Igniting the Secret Wildfires of the Past: Searching for Wildfire Records in Caves to Unravel Hidden Paleo-fire Records

Abstract:

Cave environments are sensitive to environmental changes that can affect both the δ¹⁸O composition and solute concentrations of infiltrating cave dripwater and subsequently speleothem composition. The effect of wildfire on karst processes remains poorly understood. We provide a unique analysis of the effects of an intense wildfire on δ¹⁸O composition and solute concentrations of dripwater in a shallow cave, at a forested site in southwest Australia. By determining the local controls on dripwater chemistry, i.e. vegetation cover, evapotranspiration and carbonate mineral reactions, we determine a cave dripwater signature for wildfires in semi-arid regions. The dripwater response to the wildfire is clearest in the combined δ¹⁸O and Cl signature due to increased evaporation, and decreased transpiration in the case of Cl. Other solutes such as Mg, Sr and Ca strengthen this argument when coupled with δ¹⁸O over this time period. In fact our fire response from ¹⁸O shows an approx. 2‰ increase equivalent to some of the greatest changes seen in the Quaternary record. This highlights the significance of multi–year signatures that could be mistakenly interpreted as climate variability especially when combined with other proxies that would also be affected by fire, such as speleothem growth rate. Furthermore, the potential preservation of δ¹⁸O and Ca (as a growth rate determinant) in speleothems, combined with trace elements (Mg, Sr, Cl to a lesser extent), opens a new avenue for paleo-fire records.