Chemicals in stalagmites unlock secrets of ancient fires

New research shows chemicals in stalagmites could hold the key to understanding fire activity from thousands of years ago. 

Those chemicals, called polycyclic aromatic hydrocarbons (PAHs), are formed during the burning of organic matter and were found within the layers of stalagmites collected from a cave in the Australian tropics. PAHs are pyrogenic, meaning they are produced when biomass, like grasses or trees, burns. PAHs formed by fires above the cave were washed into the cave by rainwater over previous decades and centuries, and once in the cave, the PAHs were incorporated into stalagmites that crystallized from that water. Through this chemical signal, the stalagmites appear to track the timing of past burning events.

“With all the fire activity happening around the globe, understanding how fires respond to climate change is a really important question,” said Cornell College Professor of Geology and lead researcher Rhawn Denniston. “One way that scientists are trying to answer it is by looking at fires, not just today, but also going far back into the past and looking at how fires behaved in times when climates or human impacts on the landscape were quite different.”

Denniston, along with a team of student researchers that included Huong Quynh Anh Nguyen ’19 and Cornell College senior Jonathan Azenon, recently had their findings published in the journal, Geochimica et Cosmochimica Acta in a paper titled: “Polycyclic Aromatic Hydrocarbons in Tropical Australian Stalagmites: A Framework for Reconstructing Paleofire Activity.” The lead author of the study is Elena Argiriadis of Ca’ Foscari University in Venice and also includes researchers from Australia and the U.S. 

In the fall of 2022, Azenon spent a block at Ca’ Foscari working in a laboratory under Argiriadis. He studied an 80-year period within a stalagmite sample that overlapped in age with another stalagmite from the same cave. The goal was to see if the two stalagmites showed similar chemical evidence of fires (PAHs) within the same time frame.

“I went to do a replication of everything that our team had done previously in the lab with a different sample,” Azenon said. “I wanted to see if the data previously collected from the lab could be replicated using a different stalagmite overlapping that particular time frame. We drilled the stalagmite about a millimeter or two at a time to then collect our samples.” 

Azenon discovered that spikes in the amounts of PAHs in his stalagmite, which dated to the 15th century, closely resembled spikes in PAHs in the other stalagmite they had already tested–meaning the two samples showed fires happening at the same time.

“We are cautiously optimistic that we have developed a new technique for reconstructing fires at very high temporal resolution. We are able to ‘see’ fires at an almost annual time scale,” Denniston said. “With a tree ring, you can get a burn scar and you can identify that to the individual year. But in a lot of places tree rings only go back a couple of hundred years, and in the Australian tropics where I work, many trees don’t lay down annual rings, and the trees that do often only reach back a few decades. With our stalagmite record, we could theoretically reconstruct fire for thousands, possibly even tens of thousands, of years. So that gets us back to intervals with very different climate states like ice ages.”

Having a new way to reconstruct past fires is needed. Denniston said lake sediment can also contain charcoal, which acts as evidence of fires going back millennia, but many tropical locations don’t have deep lakes for this kind of exploration, or the lakes deposit sediment so slowly that you are forced to lump together many years or even decades into a single sample

This new research is also promising because it allows an exploration of not just the frequency of burning, but also the intensity of the fires.

“If we get a high molecular weight of the PAHs, then that would be a high-intensity fire, and if we get a low molecular weight of the PAHs, that would be a low-intensity fire,” Azenon said. “Therefore, periods of low-temperature grass fires could be distinguished from much more severe forest fires.”

This research could provide help to government agencies tasked with performing prescribed burns to mimic natural conditions before human-induced changes to the landscape by the introduction of cattle or the suppression of fires. It can also help ecologists better understand the long-term impacts of fire on ecosystems.

“Humanity is changing the climate and the nature of fire through a whole host of activities,” Denniston said. “We are in a world now where the ecosystems are being faced with challenges that they may never have faced before.”

Denniston said there are two portions of their research still to come. First, he and a multinational team of scientists will conduct a prescribed burn and irrigation experiment to mimic the natural fires that create the PAHs and the monsoon rains that flush them into the underlying caves. Denniston and three students are headed to Australia this summer for that project. Next, Denniston and Argiriadis will focus on writing their next paper, which focuses on a detailed record of fire around their Australian cave site reaching back about 1,000 years.