When mineral-rich water drips from a cave’s ceiling over centuries and millennia, it forms rocky cones that hold clues to the Earth’s past climate. Now, researchers in Australia and the UK have found that these structures can also help trace past wildfires that burned above the cave. Their research is published today (21 July) in Hydrology and Earth System Sciences, an open access journal of the European Geosciences Union (EGU).
Pauline Treble, a researcher at the Australian Nuclear Science and Technology Organisation and the University of New South Wales, Sydney, first got interested in Yonderup Cave as an archive of past climate. She wanted to find out whether this shallow cave system in southwest Australia was a good site to reveal past changes in rainfall and temperatures.
“We monitored two drips in the cave expecting to see responses in the data that we could attribute to climate. But the results were surprising,” says Treble. The chemistry of the dripwater, and how it changed over time, was different in the two sites. The data could not be showing a regional climate change at the surface, but rather a local change that affected the ground above the two sites in different ways.
“This is when we started to consider whether the intense wildfire that had occurred six months before monitoring started was responsible for the inconsistent data,” Treble explains.
Treble’s student Gurinder Nagra, from the University of New South Wales, was excited by the idea of finding traces of wildfires in cave dripwater. “Not only does this open up a new avenue for the fire community, but it could hold the key to our understanding of fire and climate in the past, and how this influences our warming world,” he says.
But the signal that wildfires leave in cave formations can also present a problem, because it is remarkably similar to the signal for a change in climate.
Stalagmites (which grow from the ground up) and stalactites (which hang from the ceiling) form when water at the surface seeps through the soil and drips into underground chambers over hundreds or thousands of years. The dripwater contains minerals, which can gradually accumulate to form icicle-like rocky structures that preserve environmental information from the water within its growth layers.
By looking into the chemistry of these growth layers, scientists can find clues about how rainfall and temperature were changing above ground when the water dripped into the cave. Due to the way stalagmites and stalactites grow, the layers in the middle of these structures preserve older environmental information, while those closer to the surface hold clues to the more recent past.
Oxygen is one of the key elements scientists look at to track past climate change. Specifically, they measure changes in the ratio (noted d18O) of two oxygen isotopes: the heavier 18O, which takes more energy to evaporate, and the lighter 16O. Roughly speaking, a higher ratio signals warmer temperatures and less rainfall.
At Yonderup Cave, the researchers collected dripwater samples from two sites from August 2005 to March 2011 and analysed them for d18O, as well as for trace metals such as magnesium. They then compared the oxygen isotope ratio in Yonderup dripwater with that predicted by a model (which simulated the dripwater d18O based on measurements of rainwater d18O), as well as that measured at a different cave in the region. They found that the oxygen isotope ratio in Yonderup dripwater was 2‰ (2 parts per thousand) higher than expected.
“This value means that the water was enriched in the heavier 18O isotope by two-parts per thousand,” Treble explains. “This may not sound like much, but if we were interpreting this change in a stalagmite record [of past climate], it would be equivalent to some of the largest interpreted climatic changes seen in the Quaternary record [the last 2.6 million years].”
Treble says the results could have implications for interpreting d18O in fire-prone regions, such as Australia or the southern Mediterranean. A change that could be due to a local wildfire in the land above the cave could be wrongly attributed to a change in regional or global climate.
The Hydrology and Earth System Sciences study highlights the need to carefully interpret dripwater cave data, and to also look into changes in its trace metals, as opposed to only d18O, when analysing it. But it also shows that we can learn more about the Earth’s past than we previously thought. “Our results show for the first time that wildfire changes cave dripwater chemistry, and this chemistry will be preserved in stalagmites,” says Nagra.
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Please mention the name of the publication (Hydrology and Earth System Sciences) if reporting on this story and, if reporting online, include a link to the paper (TBA) or to the journal website (http://www.hydrology-and-earth-system-sciences.net).
This research is presented in the paper ‘A post-wildfire response in cave dripwater chemistry’ to appear in the EGU open access journal Hydrology and Earth System Sciences on 21 July 2016.
The scientific article is available online, free of charge, from the publication date onwards, at http://www.hydrol-earth-syst-sci-discuss.net/hess-2016-1/ (this URL currently links to the non-peer-reviewed version of the paper, but will redirect to the peer-reviewed paper page after publication). A pre-print version of the final paper is available for download at http://www.egu.eu/news/249/fire-clues-in-cave-dripwater-researchers-find-wildfir....
The team is composed of Gurinder Nagra (Connected Water Initiative Research Centre, University of New South Wales, Sydney, Australia [CWI-UNSW]), Pauline C. Treble (Institute for Environmental Research, Australian Nuclear Science and Technological Organisation, Lucas Heights, Australia & CWI-UNSW), Martin S. Andersen (CWI-UNSW), Ian J. Fairchild (School of Geography, Earth and Environmental Sciences, University of Birmingham, UK), Katie Coleborn (CWI-UNSW) and Andy Baker (CWI-UNSW).
The European Geosciences Union (EGU, http://www.egu.eu/) is Europe’s premier geosciences union, dedicated to the pursuit of excellence in the Earth, planetary, and space sciences for the benefit of humanity, worldwide. It is a non-profit interdisciplinary learned association of scientists founded in 2002. The EGU has a current portfolio of 17 diverse scientific journals, which use an innovative open access format, and organises a number of topical meetings, and education and outreach activities. Its annual General Assembly is the largest and most prominent European geosciences event, attracting over 11,000 scientists from all over the world. The meeting’s sessions cover a wide range of topics, including volcanology, planetary exploration, the Earth’s internal structure and atmosphere, climate, energy, and resources. The EGU 2017 General Assembly is taking place in Vienna, Austria, from 23 to 28 April 2017. For information about meeting and press registration, please check http://media.egu.eu closer to the time of the conference, or follow the EGU on Twitter (http://twitter.com/EuroGeosciences) and Facebook (http://www.facebook.com/EuropeanGeosciencesUnion).
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Hydrology and Earth System Sciences (HESS) is an international two-stage open-access journal for the publication of original research in hydrology, placed within a holistic Earth system science context. HESS encourages and supports fundamental and applied research that seeks to understand the interactions between water, earth, ecosystems, and humans. A multi-disciplinary approach is encouraged that enables a broadening of the hydrologic perspective and the advancement of hydrologic science through the integration with other cognate sciences, and the cross-fertilization across disciplinary boundaries.
Honours Candidate at the University of New South Wales (UNSW)
Isotope Palaeoclimateologist at the Australian Nuclear Science and Technological Organisation & UNSW
Lucas Heights, Australia
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EGU Media and Communications Manager
EGU on Twitter: @EuroGeosciences
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Dr. Bárbara Ferreira | European Geosciences Union
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