Understanding past fire regimes and how they vary with climate, human activity, and vegetation patterns is fundamental to the mitigation and management of changing fire regimes as anthropogenic climate change progresses. Ash-derived trace elements and pyrogenic biomarkers from speleothems have recently been shown to record past fire activity in speleothems from both Australia and North America. This calls for an empirical study of ash geochemistry to aid the interpretation of speleothem palaeofire proxy records. Here we present analyses of leached ashes collected following fires in southwest and southeast Australia. We include a suite of inorganic elemental data from the water-soluble fraction of ash, as well as a selection of organic analytes (pyrogenic lipid biomarkers). We also present elemental data from leachates of soils collected from sites in southwest Australia. We demonstrate that the water-soluble fraction of ash differs from the water-soluble fraction of soils, with trace and minor element concentrations in ash leachates varying with combustion completeness (burn severity) and sample location. Changes in some lipid biomarker concentrations extracted from ashes may reflect burn severity. Our results contribute to building a process-based understanding of how speleothem geochemistry may record fire frequency and severity, and suggest that more research is needed to understand the transport pathways for the inclusion of pyrogenic biomarkers in speleothems. Our results also demonstrate that potential contaminant loads from ashes are much higher than from soils, with implications for the management of karst catchments, which are a critical water resource.

Wildfires affect 40% of the earth’s terrestrial biome, but much of our knowledge of wildfire activity is limited to the satellite era. Improved understanding of past fires is necessary to better understand how wildfires might change with future climate change, to understand ecosystem resilience, and to improve data-model comparisons. Environmental proxy archives can extend our knowledge of past fire activity. Speleothems, naturally occurring cave formations, are widely used in palaeoenvironmental research as they are absolutely dateable, occur on every ice-free continent, and include multiple proxies. Recently, speleothems have been shown to record past fire events (McDonough et al., 2022). Here we present a review of this emerging application in speleothem palaeoenvironmental science. We give a concise overview of fire regimes and traditional palaeofire proxies, describe past attempts to use stalagmites to investigate palaeofire, and describe the physical basis though which speleothems can record past fires. We then describe the ideal speleothem sample for palaeofire research and offer a summary of applicable laboratory and statistical methods. Finally, we present four case studies which detail [1] the geochemistry of ash leachates, [2] how sulphur may be a proxy for post fire ecological recovery, [3] how a catastrophic palaeofire was linked to changes in climate and land management, and [4] demonstrate that deep caves can record past fire events. We conclude the paper by suggesting that speleothem δ18O research may need to consider the impact of fire on δ18O values, and outline future research directions.