Dartmouth researcher uses cosmic rays to calculate erosion rates

November 12, 2001

People build houses, plant fields and construct cities on the top layers of the planet's surface. These layers, however, are far from solid. They are flexible and mobile, some parts more than others. Arjun Heimsath, Assistant Professor of Earth Sciences at Dartmouth, measures this dynamic land movement by calculating erosion rates in different parts of the world.

Scientists know that the earth's processes have a profound effect on human lives, and research to study and predict erosion will help people adapt to our changing planet.

In northern Australia, for example, Heimsath's work involves uranium mining. "When you extract uranium during the mining process, you produce radioactive waste. It is piled into an earthen dump that looks like a giant loaf of bread, acres across. That mound of tailings is susceptible to erosion," Heimsath said. With his colleagues, he works to determine how the waste piles will erode over time, and how that eroded material will move through the environment. Their calculations, using one of the most accurate procedures to date, quantify erosion, computing the speed at which land melts into itself, lakes, streams and eventually the ocean.

To conduct his research, Heimsath extracts cosmic isotopes from rock and sediment samples. "Cosmic rays are high energy particles coming in from both galactic and solar sources. I'm primarily interested in the galactic ones that have higher energies. When they hit minerals in a specific way, they knock electrons and neutrons off atoms and create numerous different isotopes," he said. "Concentrations of these isotopes build over time at rates dependent on their location." From those isotope concentrations, also called radionuclides, he can determine how long that material has been there and how fast it's eroding or breaking down.

Heimsath's procedure for determining the age and erosion rate of rocks located up to a few feet beneath the soil emerged from a similar method for rocks on the surface. He collects the samples using a hammer and chisel, a rock drill or a shovel. The sample size ranges from a few grams of gravel and dirt to larger rocks about the size of a gallon of milk. Back in the lab, Heimsath works to extract the chemical data found in the quartz in his samples.

"A solution of acid dissolves everything but the quartz, and then we further break down the quartz to get the radionuclides," Heimsath said. The gallon-container-size original sample is reduced to a pinhead amount of powder. It's placed in a nuclear accelerator where a beam of ions hits the sample, resulting in another beam of electrons. It's then accelerated to close to the speed of light, and the precise atomic makeup of the sample can be measured.

Once he's obtained those figures, he plugs them into a mathematical landscape simulation - where every location or point has a number representing the elevation - to predict how a given set of environmental parameters will move sediments and eroded material around the system.

"My measurements will help explain how sediment moves off the tailings dump, down the hillslope, across grassy flood plains into a stream, picked up by flowing water, moved through rapids and across tree stumps and settles in sandbars. Tree roots could take up some nutrients, while more sediment moves further into natural settling ponds. Finally it gets to the ocean, which is about 40 to 50 kilometers away," he said. The sediment's journey is known as "source to sink," and every step along the way indicates a potential for toxic infiltration.

According to Heimsath, this erosion research is critical to understanding and preserving northern Australian economic and environmental health. An important export product for Australia and a vital source of income for some aborigines, uranium is used to generate nuclear power, and it's also valuable for producing radioisotopes, which have medical uses, metallurgical and engineering applications, and are used as food preservatives.

Heimsath's fieldwork in Nepal links local farming endeavors with his more modern research techniques. In the Himalayan region, known for its striking hills and mountains, landslides plague farmers. It was once thought that thousands of years of farming damaged the countryside.

"Now we can say with relative certainty that humans play a minor role in Himalayan erosion, and it's the natural background processes that are more significant. So, if we get a measurement of long-term erosion rates and compare them to short-term erosion rates from agricultural watersheds, then we can answer the question of 'What is the role of humans?' more definitively," said Heimsath.
These studies of erosion rates contribute to the "source to sink" initiative of the National Science Foundation. For more information, go to: http://www.ldeo.columbia.edu/margins/Home.html. Heimsath's work in Australia is supported by an Australian Research Council Grant, and in Nepal, the National Science Foundation funds the research.

Dartmouth College

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