Paleo-wildfire signatures revealing co-occurrence of angiosperm-gymnosperm in the early Paleogene: Evidences from woody charcoal and biomarker analysis from the Gurha lignite mine, Rajasthan, India

Abstract

Charcoal, a common product of wildfire, is abundant in many sedimentary rocks deposited in a wide range of environments, from terrestrial to marine. The analysis of paleofloras and the related paleoecological conditions is of great importance for the understanding of past environmental and paleoclimatic events. Woody charcoal is identified from the early Paleogene sediments of the Gurha lignite mine (Palana Formation), Bikaner, Rajasthan. This evidence is in the form of fragments of tracheids and vessels that show homogenized cell walls and well-preserved anatomical details, like pitting on cell walls, characteristic features of charcoal. These charcoal remains, most of which belong to angiosperms, represent the first verified occurrence of paleo-wildfires in this region during the early Paleogene. Moreover, a gymnospermous taxonomic affinity can be established for some of the charred woods and a relationship with conifers is likely, thus providing additional evidence for the taxonomic composition of early Paleogene floras in this region. Consecutively, the presence of diterpenoid compounds together with the occurrence of (mega-)sporinite in the lignite samples also shows the presence of gymnosperm vegetation (conifers).Furthermore, the petrographic composition reveals that the lignite samples are dominated and sub-dominated by the huminite (av. 43.2 vol%) and inertinite (av. 39 vol%) groups, respectively, followed by liptinite (av. 13.9 vol%) group of macerals. Fusinite, semifusinite (and inertodetrinite) macerals are commonly considered as fossil charcoal. The relatively higher content of these macerals in the studied lignite indicates the regular events of palaeofire during the deposition. Subsequently, the presence of unsubstituted polycyclic aromatic hydrocarbon (PAH) compounds identified in samples further suggests the occurrence of fire. Additionally, the inertinite (fusinite) reflectance indicates that the charcoal/inertinite was formed at a temperature above 300 °C, while the highest reflectance value suggests a burning temperature of ∼800 °C.