EARLY TRIASSIC (INDUAN) WILDFIRE IN NORTHEASTERN PANGAEA: EVIDENCE FROM FOSSIL CHARCOALS IN BOGDA MOUNTAINS, NORTHWESTERN CHINA – IMPLICATIONS FOR RIPARIAN VEGETATION AND ATMOSPHERIC OXYGEN CONCENTRATION IN EARLY TRIASSIC

Wildfires are an integral part of modern and ancient ecosystems, destroying and/or renewing the burnt ecosystems. Charcoals indicating palaeowildfires have not been found in the Lower Triassic before. And lack of plants in the Early Triassic due to the end-Permian mass extinction and a low atmospheric oxygen level have been speculated as the major reasons for the scarcity of wildfires. This study, for the first time, discovered macroscopic charcoals in lacustrine sandstones of the Induan Jiucaiyuan Formation in northern Dalongkou Section, Bogda Mountains, NW China, located in the paleo-mid-latitude of NE Pangea. Petrographic and vitrinite reflectance data indicate that smoldering fires had occurred in a riparian area of a lake. The original plant is woody Agathoxylon-type gymnosperm trees. The charcoals are classified as seimifusinite and interpreted to be generated by a sustained wildfire in a densely-vegetated riparian lake environment. The reflectance value of the charred wood ranges from 1.17 to 2.31%, indicating that the temperature of the paleo-wildfire ranges from 350-500 ^oC. In addition, the concentration of atmospheric oxygen should be at or higher than 18.5%, in order to sustain the wildfire. The results and interpretations of this study, combing with previously-reported fossil wood and palynological data, suggest the presence of abundant woody gymnosperms in the study area and a moderately-oxygenated atmosphere during the Induan Age of Early Triassic. The wildfires probably had significantly disturbed the terrestrial ecosystems in Bogda Mountains in Early Triassic.