Isotopic and petrographic implications for fire type, temperature and formation of degradosemifusinite in fusain layers from an Early Cretaceous coal bed, Hailar Basin, Inner Mongolia, China

Fusain in coal or sediments is generally thought to originate from wildfire. In order to test this, eight fusain layers (1–30 mm thick) were analysed for organic petrography, total carbon, nitrogen content, δ¹⁵N and δ¹³C. Samples were collected from an Early Cretaceous coal seam (locally referred to as the #16 coal seam; lignite rank) within the Yimin Formation of the Hailar Basin, Inner Mongolia (China). The dominant composition of the targeted fusain bands is inertinite; however, vitrinite and liptinite were also identified in the samples during microscopic analysis. Inertinite (max. 81 vol%) is mainly comprised of fusinite (max. 31 vol%), semifusinite (46 max. vol%), degradosemifusinite (max. 22 vol%), degradofusinite (max 11 vol%) and inertodetrinite (max. 10 vol%). The vitrinite and liptinite macerals may be explained as a product of normal peat forming processes where resumed plant growth and peat accumulation would recycle and/or bioturbate the underlying fusain layer. The δ¹³C values for the fusain layers (average of −18.9‰ VPDB, standard deviation 0.50) are significantly less negative than what has been previously recorded for the same seam based on analysis of whole coal (average of –22.3‰ VPDB, standard deviation 0.46). Thus, positive shifts in δ¹³C values in the #16 seam may reflect higher proportions of inertinite, particularly fusinite and semifusinite. The only maceral that has a positive correlation with total nitrogen is degradosemifusinite. It is interpreted that it was combusted initially and then microbially altered, which is consistent with processes found to affect more modern charcoal. However, a fire origin for semifusinite is indicated because of its positive relationship with δ¹⁵N . Reflectance measurements of fusinite (1.35 % to 3.04 %, mean random) indicate that fire temperatures that resulted in the fusain layers sampled in this study ranged from 400 to 560 °C, but these estimates have a high uncertainty. Fire types (crown, surface or ground) are similarly uncertain based on comparison with temperatures recorded in modern fire conditions. The best estimate is that these fusain layers resulted from either or both surface or ground (smoldering) fires, which are the most common fire conditions in peatlands.