Speleothem ¹⁴C is unlikely to be impacted by wildfire - case studies from Western Australia and Tasmania

Full text: Accelerator Mass Spectrometry (AMS) can be used to measure ¹⁴C in speleothems (cave formations). AMS derived ¹⁴C measurements offer an alternative geochronological method to U-series dating for recently formed speleothems with low U concentrations and insufficient ²³⁰Th ingrowth, and where speleothems are impacted by allogenic thorium. However, using ¹⁴C for dating speleothems is complicated by variable ¹⁴C sources and dilution by ¹⁴C-depleted parent rock (dead carbon). Old carbon may be mobilised from soil and decaying organic matter, which can mix with new carbon from root respiration in the vadose zone. These processes and their incorporation in speleothems are non-stationary in time and can be difficult to correct for. While variable dead carbon is a complication for dating speleothems, hypothetically, a sudden decline in ¹⁴C, indicating an influx of old carbon, could be expected to occur after a bushfire. This could be because old carbon previously sequestered in soil and biomass may be mobilised, or there could be a decline in the previous oversupply of biological CO₂ while root respiration is in recovery following tree death. Using speleothems as proxy archives for past fire is an emerging field, and recent research has shown multiple proxies are needed to accurately characterise a fire event. To test the hypothesis that ¹⁴C may serve as a proxy for palaeofire, we chose four sites which have experienced bushfires; Crystal, Golgotha, and Yonderup Caves in Western Australia, and Frankcombe Cave in central Tasmania. We used the AMS facilities at the Australian Nuclear Science and Technology Organisation to quantify the ¹⁴C content along growth axes for the speleothems from Western Australia. Previously published AMS ¹⁴C data for the Tasmanian speleothem were measured at the Australian National University. Crystal and Yonderup Cave speleothems had additional age-constraints of annual laminations. The Crystal Cave speleothem shows no decline in ¹⁴C after an historical fire event. The Yonderup Cave speleothem does show a decline in ¹⁴C after a reconstructed fire event, but the decrease is within analytical uncertainty. The Golgotha speleothem ¹⁴C shows no change after a documented fire. The Tasmanian speleothem does show a significant decline in ¹⁴C during a period where the region experienced successive large bushfires, as recorded in the historical record, but the temporal uncertainty of the measurements is so high (± 15 years), that the decline cannot reliably be attributed to the bushfires. These case studies provide only limited evidence to support our hypothesis that ¹⁴C in speleothems may be a proxy for past fire events. This may be because the response is too short to be observed, sampling resolution is insufficient, or the carbon flux associated with a bushfire is too small relative to the total CO₂ flux in the vadose zone. While this is discouraging for palaeofire researchers, results offer relief to those who use ¹⁴C dates to create speleothem chronologies, as it is one less source of uncertainty to consider when correcting ¹⁴C ages. Our results may also serve to elucidate the importance of CO₂ source on ¹⁴C, and by extension, on δ¹³C.