Direct evidence for elevated UV-B radiation and ozone layer disruption during the end-Permian mass extinction

Abstract

The end-Permian mass extinction (EPME) is the largest extinction event of the Phanerozoic, but the specific causal pathways, especially in the terrestrial realm, are unresolved. Malformed pollen and spores recovered from the EPME interval have been taken as indicators of extreme environmental stress in terrestrial ecosystems. However, whether they relate to volcanism-driven ozone-layer deterioration and enhanced ultraviolet-B (UV-B) flux, or volcanogenic toxic pollutants including mercury and acid rain, or some combination of the two, remains unclear. Here, we take advantage of a novel palynological proxy, which utilises the ability of land plants to adjust the concentration of protective UV-B-absorbing compounds (UACs) in the outer wall of their reproductive propagules in response to changes in ambient UV-B flux. We analysed UAC abundances in ca. 800 pollen grains from an independently-dated Permian-Triassic boundary section in southern Tibet, in order to infer changes in UV-B-radiation flux at the Earth’s surface during the EPME. Our data reveal an excursion in UACs that coincides with a spike in mercury concentration and a negative carbon-isotope excursion in the latest Permian deposits, suggesting a close temporal link between large-scale volcanic eruptions, global carbon- and mercury-cycle perturbations, and ozone-layer disruption. Because enhanced UV-B radiation can exacerbate the environmental deterioration induced by massive magmatism, ozone depletion is considered a compelling ecological driver for the terrestrial mass extinction.