The evolution of atmospheric ozone

Abstract

A one-dimensional coupled chemistry and flow model of the earth's atmosphere is used to study the relationship between ozone content and oxygen level. The effects of the biogenic trace gases methane and nitrous oxide are considered, as well as the production of odd nitrogen in lightning discharges. The ozone column depth is found to increase monotonically with increasing oxygen content, in contrast with previous predictions that the column depth should peak at ---10 -• PAL (present atmospheric level) of 02. The critical 02 level at which an effective ultraviolet shield is produced is in the neighborhood of 10 -1 PAL. Implications of this result for the emergence of terrestrial land life are considered. INTRODUCTIONAtmospheric ozone plays an important role in shaping our modern terrestrial environment by shielding out solar ultraviolet light in the 2000 to 3000-3, range. This, together with the realization that the amount of oxygen in the earth's atmosphere must have been much lower in the past than it is today [Walker, 1977], has led several authors to investigate the evolution of ozone as a function of atmospheric oxygen content.A question of particular interest is whether or not the development of the ozone layer was responsible for the emergence of life on land during the Late Silurian period, about 420 m.y. (million years) ago.The first serious attempt to solve this problem can in a series of papers by Berkner and Marshall [1964, 1965, 1966, 1967]. They predicted that a biologically effective ultraviolet shield would have been provided by a total ozone column of 0.2 arm cm, which should have reduced the ultraviolet flux at the earth's surface to less than 1 erg cm -2 s -l (50/•)-l. Using a semiempirical method to relate ozone densities to atmospheric oxygen levels, they found that this 03 column depth should have been achieved at an 02 level of approximately 10 -l PAL (present atmospheric level). Since their scenario for the rise of atmospheric oxygen called for an 02 level of 10 -2 PAL at the dawn of the Cambrian period (-600 m.y.), they suggested that the subsequent increase in oxygen to 10 -l PAL and the accompanying development of the ozone screen was 4irectly responsible for the spread of life onto land during the Silurian.The estimation of ozone column depth versus oxygen content was repeated in a more realistic fashion by Ratnet and Walker [1972], who calculated ozone densities by assuming photochemical equilibrium in an oxygen-nitrogen atmosphere. Their model predicted that the ozone column depth should increase as the oxygen content decreased from I to 10 -l PAL, as a result of the speeding up of the three-body ozone formation reaction as the 03 peak moved downward in the atmosphere. Below 10 -l PAL of 02 the ozone column depth began to decline, but much more slowly than had been predicted by Berkner and Marshall. Thus in spite of their use of a stricter biological tolerance limit on ultraviolet flux, requiring a minimum 03 column depth of 7 x 1018 cm -2 (0.26 atm cm), they found that the critical l...

Dynamic model constraints on oxygen-17 depletion in atmospheric O 2 after a snowball Earth

“…Here we constructed a coupled, four-box, and quick-response biosphereatmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO 2 concentrations proposed by the "snowball' Earth hypothesis produce a typical MOSD duration of less than 10 6 y and a magnitude of 17 O depletion reaching approximately −35‰. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard.…”

“…Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO 2 and low O 2 concentration (e.g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate 17 O depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean-atmosphere exchange only if partial pressure of atmospheric O 2 is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown.…”

“…A large perturbation in atmospheric CO 2 and O 2 or bioproductivity will result in a drastic pulse of 17 O change in atmospheric O 2 , as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated.…”

“…non-mass-dependent | photochemical reaction | O 3 | postglacial cap carbonate | stratosphere A tmospheric O 2 carries non-mass-dependently depleted 17 O signatures owing to photochemical reaction involving O 3 , CO 2 , and O 2 in the stratosphere (1)(2)(3)(4)(5). The magnitude of 17 O depletion in tropospheric O 2 is regulated largely by partial pressure of atmospheric CO 2 and O 2 (i.e., pCO 2 and pO 2 ) and biological O 2 production fluxes that determine the residence time of atmospheric O 2 (3,6).…”

Potential of ozone formation by the smog mechanism to shield the surface of the early Earth from UV radiation

“…comparable with our study. Kasting and Donahue (1980) suggested that 3 ppbv may be too high. Since then, additional NOx sources e.g.…”