Roman aqueduct
Les Ferreres Aqueduct, in Catalonia, Spain. A few years ago, scientists figured out that they could use the layers of carbon- and oxygen-rich sinter in ancient Roman aqueducts like this one to study past climates. Credit: Leonid Andronov/iStock/Getty Images Plus/Getty Images

It’s no secret that Earth scientists are obsessed with the past—what did our planet look like, how did its mountains and valleys evolve, where did it rain or dry up? For that last question, scientists can turn to proxies that reflect whether the climate was hot, cold, dry, or wet.

Ratios of heavy and light isotopes, especially those of carbon, nitrogen, and oxygen, often serve as those proxies. Scientists measure these ratios in the layers of many different natural archives, such as ice cores, cave formations, tree rings, corals, and even ocean and lake sediments.

But even though these materials have been integral to our understanding of the past, they have their limits—for example, you can’t find corals in all parts of all oceans, and they aren’t on land. Long-lived trees aren’t necessarily found everywhere. You can’t take an ice core in a desert. As a result, evidence for climate fluctuations on local and regional scales is lacking around the world.

To solve this puzzle, scientists get creative. They’re examining nature for whatever they can find that lays down layer after layer over a large chunk of time in relatively undisturbed environments. And in the process, they’ve stumbled on some new, intriguing archives.

Here are five of the weirdest ones we’ve found.

Bat Guano: Smelly but Useful

For some folks, the things they’ll do for science include slogging up a mountain of poop. Bat poop, to be exact.

Pile of bat guano
Two researchers stand beside a hill of bat guano in Romania’s Zidită Cave. Credit: Bogdan Onac

Scientists have found that the piles of bat guano in caves all over the world record a relatively high resolution snapshot of local climate. The poop hills amass in layers as thousands of bats dangle from cave ceilings and do their business. Repeat every few hours for thousands of years, and you’ve got yourself a robust poop record.

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. Onac and his team are at the forefront of developing a guano paleoclimate archive.

The archive works like this: Microbes fix atmospheric nitrogen into soil. This nitrogen then moves up the food chain to plants, to insects that eat the plants, and to the bats that eat the insects. The bats, of course, poop.

As the nitrogen moves from the soil to the plants, its lighter isotope is more readily washed away by the rain, unless it’s a dry period. Thus, an abundance of heavy nitrogen (15N) in any given layer of bat poop means that the bat ate an insect that ate a plant that grew when water was abundant, that is, a rainy period.

All you need now is a core to collect the guano. And a strong stomach.

Roman Aqueducts

It seems like fate: Following the trail of a paleoclimate archive inevitably led to Rome. And that trail is overhead, in aqueducts. These ancient Roman plumbing systems could help today’s climate scientists piece together past precipitation puzzles.

Aqueduct sinter
Two samples of sinter from Roman aqueducts, showing distinct banded layers that researchers can study to figure out a history of local climates. These layers typically represent seasonal cycles. Credit: Cees Passchier

Aqueducts—some 4,000 spread out on all sides of the Mediterranean—have hundreds of years’ worth of calcium carbonate layers. The water rushing through the aqueducts came from limestone cave springs, which means it was loaded with dissolved calcium carbonate.

As the water came into the open air, some of it evaporated, causing this calcium carbonate to precipitate out and form a layer of sinter. Heavier oxygen precipitates out first, so in layers composed during colder weather—when less water evaporated—scientists find more of the heavier isotope of oxygen in the aqueducts’ sinter. In warmer weather, more calcium carbonate will precipitate out, which means scientists will find a higher concentration of the lighter oxygen isotope.

Some sequences of sinter coating the aqueducts can be about a meter thick, representing hundreds of years of deposition, noted Cees Passchier, a structural geologist at Johannes Gutenberg University in Mainz, Germany, who started studying the aqueducts as a paleoclimate archive a few years ago. Although dating these layers does get tricky—unlike tree rings, each band does not necessarily represent a set time interval—the archive could help scientists tease out fine details of past droughts, rainy periods, and even signatures of early air pollution.

Snail Shells

Tired of taking cores? You could also use snail shells to study the past!

As snails consume water, oxygen from the water gets incorporated into the calcium carbonate that makes up the animals’ shells. Scientists recently figured out that they could use the different isotopes of oxygen as a proxy to tell them something about whether the weather was dry or rainy when the snail built that part of its shell.

