From a sample of bat guano spanning across 1,200 years, researchers have uncovered an unprecedented climate record for a large swathe of the European continent.
As their findings show, digging through poop can offer a great reconstruction of climatic changes in parts of the world where researchers can't drill for a more typical ice core or ocean sediment.
In the Metaliferi Mountains in northwestern Romania, for more than a thousand years generations of bats have roosted and congregated in a huge cave as their poop piled up on the rocky floor.
The guano mountain now stands at 3 metres high (nearly 10 feet) - a steaming, fungus-infested pile squirming with cockroaches and other insects feeding off the wet faeces on top of it.
But through its bountiful layers, the guano provides a sediment record not unlike what you'd get from, say, a stalagmite that's slowly grown from a cave floor as sediment drips from above. Except much stinkier, and on a smaller, more useful timescale.
A team led by geoscientists from the University of South Florida has now used this poop mountain to drill an impressive sediment core and learn about long-term climate conditions in Europe by tracing chemical fluctuations in the sample.
"Bat guano is primarily composed of loose organic material, such as insect chitin, with a geochemistry characterised by an abundance of transition metals," the team writes in the study.
Chitin, the major ingredient in things like insect exoskeletons, is so abundant in bat poop that it makes for the most significant source of nitrogen isotope measurements.
In nature, nitrogen comes as one of two stable isotopes, N-14 and N-15. The latter of these is heavier, and its ratio to N-14 (measured as δ15N) can be used to track changes in the nitrogen cycle - the great process of biochemistry encompassing our planet.
Using the topmost 10 centimetres (4 inches) of the guano sediment and weather records in the area, the team was first able to establish a very clear pattern of how nitrogen isotope ratios in the poop corresponded to winter precipitation.
In wet periods, more N-15 was present in the poop, which makes sense given that the availability of water has an impact on how much nitrogen travels through the food chain from soil through plants and all the way to bats and their droppings.
With this data on hand, they were then able to extrapolate the weather conditions all the way back to 1650 CE, reconstructing how the weather phenomenon known as the North Atlantic oscillation (NAO) has been influencing the local climate.
Having such a long-term record is extremely useful for climate scientists working on predictions of how NAO will act in the future as it brings both wet and dry winter seasons to the European continent.
But on top of that, this new bat-poop method has given other geoscientists a powerful new tool for reconstructing past climatic fluctuations well beyond what we have in modern records.
"As such, our results suggest that the δ15N values of guano can be utilised to reconstruct past phases of the NAO beyond the instrumental record," the team concludes.
The study was published in Scientific Reports.