Mineralogical and chemical markers of fire exposure in ancient, sandy podzols

The Fraser Island and the Cooloola dune fields are dominated by giant (>20 m deep) podzols, with numerous buried and well-preserved soil B horizons. Due to their cohesive structure and composition, these horizons can provide extensive information on the past weathering and environmental events that contributed to their formation. Previous work has attributed differences in the consistency and appearance of these B horizons to their mode of formation and past environmental setting. However, the role of post-erosional weathering (weathering after the removal of the overlying A and E horizons in response to climate change), such as fire, in altering these properties has not yet been explored. This laboratory study investigated the effects of simulated fire conditions on B horizon samples from Fraser Island and the Cooloola coast, and how fire may affect their efficacy as weathering and soil formation proxies. Fire simulations were found to alter the mineralogy and geochemistry of the B horizons. More than 94% of soil C in all samples was lost through volatilisation upon heating to 500 °C. Key mineral transformations included the dehydroxylation of gibbsite, kaolinite and lepidocrocite, and the formation of hematite when samples were heated to 500 °C and above. Mineral transformations and soil C loss during heating were reflected in the extractable Al, Fe and Si concentrations, indicating decreased Al, Fe, Si-organic complexes and increased poorly crystalline fractions due to additions from newly-formed dehydroxylated minerals. Results indicate that fires have the ability to affect both the geochemistry and mineralogy of B horizons, and therefore the present study recommends preliminary work should be carried out to determine if soils have been fire affected, before measuring for and relying on B horizon properties as markers for past soil formation conditions.