), a methane–ammonia atmosphere, for example, would have a maximum lifetime of about 1 million years. This finding is of interest because it has been suggested that life originated from mixtures of organic compounds synthesized by nonbiological reactions starting from methane and ammonia. Recognition of the short atmospheric lifetimes of these materials poses grave difficulties for such a theory. Water, too, is not stable against sunlight that has not been filtered by overlying layers containing ozone or molecular oxygen, which very strongly absorb much of the Sun's ultraviolet radiation. Water molecules that rise above these layers are degraded to yield, among other products, hydrogen atoms (H·).

      Hydrogen molecules (H2) and helium, or products like H·, tend to have velocities high enough so that they are not bound by Earth's gravitational field and are lost to space from the top of the atmosphere. The importance of this process extends beyond the very earliest stages of Earth's history because continuous sources exist for these light gases. Helium is continually lost as it is produced by the decay of radioactive elements (isotope) in the crust.

      A combination of photochemical reactions and the subsequent escape of products can serve as a source for molecular oxygen (O2), a major component of the modern atmosphere that, because of its reactivity, cannot possibly have derived from any of the other sources so far discussed. In this process, water vapour is broken up by ultraviolet (ultraviolet radiation) light and the resulting hydrogen is lost from the top of the atmosphere, so that the products of the photochemical reaction cannot recombine. The residual oxygen-containing products then couple to form O2.

Solar-wind stripping
      The Sun emits not only visible light but also a continuous flow of particles known as the solar wind. Most of these particles are electrically charged and interact only weakly with the atmosphere, because the Earth's magnetic field (Earth) tends to steer them around the planet. Prior to the formation of Earth's iron core and consequent development of the geomagnetic field, however, the solar wind must have struck the top layers of the atmosphere with full force. It is postulated that the solar wind was much more intense at that time than it is today and, further, that the young Sun emitted a powerful flux of extreme ultraviolet radiation. In such circumstances, much gas may have been carried away by a kind of atomic sandblasting that may have had a marked effect on the earliest phases of atmospheric development.

Biogeochemical cycles (biogeochemical cycle)
 Interactions with the crust and, in particular, with living things—the biosphere—can strongly affect the composition of the atmosphere. These interactions, which form the most important sources and sinks for atmospheric constituents, are viewed in terms of biogeochemical cycles, the most prominent and central being that of carbon. The carbon cycle includes two major sets of processes: biologic and geologic.

Biologic carbon cycle
      The biologic processes of photosynthesis and respiration mediate the exchange of carbon between the atmosphere or hydrosphere and the biosphere,

      In these reactions, CH2O crudely represents organic material, the biomass of bacteria, plants, or animals; and A represents the “redox (oxidation–reduction reaction) partner” for carbon (reduction + oxidation → redox), the element from which electrons are taken during the biosynthesis of organic material and which accepts electrons during respiratory processes. In the present global environment, oxygen is the most prominent redox partner for carbon (that is, A = O in the above equation), but sulfur (S) also can serve as a redox partner, and modified cycles based on other partners (such as hydrogen) are possible. Imbalances in the biologic carbon cycle can change the composition of the atmosphere. For example, if oxygen is the principal redox partner and if photosynthesis exceeds respiration, the amounts of O2 will increase. The carbon cycle can in this way serve as a source for O2. The strength of this source is dependent on the degree of imbalance between photosynthesis and respiration.

      The biologic degradation of organic material and the release of products to the atmosphere need not involve an inorganic redox partner such as oxygen or sulfur. Communities of microorganisms found in sediments are capable of carrying out the process of fermentation, in which electrons are shuffled among organic compounds. Many individual steps catalyzed by a variety of organisms are involved, but the overall reaction amounts to

      This process is an important source of atmospheric methane.

