Continental-scale fern savannah wildfires during end-Triassic greenhouse warming

Abstract

Two-hundred-million years ago, the emission of an estimated 100,000 gigatonnes of CO2 from the Central Atlantic Magmatic Province had dire consequences for the biosphere, driving the end-Triassic mass extinction. Palynological assemblages across the end-Triassic mass extinction obtained from drill cores in Germany, Luxemburg, Denmark and the UK show a remarkable darkening of pollen and spores that is at odds with simple thermal maturation. Here we investigate this latest Triassic ‘dark zone’ using the Palynomorph Darkness Index obtained from trilete spores derived from ferns and Classopollis pollen derived from conifers. Coinciding with forest collapse and the spread of a fern-dominated pioneer vegetation, peak darkness occurs in the Triletes Beds in Germany and equivalent strata elsewhere. Comparison of controlled heating experiments of spores from the modern clubmoss Lycopodium indicates that this latest Triassic darkening of palynomorphs was caused by surface fires passing through fern savannahs. Our analyses of microcharcoal and pyrolytic polycyclic aromatic hydrocarbons records further support frequent and widespread wildfire activity across Europe during the end-Triassic mass extinction. Continental-scale fern savannah wildfires during the end-Triassic mass extinction suggests that intense climate change, including heat stress, and periods of prolonged drought alternating with extreme precipitation events contributed to the collapse of terrestrial ecosystems. Fern savannahs spread rapidly following the collapse of the forest ecosystem during the end-Triassic mass extinction, setting the conditions for widespread wildfires, according to an analysis of the darkening of fossil pollen and spores.