Evidence of Neogene wildfires in central Chile: Charcoal records from the Navidad Formation

Mediterranean-type climate (MTC) ecosystems are characterized by recurrent wildfires. Although the majority of wildfires are human-ignited, non-anthropogenic (i.e., natural) wildfires are common in all MTC regions except central Chile. The low frequency of natural wildfires in this Chilean region is explained by the scarcity of non-anthropogenic ignition sources, basically thunderstorm-induced lightning and volcanic activity. However, from a geological perspective, the current relative absence of non-anthropogenic wildfires in central Chile is a recent phenomenon. In the transition from the Early to Middle Miocene, the climate in the region was likely warm and seasonally dry. Such climate conditions would allow the growth of fuel during the spring, becoming flammable during the dry season. This fire-prone landscape would likely have been ignited by the high volcanic activity that concomitantly occurred with the orogeny of the Andes. To evaluate this hypothesis, we sampled rocks from the three locations at the Navidad Formation, considering the fossil plant evidence deposited during this warm and seasonally dry period. We found a high concentration of charcoal in Playa Navidad, coinciding with the global warming event reported between 17 and 15Ma. The predominance of microscopic charcoal particles (between 125 and 250μm) only allows us to infer the occurrence of Neogene fires at a regional scale. The fused cell walls preserved in the charcoal anatomy likely suggest that such fire events were highly severe. The presence of pumice associated with the high charcoal concentrations supports the hypothesis of volcanic ignition sources. Very little charcoal was found in Punta Perro (Late Oligocene – Early Miocene) or in Cerro Los Pololos (late Middle Miocene), where fire may have been limited by fuel humidity and ignition sources respectively, although changes in the depositional environment would also help explain differences in the fire record throughout the Navidad Formation. In conclusion, our results provide the first quantitative Neogene charcoal record for South America. The evidence of fire occurrence in central Chile during the Neogene will contribute to understanding the evolution of the Mediterranean-type flora.