Past fires and post-fire impacts reconstructed from a southwest Australian stalagmite

Our current understanding of climate and its relationship to fires is generally confined to the recent past where instrumental records and satellite imagery are available. Speleothem records of past environmental change provide a unique opportunity to explore fire frequency and intensity in the past, and the antecedent climatic conditions leading to fire events. Here, we compare fire sensitive geochemical signals in a stalagmite from Yonderup Cave, a shallow cave in Western Australia, where well-documented wildfire events have occurred in recent decades. Principal component analysis of the stalagmite time-series revealed distinct peaks in a combination of phosphorus and metal (aluminium, zinc, copper and lead) concentrations, interpreted to have come from ash, in response to known fire events. This method is extended to identify fires during the growth interval of the speleothem (1760 CE–2005 CE). We identify lower and less variable peak phosphorus concentrations in the pre-European period that are consistent with low-intensity cultural burning by Indigenous Australians. We also identify an intense wildfire event occurring around 1897 ± 5 CE. The combination of climate and fire sensitive proxies in the speleothem indicates that this wildfire was preceded by a multi-decadal dry period. We interpret that post-fire changes in surface-cave hydrology resulted from heat-induced deformation of the shallow karst bedrock brought about by the intensity of this fire. These findings have implications for the interpretation of speleothem records from shallow caves in fire-prone climates and show the potential for speleothems to provide records of fire intensity and recurrence intervals. Further development of these records could lead to a better understanding of the climate-fire relationship and the effects of land-management practices on wildfire frequency and intensity.