Polycyclic aromatic hydrocarbons in tropical Australian stalagmites: a framework for reconstructing paleofire activity

Human activity and climate change are altering natural rates and intensities of wildfire, but the scale and extent of burning prior to the modern era are poorly understood. Paleofire activity may be reconstructed using a variety of records (e.g., charcoal in lake sediments, burn scars on tree rings), but these are not available in all environmental settings. Here we investigate the utility of a novel paleofire proxy: trace pyrogenic organic compounds in stalagmites. Polycyclic aromatic hydrocarbons (PAHs) are linked to burning of biomass and are transported downward through soil and bedrock by infiltrating rainwater and incorporated into stalagmites in underlying caves where they are preserved. In order to test links between the stalagmite PAHs and fire above the cave, we performed a series of experiments using PAH distributions at KNI-51, a shallow cave located in tropical Western Australia, where bushfire is a regular occurrence. First, the possibility of surface contamination was evaluated by measuring PAH abundances and distributions in sequential digestions and by considering other possible additional sources, such as regional oil and gas fields or proximal combustion of fossil fuels. Second, PAHs were measured in soils above the cave and in sediments from the stalagmite chamber floor as possible sources of these organic compounds, and at near annual resolution in three aragonite stalagmites to evaluate the degree of deposition and conservation. Third, possible non-pyrogenic sources of organic compounds were evaluated through alkane distribution and specific m/z ratios. Fourth, signal replication of PAHs was tested in two coeval stalagmites. Fifth, satellite-mapped fire scars were used to examine the relationship between stalagmite PAH abundances and proximity of burning to the cave. The results support the hypothesis that PAHs in KNI-51 stalagmite carbonate reflect paleofire activity within 3 km of the cave, and thus that stalagmites can serve as an important new high resolution proxy for landscape-scale fire activity. Given that karst is present in many fire-prone environments, and that stalagmites can be precisely dated and may grow continuously for millennia, the potential utility of a stalagmite-based paleofire proxy is high.