Boron Isotopes and FTIR Analysis to Determine Past Fire Occurrence in the Upper Nepean Catchment

Bushfires have shaped the Australian landscape for millennia and with increasing concerns surrounding the effects of climate change on national fire regimes, the importance of developing more extensive fire records becomes critical in management and mitigation. Existing methods are historically limited or prevent distinction of severity, hindering a continuous, long-term record of past fire activity. The aim of this study was to calibrate two novel techniques- boron isotopes and Fourier Transform Infrared (FTIR) spectroscopy- against known fire occurrence in alluvial sediments to determine their suitability as proxies. Alluvial sediments provide an insight into signal transport to reservoirs and lake sediments, which offer an enduring record of past fire activity. This study employed both geochemical and infrared spectroscopy analysis techniques to determine changes in the boron isotope ratio (δ11B) and molecular composition of the sediment profile with depth. A methods comparison was conducted with potassium bromide (KBr) discs and attenuated total reflectance (ATR)-FTIR. Clay samples underwent alkali fusion, cation exchange and boron specific chromatography and analysis by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the determination of δ11B values. The results from the FTIR proxy highlight the exposure of charcoal-rich layers to higher temperatures and pyrolysis conditions, largely effecting C-C, C-N and C-O bonds by thermal decomposition. Higher severity is also punctuated by a shift from aliphatic to aromatic compounds- a trait associated with higher temperatures. In association with this, boron isotopes highlighted a significant peak in δ11B value at the surface of the charcoal-rich layer, suggesting enrichment of 11B from ash or the temperature-induced transformations of boric acid, resulting in speciation and fractionation. This speciation is confirmed in the FTIR spectra.