An annually resolved stalagmite record of fire frequency for the last 250 years in south west Australia

Stalagmites provide records of past changes in climate, vegetation, and surface events, which can be identified through variability in their chemical composition over time. This variability is the result of changes in surface environmental properties, which are reflected in the physical and chemical properties of the water that percolates into the cave, ultimately affecting the composition of the speleothem calcite. Wildfires have the potential to alter soil properties and soluble element concentrations. Consequently, stalagmite compositions have been shown to respond to increases in soil nutrients, trace metal concentrations, and changes in soil/karst bedrock hydraulic conductivity. It is, therefore, likely that stalagmites, and particularly those grown in shallow caves for which transmission of the surface signal is rapid, capture the environmental effects of wildfires in their chemical and physical properties.

We analysed a stalagmite from a shallow cave in a region known to be affected by wildfires in south-west Western Australia. Fire proxies were assessed using a multi-proxy approach. This includes water isotopes via stable-isotope ratio mass spectrometry and trace element analyses via synchrotron X-ray fluorescence microscopy and laser ablation inductively coupled plasma mass spectrometry. This approach shows that the timing of known fire events coincided with a multi-proxy response in stalagmite chemistry, including increased concentrations of phosphorus, copper, aluminium, lead, and zinc, which are interpreted to be derived from leaching of ash from burned vegetation above the cave. We also identified lower and less variable peaks in phosphorus concentrations during the pre-colonisation period, suggesting that Indigenous land management resulted in more frequent but low intensity burning. This contrasted with less frequent but more intense fires associated with post-colonisation land-management. A particularly large paleo-fire identified in 1897 appears to coincide with a peak in 𝛿¹⁸O, interpreted to have resulted from evaporation of sub-surface water during the heat of the fire. This large fire was preceded by a multi-decadal dry period identified by trace element proxies. The intensity of the 1897 fire was then exacerbated by the combination of a multi-decadal drought and a transition away from cultural burning practices by Indigenous Australians, which resulted in build-up of vegetation and dry combustible material on the forest floor.

This research is a world-first demonstration of fire events recorded in stalagmites and shows their potential to provide accurate records of both fire frequency intervals and changes in climate. Further records of past fire events from stalagmites will help to understand how past fire regimes have varied with climate, land-use change and colonisation, and will help to better guide land management practices in the future.