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.

Subsurface hydro-geomechanical properties crucially underpin the management of Earth’s resources, yet they are predominantly measured on core-samples in the laboratory while little is known about the representativeness of in-situ conditions. The impact of Earth and atmospheric tides on borehole water levels are ubiquitous and can be used to characterize the subsurface. We illustrate that disentangling the groundwater response to Earth and atmospheric tidal forces in conjunction with hydraulic and linear poroelastic theories leads to a complete determination of the whole parameter space for unconsolidated systems. Further, the characterization of consolidated systems is possible when using literature estimates of the grain compressibility. While previous field investigations have assumed a Poisson’s ratio, our new approach allows for its estimation under in-situ conditions. We apply this method to water level and barometric pressure records from four field sites with different hydrogeology. Our results reveal the anisotropic response to strain, which is expected for a heterogeneous lithological profile. Estimated hydro-geomechanical properties (specific storage, hydraulic conductivity, porosity, shear, Young’s and bulk moduli, Skempton’s and Biot-Willis coefficients and undrained/drained Poisson’s ratios) are comparable to values reported in the literature, except for consistently negative drained Poisson’s ratios which are surprising. Closer analysis reveals that this can be explained by the fact that in-situ conditions differ from typical laboratory core tests. Our new approach can be used to passively, and therefore cost-effectively, estimate subsurface hydro-geomechanical properties representative of in-situ conditions. Our method could be used to improve understanding of the relationship between geological and geomechanical subsurface heterogeneity.