Combustion completeness and sample location determine wildfire ash
leachate chemistry
Abstract
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.