Speleothem 14C is unlikely to be impacted by wildfire – case studies from Western Australia and Tasmania
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Date
2021-11-17
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Publisher
Australian Nuclear Science and Technology Organisation
Abstract
Accelerator Mass Spectrometry (AMS) can be used to measure ¹⁴ C in speleothems (cave formations). AMS derived ¹⁴ C measurements offer an alternative geochronological method to U-series dating for recently formed speleothems with low U concentrations and insufficient ²³⁰ Th ingrowth, and where speleothems are impacted by allogenic thorium. However, using ¹⁴ C for dating speleothems is complicated by variable ¹⁴ C sources and dilution by ¹⁴ C-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 ¹⁴ C, 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 ¹⁴ C 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 ¹⁴ C content along growth axes for the speleothems from Western Australia. Previously published AMS ¹⁴ C 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 ¹⁴ C after an historical fire event. The Yonderup Cave speleothem does show a decline in ¹⁴ C after a reconstructed fire event, but the decrease is within analytical
uncertainty. The Golgotha speleothem ¹⁴ C shows no change after a documented fire. The Tasmanian speleothem does show a significant decline in ¹⁴ C 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 ¹⁴ C 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 ¹⁴ C dates to create speleothem chronologies, as it is one less source of uncertainty to consider when correcting ¹⁴ C ages. Our results may also serve to elucidate the importance of CO₂ source on ¹⁴ C, and by extension, on δ¹³C. © The Authors
Description
Keywords
Mass spectroscopy, Caves, Carbon 14, Thorium, Carbon, Rocks, Soils, Fires, Australia, Tasmania
Citation
Campbell, M., McDonough, L., Kosarac, N., Teble, P., Markowska, M., Baker, A., & Hua, Q. (2021). Speleothem 14C is unlikely to be impacted by wildfire – case studies from Western Australia and Tasmania. Paper presented to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 50). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf