Ozone, climate and biospheric environment in the ancient oxygen-poor atmosphere

Abstract

To study the climatological role of ozone in the Precambrian atmosphere and the consequences of its reduction for the ultraviolet environment of the early biosphere, a coupled one-dimensional radiative-convective and photochemical model has been developed. Oxygen levels between 10 −5 and 1 time the present atmospheric level (PAL) are considered. It is shown that when the ice-albedo feedback is taken into account, relatively important temperature decreases are associated with the ozone changes linked to the progressive decrease of the oxygen level from 1 PAL to smaller values. A similar study is performed for enhanced atmospheric CO 2 pressures ( P co 2 ). In these conditions, the ozone column is increased at low O 2 concentrations with respect to the P co 2 = 1 PAL case. Consequently, the larger CO 2 concentration in the ancient atmosphere could have contributed to strengthen the ultraviolet screening of ozone. The surface temperature response to the ozone decrease, as well as the thermal profiles are also analyzed in these CO 2-rich models. A possible evolutionary scenario of atmospheric O 2 and CO 2 is discussed. The consequences of these calculations for the ultraviolet environment of the primitive biosphere is discussed with a quantitative model calculating bacterial surviving rates. According to this model, the minimum ozone column being tolerable by unprotected bacteria would fall between ∼ 1 × 10 18 and ∼4 × 10 18 cm −2, depending on the bacterial species considered and corresponding to an O 2 level somewhat lower than 10 −2 PAL. For the coccoid blue-green alga Agmenellum quadruplicatum, this minimum ozone column would be of ∼4.5 × 10 18, a value which is only slightly less than the presently observed column in the spring time ozone hole of Antarctica.