Evidence of ancient wildfires can be stored in stalactites and stalagmites, investigation of a Western Australian cave has revealed.
The discovery creates opportunities to learn about the prehistory of fire, but also represents a wake-up call to possible inaccuracies in previous attempts to use limestone caves to measure ancient climates.
Collectively known as speleothems, stalactites, stalagmites, and flowstones provide a record of the water that formed them. Like the rings of trees, a speleothem is built up with time, providing an opportunity to study the local environment when a particular layer was laid down.
Most of the oxygen atoms in water are normal oxygen-16, but a few are heavier isotopes. “When water undergoes a phase change, either evaporation or condensation, you get a change to the ratio of isotopes in it, “ Dr Pauline Treble of the Australian Nuclear Science and Technology Organisation told IFLScience. Rain from clouds that evaporated at higher temperatures contains more oxygen-18 than that from colder environments, and this becomes incorporated into limestone caves laid down at the time.
For paleoclimatologists, this has proven invaluable. The rings of living trees or corals only go back so far. Ice cores tell us much about the climate at the poles, but little about the rest of the planet. Speleothem isotope rations have filled in the gaps.
However, when Treble studied the water dripping from two spots 23 meters (76 feet) apart in Yonderup Cave north of Perth, she noticed differences that could not have been related to the circumstances in which the rainwater vaporized.
“We started to consider whether the intense wildfire that had occurred six months before monitoring started was responsible for the inconsistent data," Treble said in a statement. Treble and her student, Gurinder Nagra, investigated and found that a tree almost directly above one of the drip sites had been killed in the fire. “That changed the transpiration at that site,” Treble told IFLScience. “It was no longer shaded so evaporation increased.”
Oxygen-18 concentrations rose by two parts in a thousand, the pair reported in Hydrology and Earth System Sciences. “This doesn’t sound like a lot," Treble told IFLScience, "but it is 10 times as large as the smallest difference we can measure, and as big as what you would see going from the middle of an ice age to an interglacial."
Although the changes to the oxygen ratios could be induced by a tree death of any cause, the fire produced a layer of ash, which in turn altered the concentrations of solutes such as chlorine, calcium, and magnesium.
Treble told IFLScience that searching for spikes or drops in elements such as these, at the same point as the speleothem records a change in oxygen ratios, could allow scientists to distinguish between climate-induced variations and the effects of fire. A capacity to detect fire in the limestone record could also help answer long-standing questions about how common fire was prior to human arrival in Australia.
In the meantime, however, Treble suggests past conclusions about climatic variation may need to be rechecked, ideally using data from widely separated caves.
The location of Yonderup Cave, the now treeless site above one of the drips, and a map of the cave itself. Nagra et al., Hydrol. Earth Syst. Sci., 2016