Bushfire clues dribble through cave dripwater

Wildfires that raze vast swathes of forests also alter the chemistry of caves below – and may muddy the waters for scientists wishing to reconstruct our climate history.

Researchers from Australia and the UK analysed water dripping from the surface through to a cave in Western Australia found after a fire, the chemistry of the “dripwater” changes considerably. The work was published in Hydrology and Earth System Science.

Stalactites hanging from a cave’s ceiling and stalagmites that rise from the ground – collectively known as speleothems – are archives of a sort, growing layers as minerals are ferried into the cave by dripwater seeping from above.

Each layer retains a “signature” of the water at the time which changes as the climate shifts. And some stalactites are nearly 200,000 years old, making them a valuable climate repository.

One key climate indicator in dripwater is the ratio between different types of oxygen.

Oxygen-16, an isotope with eight neutrons and eight protons in its nucleus, is lighter than oxygen-18, which has two extra neutrons.

Water molecules comprising oxygen-16 evaporate more easily, so cave dripwater high in oxygen-18, for instance, might indicate a hot dry spell above.

But what about fires? Bare and blackened ground absorbs sunlight, also boosting evaporation.

Pauline Treble from the Australian Nuclear Science and Technology Organisation and University of New South Wales in Sydney first wondered about fires when she analysed dripwater chemistry in Western Australia’s Yonderup cave in Yanchep National Park, near Perth.

Oxygen isotope ratios at two sites in the cave were different, even though they were only 23 metres apart – far too close to be explained by the climate above.

And when compared to oxygen isotope ratios with those predicted by a model, which used monthly rainfall and flow rate, one site didn’t add up. Its oxygen-18 ratio was two parts per thousand more than expected. 

It doesn’t sound much, but “would be equivalent to some of the largest interpreted climatic changes seen in the Quaternary period [past 2.6 million years]”, Treble explains.

“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.”

In February 2005, an intense bushfire swept through 1,200 hectares of Yanchep National Park. The flames were hot enough to fracture limestone at Yonderup cave’s entrance.

The work could affect how isotope ratios are interpreted in fire-prone regions, such as Australia or the Southern Mediterranean.

Treble, lead author Gurinder Nagra and colleagues started collecting data from their two sites in the cave six months later.

The land above the first site had no shade at all. The only tree was destroyed in the fires and the ground comprised around 70% thin soil cover and the rest, exposed bedrock.

The oxygen-18 ratio at this first site was higher than that at the second, which had much thicker soil coverage. And even though there were no trees growing directly on top of site two, some shade was provided by nearby trees.

The work, the researchers write, could affect how isotope ratios are interpreted in fire-prone regions, such as Australia or the southern Mediterranean. Scientists reconstructing the climate over thousands of years must look at other indicators such as trace elements which also shift following a fire.

For instance, they found chlorine ions at the first site dropped post-fire, perhaps because it was diluted by water that would normally be sucked up by the large tree lost in the fire.

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