The stability of the stratospheric ozone layer during the end-Permian eruption of the Siberian Traps.

The discovery of mutated palynomorphs in end-Permian rocks led to the hypothesis that the eruption of the Siberian Traps through older organic-rich sediments synthesized and released massive quantities of organohalogens, which caused widespread O3 depletion and allowed increased terrestrial incidence of harmful ultraviolet-B radiation (UV-B, 280-315nm; Visscher et al. 2004 Proc. Natl Acad. Sci. USA 101, 12952-12956). Here, we use an extended version of the Cambridge two-dimensional chemistry-transport model to evaluate quantitatively this possibility along with two other potential causes of O3 loss at this time: (i) direct effects of HCl release by the Siberian Traps and (ii) the indirect release of organohalogens from dispersed organic matter. According to our simulations, CH3Cl released from the heating of coals alone caused comparatively minor O3 depletion (5-20% maximum) because this mechanism fails to deliver sufficiently large amounts of Cl into the stratosphere. The unusual explosive nature of the Siberian Traps, combined with the direct release of large quantities of HCl, depleted the model O3 layer in the high northern latitudes by 33-55%, given a main eruptive phase of less than or equal to 200kyr. Nevertheless, O3 depletion was most extensive when HCl release from the Siberian Traps was combined with massive CH3Cl release synthesized from a large reservoir of dispersed organic matter in Siberian rocks. This suite of model experiments produced column O3 depletion of 70-85% and 55-80% in the high northern and southern latitudes, respectively, given eruption durations of 100-200kyr. On longer eruption time scales of 400-600kyr, corresponding O3 depletion was 30-40% and 20-30%, respectively. Calculated year-round increases in total near-surface biologically effective (BE) UV-B radiation following these reductions in O3 layer range from 30-60 (kJm(-2)d(-1))BE up to 50-100 (kJm(-2)d(-1))BE. These ranges of daily UV-B doses appear sufficient to exert mutagenic effects on plants, especially if sustained over tens of thousands of years, unlike either rising temperatures or SO2 concentrations.