Speleothems as archives for palaeofire proxies

Abstract

Environmental proxy archives such as tree rings, sediment cores, and ice cores are commonly used to investigate past fire regimes. Speleothems, naturally forming cave decorations mainly comprising of stalagmites, stalactites, and flowstones, have been extensively used as palaeoenvironmental archives as their physical attributes and chemical composition change with changed environment. Research has shown that cave drip water chemistry responds to fire events, and more recently, that speleothems can record past fire events due to physical and chemical processes which alter speleothem composition. These processes include changes to water stores due to evaporation, fracturing of the host rock, changed soil hydrophobicity, production of highly soluble lime, changes in soil CO2 production, destruction of vegetation and deposition of ash above the cave. These changes can result in shifts in δ18O and δ13C, altered concentrations of vegetation, soil and bedrock-derived elements, and incorporation of soluble ash derived elements (including phosphorus, aluminium, copper, zinc, and lead) in speleothems (McDonough et al., 2022; Campbell et al., 2022).Changes in speleothem chemistry are typically determined using micro-analytical techniques (such as Synchrotron X-ray Fluorescence Microscopy and laser ablation inductively coupled plasma mass spectrometry) and isotope ratio mass spectrometry. These changes can be precisely and absolutely dated via uranium-series and carbon dating, and can often be resolved at high resolution via manual counting of seasonal fluctuations in organic matter and trace element concentration. This makes speleothems, particularly those grown in shallow caves in highly seasonal climates, ideal for identifying both short-lived events such as wildfires, and longer-term changes such as shifts in climate. This novel application of speleothems as archives for coupled climate and palaeofire proxies is still in its infancy but holds great potential.Here, we present a review of this new sub-discipline. We cover its origins in cave dripwater monitoring, discuss site and sample selection, and describe the current analytical and statistical approaches used to extract fire information from speleothems. Such records will enable land managers to develop improved methods for managing fire regimes. McDonough, L.K., Treble, P.C., Baker, A., Borsato, A., Frisia, S., Nagra, G., Coleborn, K., Gagan, M.K., Zhao, J., Paterson, D., 2022. Past fires and post-fire impacts reconstructed from a southwest Australian stalagmite. Geochimica et Cosmochimica Acta. https://doi.org/10.1016/j.gca.2022.03.020   Campbell, M., McDonough, L., Treble, P., Baker, A., Kosarac, N., Coleborn, K., Wynn, P.M., Schmitt, A., 2022. Speleothems as Archives for Palaeofire Proxies [preprint], https://www.authorea.com/doi/full/10.1002/essoar.10511989.1