Climate Change News

Past Seasons Hidden Underground

Belgian paleoclimatologists study a fast-growing stalagmite to glean insight into seasonal climate from centuries past.

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Deep in the south of Belgium, near the winding Lesse River, lies a cave rich with history and geologic wonders. The Han-sur-Lesse cave system has long fascinated visitors, with its well-defined calcium carbonate formations (speleothems) marveled at for centuries.

Recently, scientists from various Belgian institutions studied one of the cave’s fastest-growing stalagmites, Proserpine, to learn more about the evolution of the region’s climate. By analyzing data such as stable isotopes and trace elements, the researchers found clear evidence of increasingly dry conditions and anthropogenic activity over the past 4 centuries.

Tiny Clues from the Past

Proserpine is formed by water dripping from bedrock above the cave. Once the drip water lands on the surface of the stalagmite, it releases carbon dioxide and precipitates into a mixture of calcium carbonate and any other minerals that hitched a ride on the way down through the limestone above. With a flat, 2-square-meter face growing at roughly half a millimeter per year, this special stalagmite displays clearly distinct layers with seasonal variability, making it a perfect proxy to study past climate.

In a recent paper published in Climate of the Past, researchers analyzed three distinct sections of Proserpine, labeling them P16, P17, and P20, according to the centuries they represent. Because the stalagmite’s layers show a distinct seasonal variability in chemical composition, the researchers were able to make several conclusions about how Belgian climate and land use have changed seasonally over many centuries.

“Caves and speleothems are rather local phenomena, but they can record patterns of regional climate quite accurately,” said Niels de Winter, a postdoctoral researcher at Vrije Universiteit Brussel and one of the lead authors on the study. When added to paleoclimate data from proxies such as tree rings, ice cores, and peat bog records, speleothems add a great deal to the bigger picture of climate history. Their samples cover a much longer period of time than tree ring data as well as a broader range of environments than ice cores or peat bog records.

De Winter and his colleagues used laser spectroscopy to detect trace elements, among the most important data found within speleothems. Some trace elements, like lead, appeared in significantly higher amounts in more recent samples, hinting at anthropogenic pollution.

Trace elements also revealed more subtle properties about the cave and the region’s past climate. For example, elevated levels of magnesium, barium, and strontium suggest higher rates of prior calcite precipitation in the modern drip waters. Prior calcite precipitation describes the process of drier conditions and hotter temperatures causing water to percolate more slowly through the bedrock, giving the water more time to precipitate calcite and pick up trace elements before entering the cave.

Another, more obvious sign that the Belgian climate has gotten drier over time can be seen in the stalagmite layers themselves. Thicker layers are found in the Little Ice Age samples because rainfall levels and drip water precipitation on Proserpine were higher.

Digging Deeper

One of the study’s most intriguing discoveries was a stark contrast in trace elements between the P16 and P17 samples. This marked shift suggests a change in vegetation cover, possibly introduced by late 17th century farmers—but no historical records were found to independently confirm this hypothesis. What’s certain is that something abruptly affected the drip water’s path through the ground at this period.

In the words of the researchers, the change itself “can serve as a valuable palaeoclimate proxy.” Many speleothem studies take place over decadal to millennial scales, making these abrupt changes harder to resolve and place in context. To get around this hurdle, the researchers suggest sampling well-expressed seasonal speleothem layers at “strategic places” and superimposing them over longer timescales. However, not all speleothems have well-expressed layers.

According to Sophie Verheyden, a postdoctoral researcher at the Royal Belgian Institute of Natural Sciences in Brussels and lead researcher on the study, another way to measure seasonality is by detecting invisible layers in speleothems that reveal themselves through ultraviolet light, a phenomenon known as luminescence. Other methods, such as measuring wiggles in amounts of phosphorous, can be used as well.

Just like telescope engineers, scientists will continue to upgrade their paleoclimate techniques to peer deeper into the past. “This research is very difficult because there are a lot of things that can happen to the rainwater before it reaches the stalagmite. Understanding all these processes requires a mixed understanding of biological, geological, hydrological, and chemical processes,” de Winter emphasized.

Ian Fairchild, professor emeritus at the University of Birmingham in the United Kingdom and a longtime speleothem expert, said, “This looks to be a very well executed study that makes excellent use of preserved high-resolution information.” And although he admits that many speleothems can’t be studied in such detail, he notes that when they are, “a powerful understanding can be revealed.”

—Christian Fogerty (@ChristianFoger1), Science Writer

Citation: Fogerty, C. (2020), Past seasons hidden underground, Eos, 101, https://doi.org/10.1029/2020EO141234. Published on 13 March 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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