Geochemical changes of peat swamp sediments following fire in Southeastern Australia

Retrospective measurements of fire characteristics, such as fire severity, can increase model data input and improve predictions of future fire events. However, existing records of past fire severity are limited to the recent past; therefore, there is a need to develop new proxies that can significantly extend our fire records. Boron isotopes and major element concentrations of the clay-sized fraction of sediments were used to assess fire severity and changes in water quality indicators as a result of fire, respectively, in three upland swamps in the Blue Mountains, southeastern Australia. The duration of the record varies across the three sites between 80 and 700 years as a result of varied sedimentation rates. Each site has a different fire history over the last 50 years, and the known fire record was used to corroborate fire signatures. Small increases in the Mg/Al and Ca/Al ratios were associated with fire-affected sediments, possibly due to increased wood ash input. However, the increased Ca/Al ratio was not preserved for older fire events. The P/Al and Fe/P ratios were shown to decrease with increasing depth, suggesting greater water saturation, whilst fire-affected sediments at two sites showed more oxic conditions, likely due to altered microbial activity. Mg and Al concentrations were shown to reflect the mineralogy of the basement, which is dominated by quartz with kaolinite and some calcite. The δ 11 B value showed no significant correlation with known fires, possibly due to higher organic matter contributions to the clay-sized fraction overprinting the signature imparted to the clays from the fire. These results suggest that fires invoke changes in the concentration of major elements in sediments. However, further research is needed when applying the B isotope ratio to organic-rich sediments and for analysing changes in major element concentrations following older fire events.