A new tool for climate science

6 November 2018 18:25
Researchers standing in a cavern

As is often the case, it was apparently contradictory observations that started Dr. Peter Wynn and colleagues thinking about whether concentrations of sulphur in stalagmites could provide evidence of changing seasonal patterns in climate over thousands of years.

Stalagmites are formed when rain water passes through vegetation, soil and rock to drip onto a cave floor - at each stage picking up a chemical signal. The rain water dissolves calcium carbonate from the rock which solidifies on the cave floor, building up a stalagmite from the ground up.

Stalagmites grow over hundreds, even thousands, of years. So stalagmites, a bit like tree rings and ice cores, offer an excellent timeline of changing atmospheric conditions over long periods.

Dr Peter Wynn, from the Lancaster Environment Centre at Lancaster University, is a world-leading expert on sulphur in stalagmites. He uses the sulphur record to provide information about changes in atmospheric pollution levels, identify volcanic events and interpret environmental changes. While conducting this research in a cave in north east Italy he came up with some surprising results.

“When monitoring the sulphur content of drip waters coming into a cave over several years, we found almost the same concentration every month.

“However, when we took the associated stalagmite and measured the sulphur concentration within it, we saw a seasonal variation. Why was the Sulphur concentration in the drip constant, but that in the stalagmite varying seasonally? This was a puzzle.

“Something must be happening between the point at which the drip enters the cave and the point at which it is captured in the stalagmite. What is driving these seasonal cycles of sulphur within the stalagmites? It must be something to do with the cave itself.”

Peter discussed the issue with colleagues and came up with a hypothesis.

“Caves breathe on a seasonal basis,” he explains. “Seasonal temperature differences change the density of the cave air, thereby switching the air flow between cave interior and exterior between seasons. This affects the carbon dioxide (CO2) content of the cave air, leading to high CO2 within the cave during the summer and low CO2 within the cave during the winter.

“This in turn changes the chemistry of the drip waters - when cave air CO2 is low, the drips contain lots of dissolved calcium carbonate and vice versa. This carbonate, when incorporated into the growing stalagmite, competes with sulphur. So the sulphur content of the stalagmite is low in winter because it is diluted by carbonate and high during the summer when it is less diluted.”

This, Peter believed, could explain why the sulphur levels in stalagmites fluctuate depending on the time of year, but the drip waters do not.

Peter and his colleagues decided to test this elegant theory in the laboratory growing mini stalagmites in glass jars containing solutions with different concentrations of calcium carbonate, spiked with sulphate.

The results supported the theory and suggest that levels of sulphur in stalagmites could provide information going back many hundreds of years about changes in the length of the seasons. These findings have now been published in the journal Geochimica et Cosmochimica Acta.

To see if the theory works in the real world, Peter is now studying stalagmites from Italy, Austria and China. He wants to see if the changes in seasonal climate indicated through the patterns of sulphur concentration reflect the actual seasonal changes shown by meterological records.

If the relationship holds up, this work provides a new tool in the climate scientists tool box allowing reconstruction of seasonal climate characteristics over thousands of years. Peter is excited by the possibilities that this new work promises.

Read the paper Sulphate partitioning into calcite: Experimental verification of pH control and application to seasonality in speleothems in the journal Geochimica et Cosmochimica Acta.

The other authors are: Prof. I.J. Fairchild, University of Birmingham, UK; Dr. A. Borsato and Prof. S. Frisia, University of Newcastle, Australia; Dr. C. Spötl, Leopold-Franzens-Universität Innsbruck, Austria; Dr. A. Hartland, University of Waikato, New Zealand; Prof. A. Baker, UNSW Sydney, Australia; and Dr. J.U.L.Baldini, University of Durham, UK.

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