      The geologic portions of the carbon cycle can be described most conveniently by following a carbon atom from the moment of its injection into the atmosphere in the form of carbon dioxide released from a volcano. The carbon dioxide—any CO2 in the atmosphere—will come in contact with water in the environment and is likely to dissolve to form carbonic acid:

      This weak acid is an important participant in weathering reactions that tend very slowly to dissolve rocks exposed to precipitation and groundwater at Earth's surface. An exemplary reaction showing the conversion of a solid mineral to soluble products would be

      where s indicates solid and aq stands for aqueous solution. Along with the other products of this reaction, bicarbonate ions (ion) (HCO3-) derived from the volcanic CO2 would eventually be transported to the ocean. At all points in the hydrosphere, bicarbonate would be in equilibrium with other forms of dissolved CO2 through chemical reactions that could be depicted as follows:

      In settings where its concentration was enhanced, carbonate ions (CO32-) produced in this way could unite with calcium ions (Ca2+), which are naturally present in seawater due to weathering reactions, to form solid calcite (CaCO3), the principal mineral in limestone. The dissolved carbon dioxide might return to the atmosphere or remain in the hydrosphere. In either case, it eventually could enter the biologic carbon cycle and be transformed into organic matter. If the CaCO3 and the organic matter sank to the bottom of the ocean, they both would be incorporated in sediments and could eventually become part of the rocky material of the crust. uplift and erosion, or very deep burial and melting with subsequent volcanic activity, would eventually return the carbon atoms of the CaCO3 and the organic matter to the atmosphere.

Interaction of biologic and geologic cycles
      The pace of the biologic carbon cycle is measured in the lifetimes of organisms, while that of the geologic cycle is measured in the lifetimes of sedimentary rocks (sedimentary rock) (which average about 600 million years). Each interacts strongly with the atmosphere, the biologic cycle exchanging CO2 and redox partners and the geologic cycle supplying CO2 and removing carbonate minerals (carbonate mineral) and organic matter—the eventual source of fossil fuels (fossil fuel) (such as coal, oil, and natural gas)—in sediments. An understanding of the budgets and pathways of these cycles in the present global environment enables investigators to estimate their effects in the past, when conditions (the extent of evolution of the biota, the composition of the atmosphere, and so on) may have been quite different.

       Carbon in Earth's crustThe quantitative importance of these processes, now and over geologic time, can be summarized by referring to the table (Carbon in Earth's crust). Carbon in the atmosphere as carbon dioxide is almost the smallest reservoir considered in this tabulation, but it is the central point from which processes of the biogeochemical cycle have distributed carbon throughout Earth's history. Reconstructions of atmospheric development must recognize that the very large quantities of carbon now found in sedimentary carbonates and organic carbon have flowed through the atmosphere and that the organic carbon (which includes all fossil fuels as well as far more abundant, ill-defined organic debris) represents material produced by photosynthesis but not recycled by respiration. The latter process must have been accompanied by the accumulation of the oxidized forms (such as molecular oxygen, O2) of carbon's redox partners.

       Carbon in Earth's crustThe table (Carbon in Earth's crust) also emphasizes the dissolution of atmospheric gases by the ocean. The carbon dioxide in the atmosphere is in equilibrium with, and far less abundant than, the oceanic inventory of carbon dioxide, bicarbonate ions (HCO3-), and carbonate ions (CO32-). If all carbon dioxide were somehow suddenly removed from the atmosphere, the ocean would replenish the supply within a few thousand years (the so-called stirring time of the ocean). Likewise, any change in the concentration of CO2 in the atmosphere is accompanied by a quantitatively far larger change in the amount of CO2, HCO3-, and CO32- in the ocean. Similar equilibriums prevail for molecular nitrogen (N2) and molecular oxygen (O2). The atmosphere contains about 3,940,000 petagrams (Pg; one petagram equals 1015 grams) of nitrogen as N2, with about 22,000 Pg being dissolved in the ocean. oxygen is distributed in such a way that 1,200,000 Pg of O2 are in the atmosphere while 12,390 Pg are in the ocean.

weathering reactions
      No matter what their origins, reactive gases in the atmosphere are likely to interact with other parts of the crust through what are termed weathering reactions. Not just carbonic acid associated with the carbon cycle but any acid becomes involved in acidic dissolution of susceptible rocks. As it does so, its concentration in the atmosphere declines, eventually reaching zero unless some process keeps replenishing the supply.

      Even if respiration were suddenly to cease, oxygen produced by photosynthesis, or any oxidant in the atmosphere, would be consumed if oxidizable materials were present. The corrosion of metals is the most familiar example of this process in the modern world, but there are other examples involving natural forms of iron, sulfur, and carbon as well. Much of the iron bound in minerals is in the ferrous form (Fe2+). As this material is exposed by uplift and erosion, it consumes atmospheric oxidants to form ferric iron (Fe3+), the red, fully oxidized form of iron commonly identified as rust (Fe2O3). Sulfide minerals ( pyrite, or fool's gold, being the most familiar example) also consume oxidants as the sulfur is oxidized to produce sulfate. Finally, natural exposure of sedimentary organic matter, including coal beds or oil seeps, results in the consumption of atmospheric oxidants as the organic carbon is oxidized to produce carbon dioxide.

