Stalagmite shows history and effects of deforestation


Researchers from Ghent and Brussels have determined time periods of deforestation and temperature changes dating back to the 17th century

Undisturbed by cave tourists

Researchers from Brussels and Ghent have confirmed that summers in what is now Belgium have been getting warmer and the winters drier since the 17th century. The scientists, from Vrije Universiteit Brussel (VUB), Ghent University and the Royal Institute for Natural Sciences, can also pinpoint the time at which humans began to have a visible influence on the environment.

To carry out their research, they took samples from a stalagmite in the caves of Han-sur-Lesse, a popular tourist attraction in Wallonia. The samples came from a wide, fast-growing stalagmite known as Proserpine (pictured), which is deep enough in the cave system to be virtually free from the influence of tourists.

“We could determine that even in the 17th century, humans were beginning to leave their mark on the landscape,” said Niels de Winter of VUB’s Analytical, Environmental and Geochemistry research group. “Cutting down the forests meant that more water entered the cave because the soil above it thinned out or disappeared entirely. The stalagmite grew at a faster pace during that period.”

That effect is no longer seen in the 20th century, but “we see that the winters become drier and the summers warmer,” he said.

Well-defined rings

The rings of a stalagmite can be measured in the same way as those of a tree, to gauge their age and examine the conditions they have experienced over their lifetime. Proserpine’s rings are particularly clear.

The researchers took three samples and dated them using the Uranium-Thorum method. Along with counting the rings, this allowed them to establish that the samples came from the periods 1960-2010, 1635-1646 and 1593-1605.

“We examined these samples for the presence of carbon and oxygen isotopes, which come from the water that seeps into the cave,” said De Winter. “By the variations in the amount of water and the isotope composition of that water, we can deduce how rainfall evolved by season and by year.”

The higher the rainfall or the thinner the soil above the cave, the faster rainwater enters, collecting other elements that end up in the stalagmite. The researchers were able to demonstrate that conditions have become warmer and drier since the 17th century: The most recent samples contain much higher concentrations of magnesium and strontium, due to water entering the cave more slowly.

The study was published in the scientific journal Climate of the Past.

Photo ©Sophie Verheyden/KBIN