The team led by Dr. Stef Vansteenberge of the VUB research group Analytical, Environmental and Geo-Chemistry (AMGC) used for their research "Proserpine", a broad, fast-growing stalagmite located deep in the Han caves, so that no contamination could be expected from tourism in the popular cave.
Stalagmites grow every year, similar to the growth rings of trees. Those annual rings are not as clear in all stalagmites as in "Proserpine", which made it a very suitable sample for this type of research. The researchers took three samples from the stalagmite, which they then examined using the Uranium-Thorium method. This method, together with counting of the annual layers, made it possible to determine that the samples spanned the following dates: a period from 1960 to 2010, a seventeenth-century sample for the years 1635 to 1646 and one from the sixteenth century from 1593 to 1605. .
"We examined these drill samples for the presence of carbon and oxygen isotopes, which are supplied annually via the water that seeps into the cave," says Dr Niels de Winter of the VUB research group AMGC. "From the variations in the amount of water and the isotopic composition of that water, we can deduce how rainfall evolved per season and per year".
If there is more rainfall or if the soil above the cave is less thick, water enters more readily and faster into the cave. The water then absorbs other elements which end up in the stalagmite. For example, much higher concentrations of the elements magnesium and strontium in today's samples show that it is warmer and drier now compared to the 17th century.. These elements enter the cave when the water takes longer to seep into the cave.
"In our research, we were able to establish that man began to leave his mark on the landscape as early as the seventeenth century. By cutting down the forests above the cave, more water suddenly came into the cave because the soil disappears or becomes thinner. In that period the dripstone grew at a higher rate. That effect disappears again in the 20th century: we see that the winters become drier and the summers warmer".
Niels The Winter
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