Sequence of events in the development of the atmosphere

Absence of a captured primordial atmosphere
       Abundances of elementsIf the planet grew large (and had, therefore, a substantial gravitational field) before all gases were dispersed from its orbit, it ought to have captured an atmosphere of nebular gases. The size and composition of such an atmosphere would depend on temperature as well as planetary mass. If the solid planet had reached full size and if temperatures were greater than 2,000 K, the minimum molecular weight that could be retained might have been high enough that the very abundant gases with molecular weights between 10 and 20 (methane, ammonia, water, and neon) would have been collected inefficiently, if at all. A thinner primordial atmosphere consisting of nebular gases with higher molecular weights (such as argon and krypton—see the table (Abundances of elements)), however, ought still to have been captured.

      In spite of this, characteristics of the present atmosphere show clearly that a primordial atmosphere either never existed or was completely lost. Explanations offered for both of these possibilities are linked to the development of the Sun itself. Astronomical observations of developing stars (that is, bodies similar to the early Sun) have shown that their early histories are marked by phases during which the gas in their surrounding nebulas is literally blown away by the pressure of light and particles ejected from the stars as they “turn on.” (After this initial intense activity, young stars begin life with an energy output significantly below their mid-life maximum.) If the removal of gases occurred in the solar system after nonvolatile solids had condensed but before the inner planets (Mercury, Venus, Earth, and Mars) accreted, it would have been impossible for Earth to capture a primordial atmosphere. Alternatively, if planetary accretion preceded ejection of gases and Earth had accumulated a primordial atmosphere, perhaps the early solar radiation, particularly the solar wind, was so intense that it was able to strip all gases from the inner planets, meeting the second condition described above—namely, complete loss.

Secondary atmosphere
      The atmosphere that developed after primordial gases had been lost or had failed to accumulate is termed secondary. Although the chemical composition of the atmosphere has changed significantly in the billions of years since its origin, the inventory of volatile elements on which it is based has not.

       Abundances of elementsThe elemental composition of the volatile inventory reveals its secondary origin. Abundances are given in the table (Abundances of elements) for 12 nuclides (nuclide) (species of atom) that can be associated with four groups:
● chemically active volatiles: hydrogen (H), carbon (C), nitrogen (N), oxygen (O), and sulfur (S)
● primordial noble gases: helium (4He), neon (20Ne), argon (36Ar), and krypton (84Kr)
● elements that form nonvolatile minerals: oxygen (O), magnesium (Mg), sulfur (S), and iron (Fe)
● a noble gas derived by the radioactive decay of a nonvolatile element: potassium-derived argon (40Ar).

      A comparison of entries in the table above shows that these groups have been collected by the planet with sharply varying efficiencies. The column headed “collection efficiency” has been derived by the division of the abundance of each element on Earth by its abundance in the solar system and multiplying by 100. If the collection efficiency is close to 100 percent, the abundances are nearly equal and the transfer of this element from the solar system's initial reservoir to the planet was highly efficient. If the collection efficiency is low, most of the element was lost and is “missing” from Earth's inventory. It is evident from the table that efficiencies of collection are correlated primarily with chemical characteristics, not mass. This is the pattern expected if volatiles were retained by chemical interactions that yielded nonvolatile phases rather than by gravitational attraction. Collection efficiencies for O, Mg, S, and Fe (which are included here only as representatives of the broad range of elements that were largely bound in nonvolatile solid phases as the solar nebula cooled) are high. Those for the chemically active volatiles that could not form minerals stable at high temperatures (H, C, and N) are much lower. Spectacularly decreased efficiencies of collection are associated with the primordial noble gases.