The proxy works like this: When it’s wet out, snails have to use less energy to access oxygen through water and will preferentially draw up water with the lighter oxygen isotope. When it’s dry out, the snails will take up whatever water is available, including water with the heavier oxygen isotope. So dry climates correspond to higher abundances of the heavier isotope.

Snail shells
Three different Achatina fulica shells. Credit: Siong Kiat Tan (CC BY 3.0)

Scientists are still trying to understand climate patterns that bring different wet and dry conditions to different parts of the world. And in India, the invasive giant African land snail species Achatina fulica may have just the shells needed to help figure it out. Wet-dry conditions resulting from these patterns “can be easily captured using isotopic fingerprinting of carbonates in growth bands of Achatina fulica snails,” explained Prosenjit Ghosh of the Indian Institute of Science in Bangalore, India. Ghosh and collaborators are using current and museum specimens of snail shells to piece together a detailed history of Indian summer monsoons.

Sometimes, archaeologists find old snail shells near ancient human dwellings. People did (and still do) eat snails, so could boiled snail shells still preserve a climate record? Paleoclimate scientists from Germany put this to the test. Turns out, the shells do!

Whale Earwax

Do your ears hold a valuable archive of paleohistory? Not unless you’re a whale.

Whale ear plugs, collected from dead whales as oddities and found in museums and private collections, are usually read like tree rings—counting their layers reveals clues on a given whale’s age. But some scientists wondered whether the plugs could be used for more.

New research shows that they can. Specifically, carbon isotopes within the layers of the waxlike substance that builds up in a whale’s ear canal can reveal its diet and migratory behaviors, explained Farzaneh Mansouri, an environmental scientist at Baylor University in Waco, Texas. Mansouri and colleagues have studied 20 ear plugs from museums in the United States and the United Kingdom.

“It’s like a sediment core, or an ice core,” Mansouri said. For example, the heavy carbon isotope 13C tends to decrease in the ocean near the poles, so more 13C in a layer of whale earwax tells researchers that the whale traveled to colder waters.

Patterns beyond simple migratory behavior can pop out of the data, too. One trend scientists noticed in a group of samples was that the presence of 13C decreased steadily over a decade; they wonder if that means the whales moved their activities northward to escape from shipping-filled or warmer oceans.


Sponge spicule
A researcher holds a 2.7-meter-long silica spicule of M. chuni. The specimen was obtained from a depth of 2,100 meters in the South China Sea. Credit: W. E. G. Müller

One of Earth’s oldest animals preserves information about past climates, spanning several millennia, in its skeleton.

The animal is a sea sponge called Monorhaphis chuni, and it grows a giant basal spicule that can reach 3 meters long. Specimens plucked from the deep reaches of the Pacific Ocean show that individual M. chuni can grow their spicules for some 18,000 years. And much like a tree, cross sections of the spicule reveal growth rings.

New research shows that the rings’ silicon isotope ratios give clues about the concentrations of silica in the seawater that surrounded the sponge when the ring formed.

To understand how the archive works, let’s move this story to the surface. At the surface of the ocean, populations of microscopic algae called diatoms use available silica to build their shells. At the same time, these diatoms are photosynthesizing and taking up carbon. When the diatoms die, they—with their silica and carbon—sink to the ocean floor, making these populations an important carbon sink and regulator of greenhouse gases in the atmosphere.

Sponges like M. chuni, in turn, use the silica that the diatoms deliver to the deep ocean to build their spicules. Thus, information on the silica available at different times in M. chuni’s long life gives clues about the abundance of diatoms that were present at the surface and thus how much atmospheric carbon dioxide those diatoms converted into organic forms.

“These deep Pacific Ocean data help to fill an important global gap in paleo‐nutrient records,” wrote Klaus Jochum and coauthors in a Geophysical Research Letters paper published in November 2017. For example, the study shows that during a period when glacial conditions switched to interglacial conditions around 15,000 years ago, the Pacific held higher concentrations of dissolved silica than in current times, likely reflecting changes in the time period’s global carbon budget.

“Either continental sources supplied more silica to the deglacial ocean and/or biogenic silica burial was lower, both of which may have affected atmospheric CO2,” they continued.

Have you encountered any other weird paleoclimate archives? Do you have any ideas of other natural records that might exist? Let us know in the comments.

—JoAnna Wendel (@JoAnnaScience), Staff Writer; Mohi Kumar, Scientific Content Editor


Wendel, J.,Kumar, M. (2018), Five weird archives that scientists use to study past climates, Eos, 99, Published on 30 March 2018.

Text © 2018. The authors. CC BY-NC-ND 3.0
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