Over the ages, armies have fought wars over guano, a stinky, cockroach-infested treasure that’s rich in nitrogen and phosphate. Although still valued for its fertilization qualities, this natural resource—once used to make gunpowder—may offer a new application as a window into climate change.
Scientists investigate past climates by studying substances that accumulate slowly over time. Trees, stalactites and stalagmites, peat, ice, lake sediments, ocean sediments, and corals all grow or accumulate in ways that reflect the environment surrounding them. Specifically, the isotopes within their structures fingerprint the conditions prevalent at the time of formation, be they wet, dry, cold, or warm.
But paleoclimate records aren’t limited to tree rings and the like. As scientists search for records that provide regional coverage, they’re starting to turn to some unusual sources: sinter in ancient Roman aqueducts, for example, or even piles of bat guano, excreted in caves around the world, accumulating for thousands of years.
There are “a number of caves with large such deposits in areas where no other climate proxies are readily available,” said Bogdan Onac, a paleoclimatologist at the University of South Florida in Tampa.
So he wondered, What climate story could bat guano reveal? A robust one, it turns out. His graduate student, Daniel Cleary, will present the research in a poster session today at the American Geophysical Union’s Fall Meeting in San Francisco, Calif.
A Secret in Nitrogen?
In traditional cores, from trees, ice, or sediments, scientists look at oxygen isotopes to tease out climate clues. However, unlike ice, wood tissues, or carbonates, guano does not have oxygen locked into its structure. Moreover, rainwater percolates through piles of guano, contaminating layers with outside oxygen.
What guano does have aplenty is nitrogen. The atmosphere, geosphere, and biosphere are constantly cycling nitrogen through the food chain—bacteria convert atmospheric nitrogen into molecules that plants absorb, and then insects and animals eat those plants and excrete nitrogen-rich feces and urine, returning it back to the geosphere.
Onac and Cleary wondered whether nitrogen isotopes in a core of bat guano could reflect environmental conditions. They already knew that the ratio of the heaver nitrogen isotope 15N increases as nitrogen moves up the food chain—this is because animals release nitrogen mainly through urine, and the lighter isotope tends to leave the body first.
This could complicate the viability of nitrogen as an environmental proxy, but the researchers were determined to learn more.
The Power of Poop
Animal poop is valuable currency in the scientific world. Scientists study both past and present poop to find out where an animal lived and what it ate, among other things.
And in any cave that bats inhabit, there will be guano—huge, relatively undisturbed piles rising several meters high, sometimes thousands of years old, with the youngest samples steaming on top, providing food for all sorts of many-legged critters. Like the layers in stalagmites and stalactites, the layers of pungent guano represent distinct periods of time.
But unlike cave formations, which form from slowly dripping water over millions of years, the more constantly raining bat guano offers the potential for a much more highly resolved record, Cleary said. Similarly, although tree ring data are invaluable to climate research, scientists don’t find many trees older than 100–200 years, so records must be stitched together. Not so with guano.
With the potential of bat guano on their minds, Cleary and Onac went in search of fresh poop. An already existing core from a 3-meter-high pile in the Zidită Cave in western Romania indicated that the guano pile was about 1000 years old, a potentially valuable time period for studying recent changes in climate.
The team studied fresh guano from the top of Zidită Cave’s guano pile, which towers several feet above Cleary’s 5-foot, 6-inch frame. Luckily the cave walls offered ledges to climb, a much cleaner feat than scrambling up 3 meters of cockroach-infested guano.
Sample acquisition is a dirty process, Cleary said. The guano pile is covered in fungus and threatens researchers with harmful viruses, forcing them to sometimes wear masks, but it’s all worth it “for the advancement of science.”
Peeking into the Past
Clearly and his team studied the nitrogen isotopes in the top 10 centimeters of the guano and compared them to recent monthly precipitation records from the area around the cave.
The researchers found that during recent wet periods, more 15N showed up in the guano. This is likely because water availability affects how nitrogen travels up the food chain from soils to bats. In the soil, heavy rainfall flushes away nitrogen’s lighter cousin 14N, leaving more 15N behind for plants to slurp up. During dry periods, the plants, which would rather use the lighter isotope, end up absorbing more 14N. These isotopic signatures remain generally constant as the nitrogen travels from plant material to insects to the stomach and digestive tract of bats.
Extrapolation from Excrement
The team then turned to their guano core, which they had studied previously as a paleoclimate record using carbon.
Like with nitrogen, carbon’s isotopic signature is dictated by precipitation. During wet climates, scientists found a higher ratio of the lighter isotope 12C in the core because plants prefer using it. During dry conditions, however, when water is less available, plants have to use more of the heavy carbon isotope. This ratio is preserved throughout the food chain and into the bat’s guano.
The researchers found that the nitrogen isotope composition in the core reflected the same wet/dry patterns as the carbon isotope data for the past 300 years, which told them that nitrogen can be a viable paleoclimate proxy worthy of study.
“For scientists who use stalagmites in caves to reconstruct past hydroclimate, bat guano is usually a bad sign because the preservation of carbonate stalagmites might be questionable,” said Kaustubh Thirumalai, a paleoclimatologist at the University of Texas at Austin who wasn’t involved in the research. “I have been to several caves that take a long time to find and reach, only to be disappointed by the sight of bats and guano.”
But not anymore. “Research such as Cleary et al.’s inspires me to carry a sediment corer next time I head out for cave research,” he said.
—JoAnna Wendel (@JoAnnaScience), Staff Writer