Tiny ants are heroic weathering agents

by Mary Caperton Morton
Thursday, October 2, 2014

Over geologic timescales, weathering of silicate minerals — which account for more than 90 percent of all crustal rocks on Earth — plays a significant role in decreasing carbon dioxide in the atmosphere, as carbon dioxide is consumed in chemical weathering reactions and the resultant carbonate becomes sequestered in the form of limestone and dolomite.

Now, a new study has identified an unsung all-star of weathering agents: ants, which appear to be among the most powerful biological agents of mineral decay yet observed. Whether the weathering prowess of the ubiquitous tiny titans can be harnessed to draw down carbon dioxide levels, however, remains to be seen.

“When I started this study 25 years ago, ants weren’t at all the focus of the project, but over time, it became apparent that they were the stars of the show,” says Ronald Dorn, a geomorphologist at Arizona State University and lead author of the new study published in Geology.

Over time, silicate minerals in the subsurface are degraded from exposure to water, chemical dissolution, and physical and chemical weathering by tree roots and even insects such as ants and termites. To study the effects of weathering over time on these silicate minerals, Dorn buried basalt sand at six test sites in Arizona and Texas and dug up the samples every five years to measure how much degradation occurred on the minerals olivine and plagioclase. Over a 25-year time span, Dorn found that the ants broke down the minerals 50 to 300 times faster than his controls.

“Of course, the 600-pound-gorilla question is: ‘What are ants doing to the basalt?'” Dorn says. “Are they licking the grains or digesting them or could the breakdown be due to the bacteria and fungi that are associated with ant colonies? At this point, we don’t know.”

The effect of ants on soil particles is no surprise, although this is the first time that somebody has tried to quantify it in this way, says Walter Tschinkel, an entomologist at Florida State University. “Ants are great bioturbators of soil,” he says. “Over many generations, ant colonies can have a dramatic effect on their surrounding environment.”

Further investigation by Dorn revealed that the ants were gradually building up limestone within and around their nests, possibly salvaging calcium and magnesium from the basalt sand to form the deposits and trapping carbon dioxide in the rock in the process. At least eight species of ant are known to accumulate limestone, although how and why they do it is a mystery.

Given the sheer number of ant colonies that have lived on Earth since the insects evolved and started proliferating about 65 million years ago, ants may play an important role in sequestering carbon over geologic time, Dorn says.

“It wouldn’t surprise me if ants play a significant role in the carbon cycle,” Tschinkel says. “Ants make up a very large fraction of the biomass in many ecosystems, and in certain places, the biomass of ants is greater than the biomass of all the vertebrates put together.”

If the mechanisms of weathering can be determined, bioengineers may be able to harness the power of ants for sequestering carbon dioxide, Dorn says. “If we can figure out the mystery mechanism that the ants are using, and if it can be engineered on a bigger scale, it might be a commercially viable way to sequester carbon dioxide.”

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