Fire-induced shifts in stalagmite organic matter mapped using Synchrotron infrared microspectroscopy

Understanding organic matter (OM) in cave mineral deposits (speleothems) is essential for interpreting land use and climatic changes, and the incorporation of trace elements associated with organic compounds. However, the sources and composition of OM in speleothems are poorly understood due to challenges associated with measuring OM at low concentrations and the destructive nature of most speleothem OM analysis techniques. Synchrotron Fourier-transform infrared (FTIR) microspectroscopy is a promising non-destructive technique that can be used to investigate stalagmite OM composition. We use FTIR to analyse vegetation, soil, calcium carbonate and ash end-members and demonstrate the use of Synchrotron infrared microspectroscopy (IRM) mapping to detect temporal changes in the OM composition of a stalagmite from a shallow cave in south-west Western Australia. Our analysis reveals predominant FTIR peaks in the stalagmite linked to amides and CH2 groups, suggesting potential microbial contributions, with smaller proportions of aromatic, CH3 and C[dbnd]O groups. High-resolution transmission electron microscopy revealed that this OM is likely hosted in sets of nanopores spaced hundreds of nanometers apart, aligned along calcite crystallographic orientations. Furthermore, we assess the impact of known wildfire events as discrete short term environmental changes on the stalagmite's OM composition. The temporal variability in OM functional group composition after fires implies complex fire-soil-vegetation-microbial interactions. This research demonstrates the effectiveness of Synchrotron IRM mapping in providing insights into the short and long-term environmental influences on stalagmite OM composition. Expanding this research to other regions and climates could further enhance the interpretation of OM changes in speleothem-based palaeoclimate reconstructions.