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

Abstract

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 surfacecave 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.October 2019 and February 2020, during which nearly 17 million hectares of land were burned are occurring with increasing frequency. In the Northern hemisphere, in 2020 more than 4% of the total 100 million acres of the state of California land were burned. Karst systems, where caves host speleothems (e.g., stalagmites, stalactites and flowstones), can provide long-term records of changes in climate and the environment (e.g. Bar-Matthews et

Monitoring of Sierra Nevada Caves Reveals the Potential for Stalagmites to Archive Seasonal Variability

“…For example, precipitation source, precipitation amount, and precipitation type (i.e., snow or rainfall), as well as relative humidity, and temperature all can influence drip-water and stalagmite δ 18 O values (Banner et al, 2007;Fairchild and Baker, 2012;Oster et al, 2012;Luo et al, 2014;Baker et al, 2019). Furthermore, studies have shown that vegetation density and wildfire activity can impact hydrologic recharge to a cave system leading to dripwater δ 18 O variability (Nagra et al, 2016;Treble et al, 2016;McDonough et al, 2021). Trace element variability (Mg/Ca, Sr/ Ca, and Ba/Ca) and δ 13 C compositions are also influenced by precipitation amount as well as bedrock composition and pCO 2 ventilation regimes in a cave (Fairchild and Treble, 2009;Wong et al, 2011;Oster et al, 2012;Oster et al, 2014;Casteel and Banner, 2015;Oster et al, 2015).…”