      The evidence points decisively to a process in which the elements to be retained in the terrestrial inventory were separated from those to be lost by a separation of solids from gases. The chemically active volatile elements could be incorporated in solids by formation of nitrides (nitride) and carbides (carbide), by hydration of minerals, and by inclusion in crystal structures (such as in the form of ammonium [NH4+] and hydroxide [OH-] ions) and could form some relatively nonvolatile materials independently (organic compounds with high molecular weights are found in meteorites and were probably abundant in the cooling solar nebula); yet, none of these mechanisms was available to the noble gases. Formation of a group of solids rich in chemically active volatiles, but not large enough to retain noble gases, followed by a loss of all materials still in the gas phase and an incorporation of the volatile-rich solids in the planet, would be consistent with the chemical evidence and with the processes described above as outgassing and importation.

       Abundances of elementsThe special case of 40Ar (argon) is particularly indicative of the derivation of the atmosphere through outgassing. Whereas the other noble-gas isotopes, 4He, 20Ne, 36Ar, and 84Kr, are primordial in origin, 40Ar derives primarily from the radioactive decay of the isotope 40K. Therefore, even though the solar system abundance of 40Ar is much lower than that of 36Ar, its abundance on Earth is much higher because, uniquely among the noble-gas isotopes listed in the table (Abundances of elements), its source—the rock-forming element potassium (K)—is part of the solid planet. As radioactive potassium in rocks decayed over Earth's history, the 40Ar produced first became trapped within mineral crystals at sites formerly occupied by K+, then was released when the crystals were melted in the course of igneous activity (igneous rock), and eventually reached the surface through outgassing. Given the abundance of potassium in Earth's crust, it would be impossible to attribute the origin of the atmosphere to outgassing if the abundance of 40Ar was far lower than that of 36Ar, as in the solar system.

Early composition
      The most critical parameter pertaining to the chemical composition of an atmosphere is its level of oxidation or reduction. At one end of the scale, an atmosphere rich in molecular oxygen (O2)—like Earth's present atmosphere—is termed highly oxidizing, while one containing molecular hydrogen (H2) is termed reducing. These gases themselves need not be present. Modern volcanic gases are located, for example, toward the oxidized end of the scale. They contain no O2, but all hydrogen, carbon, and sulfur are present in oxidized forms as water vapour (H2O); carbon dioxide (CO2); and sulfur dioxide (SO2); while nitrogen is present as molecular nitrogen (N2), not ammonia (NH3). A relationship prevails between the oxidation or reduction of outgassing volatiles and the inorganic material with which they come in contact: any hydrogen, carbon, or sulfur brought into contact with modern crustal rocks at volcanic temperatures will be oxidized by that contact.

      The abundance of hydrogen in the solar nebula, the common occurrence of metallic iron in meteorites (meteorite) (representative of primitive solids), and other lines of geochemical evidence all suggest that Earth's early crust was much less oxidized than its modern counterpart. Although all iron in the modern crust is at least partly oxidized (to Fe2+ or Fe3+), metallic iron may have been present in the crust as outgassing began. If the earliest outgassing products were equilibrated with metallic iron, hydrogen would have been released as a mixture of molecular hydrogen and water vapour, carbon as carbon monoxide, and sulfur as hydrogen sulfide. The presence of metallic iron during the last stages of outgassing is, however, unlikely, and, because H2 is not gravitationally bound, it would have been lost rapidly. At an early point, hydrogen would have been almost completely in the form of water vapour and carbon in the form of carbon dioxide. Nitrogen would have been outgassed along with the carbon and hydrogen. As carbon dioxide was consumed by weathering reactions and water vapour condensed to form the oceans, molecular nitrogen must have become the most abundant gas in the atmosphere. It is certain that molecular oxygen was not among the products of outgassing.

      Among the oldest rocks are water-laid sediments with an age of 3.8 billion years. Neither they nor any other ancient rocks contain metallic iron, though nearly all contain oxidized iron (Fe2+). Carbon is present both as organic material and in a variety of carbonate minerals (carbonate mineral). The existence of these sediments requires atmospheric pressures and temperatures consistent with the presence of liquid water. The nature of the iron minerals and their abundance suggest that Fe2+ was a significant component of ocean water and that concentrations of O2 had to have been essentially zero because Fe2+ reacts very rapidly with O2.

