Combustion Completeness and Sample Location Determine Wildfire Ash Leachate Chemistry.

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. Plain Language Summary: Understanding past fire activity is necessary to develop effective land management strategies to manage activity. Recently, stalagmites (naturally forming cave decorations) have been shown to record past fire information through chemical changes. The chemical changes are due to post‐fire leaching of wildfire ash. By investigating wildfire ash chemistry, we will be able to improve our interpretations of the stalagmite past fire signal. Our results show that ash chemistry from Australian fires varies with burn severity and sample location, and that the ash chemistry and soil chemistry differ. Results suggest that stalagmites may record burn severity as well as fire frequency. We also suggest that the potential impact of high concentrations of potential contaminants in wildfire ash on karst aquifers should be further investigated. Key Points: Stalagmites record past fire activity through changes in inorganic and organic chemistry sourced from ashesWildfire ash leachate chemistry will aid interpretation of proxy fire dataResults show that inorganic chemistry varies with burn severity and sample location, and the pyrogenic biomarker signal is less clear [ABSTRACT FROM AUTHOR]