Geologists supported by the Swiss National Science Foundation have developed a new technique for mapping an entire glacier. They could confirm a theoretical model that describes how climate change affects erosion.
Supported by the Swiss National Science Foundation (SNSF), a team led by Frédéric Herman of the University of Lausanne has mapped the Franz Josef Glacier in New Zealand. The researchers have developed a new technique to study more precisely the relationships between global warming, glacier movement and erosion of rocks located below the ice mass. "The glacier is over 10 kilometres in length and fairly similar to those found in Switzerland," explains Frédéric Herman. "We selected it because of its location on a tectonic fault, with contrasting geological layers that contain graphite, an element that provides us with information on erosion." The results of the study have been published in Science.
Probing the geological history
The researchers used a combination of two techniques to map the glacier. First, stereoscopic satellite imagery allowed them to estimate the speed of movement at the surface. They were then able to extrapolate the speed at which the lower layer is sliding over the bedrock (between 30 and 300 metres each year).
At the same time, the study sought to quantify the intensity of erosion below the glacier -- the extent to which the glacier erodes the rock below it as it slides along. The research team took an indirect approach, as Frédéric Herman explains: "We studied the crystalline structure of the graphite -- carbon formed from fossilised organic matter -- contained within the rock flour retrieved downstream of the glacier. It provides us with quite precise information on the conditions at the time the graphite was formed, in particular its temperature, which was between 300 and 700 degrees. When we compare this with samples taken from around the glacier, we can work out the origin of the flour. Since the quantity of flour is directly linked to the rate of erosion, it is possible to draw a map showing the intensity of erosion beneath the glacier."
The researchers used the Raman spectroscopy technique to analyse the crystalline structure of the material. "Until now, geologists relied on isotope analysis, which requires very heavy equipment," continues the researcher. "It could take years just to obtain forty samples. With our technique, our Master's student Mattia Brughelli successfully analysed 4000 samples in two weeks, and then produced a very precise map of the glacier with a resolution of 1 metre."
A theory validated
The measurements confirm a theoretical model that was proposed in 1979, predicting that erosion is not simply proportional to the speed of movement of the glacier, but is related to its square. "In the last few decades, we have been able to observe that glacier movement is accelerating," says Frédéric Herman. "Our model indicates that erosion will intensify in a non-linear fashion with global warming." That means there will be increased sediment levels in alpine streams, which will increase the risk of debris flow, a mix of water and mud. "Our work shows that natural systems can be very perceptible to changes in the environment, even mountains."
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