      The presence of organic carbon and carbonate minerals in the sediments dated 3.8 billion years old would be consistent with the development of a biologically mediated carbon cycle by that point in time, but the degree of preservation of these materials (which were heated to temperatures near 500 °C [932 °F] for millions of years at some point in their history) is so poor that the question cannot be settled. Relatively well-preserved sediments with an age of 3.5 billion years are far more abundant. In addition to abundant organic carbon and carbonate minerals, these sediments contain microfossils and other sedimentary features that demonstrate convincingly that life had arisen on Earth by that time. The distribution of the stable isotopes (isotope) of carbon (carbon-12 and carbon-13) in sedimentary materials younger than 3.5 billion years ago demonstrates that living organisms were effectively in control of the global carbon cycle from that time onward.

      The existence of sedimentary carbonates is direct evidence that carbon dioxide was present in the atmosphere. Its precise abundance is not known, but the best estimates are that it was substantially higher, perhaps by as much as 100 times, than the present atmospheric level. A strongly enhanced greenhouse effect (see the sections on carbon budget and energy budget in atmosphere), leading to more efficient retention of heat derived from solar radiation, would be expected. For many students of Earth's history, the fact that the early oceans did not freeze in spite of the dim Sun is evidence that the abundance of atmospheric carbon dioxide was high enough to provide the enhanced greenhouse effect.

Rise of molecular oxygen
 Recognition of the nature of Earth's pre-oxygenic environment is critical to consideration of this problem. If humans could somehow travel back in time to Earth of 3 billion years ago, they would find that space suits would have been required. More dramatically, if those time-traveling astronauts were somehow able to take with them all of the oxygen from the modern atmosphere, they would find that it would disappear soon after release. Not only was oxygen absent in the early atmosphere, but potent sinks for O2 were abundant as well. Oxidizable materials such as ferrous iron, sulfides, and organic compounds littered environments from which they are now absent. These chemicals absorbed O2 almost immediately after its release. Moreover, as the oxygen-absorbing capacity of such compounds was exhausted, new material that had been eroded from the unoxidized crust took their place. This process continued until the rock cycle (sedimentation, burial, igneous activity, uplift, and erosion) had exposed all oxidizable materials in the crust. No matter what the supply of O2, the process must have taken time (about half the rock volume of the crust is recycled every 600 million years). It is, therefore, very important to distinguish clearly between the first biologic production of O2 and its persistent accumulation in the atmosphere. It is conceivable, even likely, that these events were separated by hundreds of millions of years. The abundance of O2 at each point is expressed in terms of its approach to the present atmospheric level (PAL). For example, because the pressure of O2 in the present atmosphere is 0.21 atmosphere (3.1 pounds per square inch or 212.7 millibars), a planetary atmosphere containing 10 percent of that amount, 0.021 atmosphere (0.3 pound per square inch or 21.3 millibar), would be described as having an oxygen level of 0.1 PAL.

      The strength of this source is limited by the requirement that water vapour rise in the atmosphere to altitudes at which solar ultraviolet radiation capable of cleaving water molecules has not yet been absorbed by other atmospheric constituents. The transport of water vapour to high altitudes is severely impeded by a cold layer in the atmosphere. Water vapour freezes in this layer, and the rate of photochemical (photochemical reaction) production of O2 is thus limited. The severity of this limitation is not precisely known, but it is evident that atmospheric levels of oxygen did not rise until oxygenic photosynthesis was well established. This does not indicate that photochemical production of O2 was insignificant. Rather, it demonstrates that the strength of the process as a source was exceeded by the strength of the contemporary oxygen sinks (chiefly oxidative weathering reactions at Earth's surface) and that residence times for O2 were so short that significant atmospheric concentrations could not accumulate. The best estimate is that pressures of O2 at sea level and ground level were less than 5 × 10-8 PAL.

Onset of oxygenic photosynthesis
      The development of a biologically mediated carbon cycle prior to 3.5 billion years ago virtually requires that some form of photosynthesis had arisen by that time, but the possibility remains that sulfur or hydrogen, not oxygen, was serving as the redox partner (the agent removing electrons from carbon during the oxidation process). It also has been noted that some sediments 3.5 billion years in age contain microfossils with shapes resembling those of modern oxygenic photosynthesizers. This is suggestive, though not compelling, evidence that oxygenic photosynthesis had developed by 3.5 billion years ago. Shape is an infamously imprecise indicator of biochemical characteristics of microorganisms. More specifically, while it might be possible to recognize a photosynthetic organism from its shape, it is very difficult to determine exactly what redox partners that organism employed.

