Hydrological conceptual model for reconstructing fire history from cave stalagmites

Cave stalagmites (speleothems) are highly-valued archives of environmental information owing to their preservation of climate sensitive proxies and well-defined chronologies.  Yet the reconstruction of fire history from stalagmites is a relatively unexplored approach, with some advantages over traditional fire proxy archives.  For example, stalagmites may contain annual laminae (visible or chemical) which can be exploited for seasonal to annual proxy information with precise chronologies.  Thus stalagmites have the potential to yield annually-resolved records of fire and climate that could be used to (1) better understand the fire-climate relationship, (2) fire recurrence interval information, (3) understand ecosystem resilience and (4) inform land management policy.

The development of fire proxies from stalagmites is still in its infancy. Robust interpretations of any proxy information relies on an understanding of the environmental processes that lead to the preservation of proxies in the archive.  Cave stalagmites may record fire history via dripwater, or via the cave entrance as aerosols.  The focus here is on the transportable constituents in dripwater such as solutes, colloids and suspended matter.  A fire event produces ash (a source of leachates) and can alter soil properties (hydrophobicity, pH, organic matter etc) producing temporary enrichments (or depletions) in transported constituents via dripwater.  The resulting signal may be detected in stalagmites using high-resolution methods such as laser ablation mass spectrometry, fluorescence and infrared microscopy techniques.  Cave depth is an important factor in the preservation process with the detection of a fire signal more likely to be observed in dripwater from shallow caves (e.g. 5-10 m) owing to the potential for attenuation and mixing that may occur in deeper caves (Campbell et al., 2022).  However, owing to the karstification of carbonate rocks which host caves, there commonly exists different flow types: diffuse/slow flow through the matrix, preferential/fast flow through fractures and conduits.  Fracture (or conduit) influenced flowpaths have higher permeability and enhance rapid and deep percolation of water from the surface towards the cave.  Several studies have shown that stalagmites fed by dripwater with a fracture-flow component contain higher concentrations of soil-derived trace metals and organics indicating a stronger hydrological connection with the surface.  It logically follows that fracture-influenced flowpaths are more likely to transmit proxies for fire.  Furthermore, flowpaths may be a more important factor than cave depth in some settings, e.g., Campbell et al. (2022) presented a case study of a historical fire event recorded in a stalagmite that was located ~40 m below the surface.  

Understanding the hydrological setting of a cave system including rainfall recharge and flowpaths is valuable in the interpretation of speleothem records in general.  This contribution presents a conceptual model illustrating how these factors influence the preservation of fire proxies in stalagmites and makes recommendations for ideal sample selection for fire proxy records based on cave characteristics as well as stalagmite attributes such as morphology and colour.

Campbell. M. et al., Speleothems as Archives for Palaeofire Proxies. ESS Open Archive. July 24, 2022. DOI:10.1002/essoar.10511989.1