Pyrogenic Organic Compounds in Tropical Stalagmites as a Possible New Paleofire Proxy

In the Western Australia tropics, fire represents an important component of ecological change, carbon cycling, and societal disruption. Shifts in climate and land use are altering natural rates and intensities of burning, but contextualizing these changes is complicated by our poor understanding of the frequency and intensity of fire prior to the modern era owing to a sparsity of traditionally used proxies (e.g., charcoal in lake sediments, burn scars on tree rings) in this region. A new method for reconstructing fire activity is needed. Toward that end, we are investigating the utility of a novel paleofire proxy: pyrogenic organic compounds in stalagmites. Polycyclic aromatic hydrocarbons (PAHs) are produced by the burning of biomass, and their molecular weights are tied to the temperature of combustion. After being formed, PAHs are transported downward through soil and bedrock by infiltrating rainwater and become incorporated into stalagmites as they crystallize from dripwater in underlying caves, thereby allowing for the possibility of stalagmites to preserve evidence of the presence and intensity of fire. We present a high-resolution analysis of PAH distributions in two fast-growing (1-2 mm/year) and precisely dated (±1-30 years) aragonite stalagmites spanning most of the last 900 years from cave KNI-51, tropical Western Australia. To calibrate the record, we compared satellite imagery of burn scars with PAHs in recently deposited stalagmite carbonate to evaluate the influence of fires occurring near to the cave. We also evaluated changes in stalagmite PAH abundance and molecular weight in relation to shifts in rainfall regime reconstructed from oxygen isotope ratios in the same stalagmites. Finally, because KNI-51 is subject to flooding during extreme rainfall events, we measured PAHs in both stalagmite carbonate and in secondary sources (e.g., flood-derived sediment incorporated within the stalagmites). The results from each analysis support the contention that PAHs in stalagmites are capable of serving as a novel, high-resolution, and extended proxy for paleofire activity. However, further tests of this method are required. These include replication of PAH abundances, ratios, and trends in coeval stalagmites and also field tests of the hydrologic links of PAHs between land surface and stalagmite.