      Geochemical and paleontological features of sedimentary rocks 2.8 billion years in age offer stronger evidence that oxygenic photosynthesis had arisen by that time. At 2.8 billion years, the abundance of carbon-13 in sedimentary organic carbon decreases sharply from levels maintained between 3.5 billion and 2.9 billion years ago, then slowly rises, regaining those levels about 2.2 billion years ago. This has been interpreted in terms of a transient in the biogeochemical carbon cycle in which biogenic methane (CH4), which is strongly depleted in carbon-13, served as an important mobile constituent of the cycle during the interval from 2.8 billion to 2.2 billion years ago. According to this interpretation, methane was able to play this role only after O2 became available and facilitated its metabolism. As O2 sinks decreased in strength and the atmosphere became oxidizing, however, the mobility of methane was reduced and the methane cycle took on its modern form, which seldom leads to strongly decreased abundances of carbon-13 in sedimentary organic matter.

      Microfossils resembling modern oxygenic photosynthesizers also appear in sediments of this age, and they are accompanied by sedimentologic features (apparent “fossil gas pockets”) that are interpreted as evidence of aerobic metabolism. Thus, evidence dating from about 2.8 billion years ago is more abundant and diverse (geochemically, morphologically, and sedimentologically) than that found in rocks 3.5 billion years of age. In spite of these points of consistency, this evidence is not decisive.

      Evidence from younger sediments indicates that oxygenic photosynthesis almost certainly developed earlier than 2.2 billion years ago. Whatever the precise moment of development, it marked the origin of the first so-called oxygen oasis, a restricted environment in which the abundance of O2 rose above 5 × 10-8 PAL probably quite significantly. Within such oases, aerobic metabolism could occur. At their margins, the delivery of oxidizable materials from the surrounding global environment overwhelmed the local supply of O2. Overall, the atmosphere did not become oxidizing, but, as oxygenic photosynthesizers proliferated, the number and size of the oases grew.

Transition to an aerobic environment
       pyrite and uraninite are minerals of iron and uranium, respectively, that are not stable in the presence of O2. Though they can be found in some modern river sediments, neither can survive in them for thousands of years. Yet, many sediments older than about 2.2 billion years contain well-rounded grains of these minerals. Their shapes and locations indicate prolonged exposure and tumbling in ancient rivers or as beach deposits, but there is no evidence of chemical attack by oxygen. The precise significance of this observation is best considered together with measurements of the movement of iron in fossil soil profiles.

      If soil gases (in equilibrium with the atmosphere) contain O2, iron exposed during the breakdown of soil minerals will be immobilized by oxidation and will not be leached from near-surface soil horizons. Conversely, if O2 is absent during soil development, chemical analysis of fossil soils will reveal depletion of iron near the former soil surface. Rates of the dissolution of uraninite and the leaching of iron in soil profiles also depend on the abundance of carbon dioxide (CO2). Because the patterns of dependence are different, the combination of evidence based on both phenomena allows for the estimation of abundances of both CO2 and O2. This line of interpretation leads to the conclusion that about 2.2 billion years ago the ratio of the molecular abundance of O2 to that of CO2 was about 1.3 (at present it is 635), and that the pressure of O2 was near 0.01 PAL while that of CO2 was about nine times higher than at present. Many authorities agree that the uraninite and fossil soil data indicate the development of oxidizing conditions at the surface by 2.2 billion years ago, but they place the most probable level of O2 lower by a factor of 10 or more.

 The consumption of oxidizing power by the crust is recorded by the inorganic constituents of sedimentary rocks. Iron-bearing sediments, or iron formations, are of particular interest because the collection of substantial quantities of iron in a sedimentary basin requires that iron be mobile in the world ocean. Mobility requires solubility, and, while Fe2+ is soluble, Fe3+, the form of iron that results if O2 comes in contact with Fe2+, is highly insoluble.

      Three states can be distinguished:
● The existence of iron formations containing only Fe2+ suggests a complete absence of oxygen.
● The existence of iron formations containing Fe2+ and Fe3+ indicates that levels of oxygen were low enough—essentially zero in the deep ocean—so that iron was mobile, but it also suggests that O2 (perhaps at an oxygen oasis) was important in triggering deposition of the iron, though other means of oxidation—photochemical processes, for example—are quite conceivable.
● The disappearance of iron formations from the sedimentary record suggests persistent oxygenation of the ocean.

