Evidence of widespread wildfires in coal seams from the Middle Jurassic of Northwest China and its impact on paleoclimate.

Today wildfire is an important disturbance in many continental ecosystems, and it is assumed that with ongoing climate changes the frequencies and impacts of wildfires will increase in many regions. One way to obtain information about the potential long term influences of wildfires on ecosystems and the climate system itself is to study palaeo-wildfires. A total of 140 Jurassic samples, including 122 coal samples and 18 rock samples from roof and floor of coal seams, were collected from 4 coalfields in Northwestern China. The samples were analyzed by macropetrography, microscopy, scanning electron microscopy, gas chromatography, and gas chromatography–mass spectrometry to study the evidences of wildfires and their impact on the paleoclimate. High contents of pyrogenic inertinites (charcoal), natural char and natural coke particles observed in the samples which indicate that widespread wildfire events occurred in the early Middle Jurassic in all over Northwestern China. Additionally, high-molecular-weight polycyclic aromatic hydrocarbons were detected in the coal samples. These aromatic compounds were formed under high temperature and provide further evidence of wildfires. According to a carbon emission model for modern forest fires, the total carbon and gas emissions from wildfires in peat swamps of the Middle Jurassic in Northwest China were calculated by the data from 21,575 coal samples of number of 1567 drill core samples. The total carbon released from peat-swamp forest wildfires was at least 1207 Gt, corresponding to emissions of CO 2 , CO and CH 4 of 1926 Gt, 163 Gt, and 8.8 Gt, respectively. The huge amount of CO 2 released by forest fires in the peat swamp systems of the Middle Jurassic in Northwestern China alone was equivalent to 64.20% of the total CO 2 in the atmosphere at present (3000Gt). The large amounts of greenhouse gases released by wildfires must have led to long-term changes in the atmospheric composition and have caused global warming, which could have the reason of affected the paleoclimate. • High contents of charcoal indicate that wildfire events occurred in the early Middle Jurassic. • The main types of wildfires were surface fires and ground fires, with less crown fires. • The oxygen levels could have reached from 27% to 29% in the Middle Jurassic. • The greenhouse gases released by wildfires in the Middle Jurassic in Northwest China reached at least 2097.6 Gt • Greenhouse gases released by wildfires might have affected the paleoclimate and paleoecosystems. [ABSTRACT FROM AUTHOR]