A stalagmite from Yonderup Cave, a shallow cave in Western Australia, preserved a record of fire events and climate conditions.
Speleothems in Yonderup Cave, Yanchep National Park, Western Australia. Image credit: Andy Baker.
“We found that the largest fire event in the stalagmite record, in approximately 1897 CE, coincided with a decades-long drought period known as the Australian Federation drought,” said Dr. Liza McDonough, a researcher at Australia’s Nuclear Science and Technology Organisation (ANSTO).
“The intensity of this fire was likely caused, at least partially, by these dry conditions.”
“We also know that this fire occurred a few decades after Indigenous cultural burning would have been suppressed by Europeans, so the fire was also probably exacerbated by a build-up of understorey vegetation and dry combustible material on the forest floor due to removal of Indigenous land management practices.”
Dr. McDonough and her colleagues interpret the pre-European period captured in the stalagmite record as characterized by regular, low-intensity fires, while its post-European record depicts infrequent, high-intensity fires, which they speculate could be due to management practices.
“Nutrients such as phosphorus, and trace metals are found in bushfire ash and, in theory, can dissolve into waters that eventually infiltrate underground caves,” she said.
“Our research provides the first evidence that water containing high concentrations of these dissolved ash-derived elements can also alter the chemistry of a stalagmite and result in the preservation of signals from past fire events.”
Why had stalagmites not been previously discovered as archives of past fires?
“We realised we needed to use the highest resolution geochemical techniques available, as stalagmites grow very slowly,” Dr. McDonough said.
“In one year, a stalagmite increases in height by the same thickness as that of a sheet of paper. The geochemical trace left by a fire would be even thinner.”
It’s not just historical fires that are recorded in stalagmites but also the annual accumulation of years, much like tree rings.
“In regions with high seasonality, wet winters can lead to a flush of organic matter into the dripwaters that form stalagmites,” Dr. McDonough said.
“This causes annual dark bands alternating with light calcite bands in summer. This means that these stalagmites can be easily and precisely dated by counting back the annual layers.”
While the particular portion of the Yonderup stalagmite is relatively young, allowing scientists to peer back just 260 years, the range of time promised by other stalagmites and other speleothems (cave ornaments) stretches back much further, thousands or even tens of thousands of years.
“This new technique opens the possibility of speleothems, and their chemical record, to describe historical fire and climatic events around the world potentially anywhere we might find caves,” Dr. McDonough said.
“The technique also grants new perspectives on climate change.”
“Speleothems record increasing or decreasing rainfall rates and changes in evaporation and their potential influence on local fire events, whether they’re becoming more or less frequent through time.”
The results appear in the journal Geochimica et Cosmochimica Acta.
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Liza K. McDonough et al. Past fires and post-fire impacts reconstructed from a southwest Australian stalagmite. Geochimica et Cosmochimica Acta, published online March 19, 2022; doi: 10.1016/j.gca.2022.03.020
