IMPLICATIONS OF OZONE SHIELD FAILURE DURING THE END-PERMIAN BIOTIC CRISIS (Invited Presentation)

While the Siberian Trap volcanism is considered to have been a primary contributor to the end-Permian crisis (~252 million years ago), the connection between its activity and the observed floral and faunal turnover is unclear. Geochemical and paleo-atmospheric modelling studies have shown that the volcanism likely caused pulses of global deterioration of the stratospheric ozone layer, which must have resulted in increased levels of ultraviolet-B (UV-B) radiation at the Earth’s surface. Besides the stepwise decline or disappearance of several plant lineages, direct signals of environmental stress are recorded in palynological assemblages as elevated levels of clusters of unseparated lycopsid spore tetrads and malformed seed gymnosperm pollen. The malformed pollen grains are hypothesized to indicate both enhanced, volcanically induced ozone shield deterioration and reduced fitness of the parent plants. We experimentally testedhow elevated UV-B fluxes affect pollen development and reproductive success in the dwarf conifer Pinus mugo'columnaris'. The three elevated UV-B regimes used caused premature death of all seed cones before fertilization, and therefore rendered all conifer specimens sterile. The highest UV-B intensitiesalso induced a fivefold increase in specific types of pollen malformation frequencies. By impairing gymnosperm reproduction, frequent and/or prolonged volcanogenic ozone shield weakening could have triggered the step-wise seed plant decline observed in Permian-Triassic palynological assemblages, and repeated terrestrial biosphere destabilization and food web collapse. These findings challenge the paradigm that biotic crises require kill mechanisms and indicates that modern conifer forests may be relatively vulnerable to anthropogenic stratospheric ozone depletion.