Geochemistry of Historical Fires Recorded in Sediments in Southeastern Australia

Fire has played a key role in shaping the Australian landscape and biodiversity for millennia; however, as climate change continues to alter the fire regime, understanding how this may impact landscape-scale events remains unknown. At present, our records of past fire events are historically limited or cannot accurately account for changes in fire characteristics, such as severity and intensity. In order to more accurately predict the behaviour of future fire events, there is an urgent need to develop new techniques that can significantly extend our existing fire record. Therefore, the aim of this thesis was to develop two novel techniques, boron (B) isotopes and Fourier Transform Infrared (FTIR) spectroscopy, to determine their suitability as proxies. Three key objectives were addressed to achieve these aims and determine the suitability of the two techniques as proxies for past fire events: 1) To link changes in the B isotope ratio and the FTIR spectra against a fire of known severity to determine their suitability as proxies; 2) Apply these techniques to sites of known fire history to determine their ability to record multiple fire events; 3) Formulate a >100-year record of fire severity and intensity and compare it with existing palaeoclimate records to determine how fire characteristics have changed through time. Sediment cores were collected from fire-prone landscapes in southeastern Australia and analysed for changes in the FTIR spectra and the B isotope ratio.

Boron isotopes showed sensitivity to fire severity, where severity was defined as the degree of canopy consumption, and mineral ash composition. High-severity fires were accompanied by increases in the δ11B value (>2SD greater than the mean) due to increased leaching of ashed leaf material enriched in the heavier isotope. Negative excursions in the δ11B value (>2SD less than the mean) were hypothesised to record increased leaching from burnt woody material enriched in 10B. These excursions were associated with erosion events, where rainfall was required to leach B from ash, facilitating adsorption onto clay minerals. In the FTIR spectra, the aromatic/aliphatic ratio showed sensitivity to high-intensity fire events, where fire intensity was defined as the rate of heat transfer from the fire. Higher aromatic/aliphatic ratio values were associated with higher-intensity fire events, suggesting increased high-intensity fire frequency in the last 200 years compared to the last 3000 years. This was hypothesised to stem from the complex interactions between climate, people and vegetation change. Combining the records from the two techniques, it was suggested that a negative excursion in the δ11B value accompanied by a positive excursion in the aromatic/aliphatic ratio was the result of a more significant contribution of bark to the mineral ash fraction, which typically requires higher temperatures for thermal decomposition compared to leaves. Both techniques were found to be suitable proxies for detecting past fire characteristics, extending our existing fire record by decades to millennia. The results provide valuable insights into past fire characteristics, allowing for improved management strategies to prevent future landscape-scale bushfires. It is recommended that future studies apply these techniques to mineral-dominated sediments in a range of environments with varying climate conditions and vegetation communities to determine trends in the fire regime at national to global scales.