      This sequence of possibilities is represented in the geologic record as follows:
● The oldest sedimentary rocks are iron formations that contained only Fe2+ at the time of their deposition.
● The first appearance of primary Fe3+ (produced during the formation of the rock rather than in later weathering) was in iron formations about 2.7 billion years ago.
● Iron formations disappeared almost completely from the record about 1.7 billion years ago (with a few isolated and very small recurrences about 1 billion years ago).

      Moreover, the abundance of iron formations increased significantly from 2.7 billion to 2.2 billion years ago, suggesting that some new factor, possibly oxidative precipitation of Fe3+, was enhancing the rate of deposition. It is for this same time interval that isotopic evidence from carbon indicates the operation of an O2-dependent methane cycle.

      Evidence for the evolution of eukaryotic (eukaryote) organisms (those containing a membrane-bound nucleus and other organelles) first appears in the microfossil record of about 1.4 billion years ago. Biochemical reactions that occur during the growth and division of such cells require oxygen levels of 0.02 PAL. Attainment of that level by 1.4 billion years ago apparently led to oxygenation of the deep sea and the cutoff of deposition of iron formations about 1.7 billion years ago.

Attainment of the modern O2 level
      The abundance of carbon-13 in sedimentary organic materials and in carbonates from 900 million to 600 million years ago indicates that unusually large quantities of organic carbon were buried without reoxidation during that interval. The burial of this carbon must have been accompanied by the accumulation of oxidized forms of carbon's redox partners. The quantities released were adequate to raise the level of O2 to 1.0 PAL or more.

      It has been calculated that oxygen requirements of the earliest animals, which developed about 700 million years ago, would have been met—if the animals had circulatory systems that incorporated oxygen carriers like hemoglobin—by O2 abundances as low as 0.1 PAL. If circulatory systems had not yet evolved, an O2 abundance of 1.0 PAL would have been required. Studies of fossils indicate that the animals were very thin (1–6 mm [0.04–0.24 inch]) in spite of great breadth and length (up to 1,000 mm [39 inches]). Such a shape seems optimized for transport of O2 by diffusion from the surrounding water to the cells in which it was needed, thus pointing to the latter higher value (namely, an O2 abundance of 1.0 PAL). Other reconstructions of O2 levels based on biologic evidence suggest that the widespread development of land plants about 400 million years ago must have driven O2 to levels near 1.0 PAL, and they show O2 levels rising smoothly from levels near 0.1 PAL at 650 million years ago to 1.0 PAL at 400 million years ago.

Variation in abundance of carbon dioxide
      The approximately hundredfold decline of atmospheric carbon dioxide (CO2) abundances from 3.5 billion years ago to the present has apparently not been monotonic. During that interval, numerous ice ages (ice age) have come and gone. Significant changes in climate can result from geographic changes, but it is generally concluded that modulation of the efficiency of Earth's greenhouse effect is also required to produce the extreme variations associated with widespread continental glaciations. In recognition of this, broad climatic variations during the past 750 million years have been described in terms of alternating “icehouse” and “greenhouse” episodes.

      Icehouse conditions—apparently associated with the depletion of atmospheric CO2, the principal greenhouse gas—have prevailed since about 65 million years ago and during two earlier periods, 650 million–530 million and 360 million–240 million years ago. It is suggested that intervening greenhouse episodes have been associated with higher abundances of CO2 in the atmosphere. It has been suggested that the modern buildup of atmospheric CO2, due in large part to modern industrial and agricultural activities, could result in the melting of the polar ice caps and the subsequent flooding of coastal areas (see the article global warming).

J.M. Hayes

Additional Reading
Astrophysical considerations bearing on the earliest stages of atmospheric development are reviewed in John S. Lewis and Ronald G. Prinn, Planets and Their Atmospheres: Origin and Evolution (1984). Aspects of subsequent development are discussed in James C.G. Walker, Evolution of the Atmosphere (1977); and Heinrich D. Holland, The Chemical Evolution of the Atmosphere and Oceans (1984). Numerous chapters in J. William Schopf (ed.), Earth's Earliest Biosphere: Its Origin and Evolution (1983), discuss biologic controls of atmospheric composition and their development over time. Development of the biogeochemical cycle of carbon and its interactions with the atmosphere are discussed in E.T. Sundquist and W.S. Broecker (eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present (1985).J.M. Hayes

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Universalium. 2010.

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