Boron isotopes and FTIR spectroscopy to identify past high severity fires

•Existing Australian fire records are limited and don’t capture fire characteristics.•We developed boron isotopes and FTIR as proxies for past fire severity.•We find increased δ11B values in fire-affected sediment layers.•Fire-affected layers show a relative increase in aromatic and aliphatic composition.•Boron isotopes and FTIR spectroscopy show good potential as proxies.