Quantifiying the fire imprint in speleothem oxygen and carbon isotope records
No Thumbnail Available
Date
2022-12-14
Journal Title
Journal ISSN
Volume Title
Publisher
American Geophysical Union (AGU)
Abstract
Speleothems are an emerging archive of past fires. Fires primarily leave an imprint in speleothem calcite from ash-derived water-soluble trace elements transported from the surface to the cave in percolation waters. But what about the influence of fire on speleothem oxygen and carbon isotopes? Oxygen and carbon isotopes are the most widely used speleothem geochemical proxies, generally interpreted as archives of past climatic and environmental change. The link between speleothem oxygen and carbon isotopes and fire, however, is still unclear.
Field monitoring before and after recent fires provides some clues to the possible fire effect on speleothem oxygen isotope composition. Recent papers present evidence that cave percolation water can have oxygen isotope composition that is enriched in oxygen-18 post-fire, due to partial evaporation of soil water and loss of shade cover. There is also evidence indicating that post-fire, percolation waters have greater variability in oxygen-18 when there is complete evaporation of soil water.
To better understand the influence of fire on speleothem oxygen and carbon isotopes, we have sampled ~10 mm from the tops of actively-depositing stalagmites from cave systems at Yanchep, Western Australia. The sampled material contains calcite that has formed over the last ~50 years, during which numerous fires have occurred at the site. The data spans the period of satellite monitoring, allowing fire size and intensity to be determined.
We use synchrotron x-ray fluorescence imaging to map the trace element composition of the stalagmites, and use this to identify the fire imprint of the ash-derived water-soluble elements in the stalagmite calcite. Using this as a time marker of the fires, we quantify the magnitude and duration of any post-fire changes in stalagmite oxygen and carbon stable isotope composition. We identify whether changes in oxygen or carbon isotopes are replicated between stalagmites and between fire events, and compare the extent to which the post-fire imprint in stalagmite oxygen and carbon isotopes is significant compared to climatic and hydrological controls on these isotopes. This information will inform paleoclimate studies where shifts in speleothem oxygen and carbon isotopes due to fires may be misinterpreted to result from changes in climate and hydrology.
Full Abstract
Are wildfires increasing in number, intensity or spatial extent compared to the past? Before the era of satellite imaging, we have to rely on geological and biological archives of past fires. One such archive is cave stalagmites. However, they are normally used as an archive of past climate through the analysis of oxygen isotopes contained within their calcite. And wildfire can affect the oxygen isotope composition of the water percolating from the surface to the cave through the burnt environment. To what extent does wildfire affect the stalagmite climate archive?
Here, we compare stalagmite records from caves in a region in Western Australia that has burnt many times in recent decades. We identify the fire imprint in the stalagmite calcite using synchrotron mapping of ash-derived elements. And we line these fire events up with the stable isotope record in the same sample. That way we can quantify the fire imprint in the stalagmite oxygen isotope record for the first time. This will allow future researchers to take this into consideration when interpretating stalagmite oxygen isotope records from fire-prone regions.
Description
Keywords
Fires, Caves, Ashes, Water, Trace amounts, Oxygen, Carbon isotopes, Environment, Climatic change, Evaporation, Western Australia
Citation
Baker, A., Campbell, M., Treble, P. C., Adler, L., McDonough, L., & Howard, D. (2022). Quantifiying the fire imprint in speleothem oxygen and carbon isotope records. Poster presented to the AGU Fall Meeting, Chicago, IL & Online Everywhere, 12-16 December 2022. In AGU Fall Meeting Abstracts (Vol. 2022, PP25C-0894). Retrieved from: https://agu.confex.com/agu/fm22/meetingapp.cgi/Paper/1044051