Past fire severity and intensity identified in SE Australian sediments using boron isotopes and FTIR spectroscopy

Landscape-scale bushfires threaten lives, property, and biodiversity. Understanding how their characteristics have changed over time proves vital in improving management strategies and understanding future ecosystem responses. Therefore, there is an urgent need to develop novel proxies to extend our existing record of past fire characteristics, such as severity and intensity. Here, we use carbon and nitrogen contents, Fourier Transform Infrared (FTIR) spectroscopy, and boron isotopes in a sedimentary archive to investigate past fire events in Namadgi National Park (southeastern Australia) and estimate their characteristics such as fire intensity and severity. Strontium and neodymium isotopes were used to assess the possible catchment-scale erosion events following fire. The aromatic/aliphatic ratio of sediments showed that fire frequency and intensity have increased in the last 200 years compared to the previous 3000. Boron isotopes were influenced by both lithology and fire severity, where negative excursions may result from higher contributions of bark to mineral ash, whilst positive excursions in the isotope ratio result from higher contributions of leaves. Negative excursions in the B isotope ratio, coinciding with positive excursions in the aromatic/aliphatic ratio, were hypothesised to record low-severity fires that experienced longer residence times. This multi-proxy approach provides valuable insights into past fire characteristics. By improving our understanding of how fire characteristics have changed in the past, the results can inform mechanistic models to improve predictions of fire severity and intensity changes in the future. • Existing records of fire in Australia are limited and do not inform of fire characteristics. • Aromatic/aliphatic ratio shows increased fire intensity in the last 200 years. • Boron isotopes informed on the severity of the fire. • Sr and Nd isotopes showed no change in parent material over the last 3000 years. • Using FTIR and B isotopes, the fire record was extended by millennia.