It doesn’t take long to be hit by a particle traveling close to the speed of light from another galaxy. Just step outside and wait all of a second to be bombarded with a couple hundred. These particles, known as cosmic rays, hit Earth from all directions, day and night. The journey for cosmic rays is unthinkably long; they often start in a distant galaxy and meander around for a billion years. When they finally smash into Earth, they embed themselves into rocks, soil, and even water. The longer a material is exposed on Earth’s surface, the more cosmic rays it accumulates, making cosmic rays perfect indicators of erosion. Geomorphologists have recently started using accelerator mass spectrometry (AMS) to measure how long a rock has been exposed on Earth’s surface. By analyzing a variety of samples from a region, they can tell just how and when a surface eroded. The theoretical methods have rapidly triggered several new applications.
Although working with cosmic rays involves lots of money and lab time, practical uses are quickly cropping up in the field. For the first time, scientists have been able to accurately measure soil production rates and calculate exactly how a given surface has eroded over thousands of years. Furthermore, they can measure long-term erosion rates in an area and compare it with short-term erosion rates, thus obtaining a clear view of just how humans have affected an area.
Areas of mountainous farming are of special interest, and new technologies are giving scientists the opportunity to examine them like never before. For the first time, we can quantify the impact of farming on such areas as the mountains of Sri Lanka and the terraces of Himalayan Nepal.
Tools of the Trade
It is not uncommon for me to fall asleep in the sun. Upon waking up, thirsty and disoriented, I can determine how long I slept in the midday blaze by measuring the redness of my sunburn. Characterization of rocks by examining cosmic rays is essentially no different. The longer rocks are exposed on Earth’s surface, the more radioactive nuclides they have resulting from cosmic rays.
Cosmic rays are actually not rays at all but instead are highly energized particles zipping around from galaxy to galaxy. They range in size from a single proton to several protons and neutrons, and they move fast–just shy of the speed of light. There is not a laboratory on Earth that can give particles that much energy. Because cosmic rays have so much energy, they can penetrate several meters of rock like tiny bullets before coming to a stop.
Scientists are still attempting to find concrete answers about cosmic rays. Where do they come from? How are they produced? How are they accelerated to such ridiculous speeds? Most agree that cosmic rays probably come from supernovae, some from our galaxy and some from others. We do know that cosmic rays usually catapult back and forth between the opposite ends of our galaxy several times before bumping into something such as Earth. We also know that they hit Earth from all directions day and night and have done so for billions of years.
The secret to using cosmic rays to study erosion rates is in measuring their traces in quartz. Quartz (SiO2) is made up of silicon and oxygen. When a cosmic ray comes rocketing in, it will occasionally hit Si and O atoms and strip off protons, a process known as spallation. In a gram of quartz, this happens only a few times each year. Since the number of protons determines the identity of the element, silicon becomes radioactive aluminum (26Al), and oxygen becomes radioactive beryllium (10Be). These isotopes are known as cosmogenic nuclides.
To measure the cosmogenic nuclide concentrations, quartz samples are brought to particle accelerators for AMS. There are two such facilities in the United States: PRIME Lab at Purdue University and Lawrence Livermore National Laboratory. The particle accelerators accelerate the quartz samples to about 50 million mph, still just a mere fraction of the cosmic ray’s original speed. Then, using enormous magnets, the accelerators get an extremely accurate count of the cosmogenic nuclides. AMS is so accurate that it can detect a single atom out of 1015 (one in a quadrillion). An entire gram of quartz only contains a few thousand 26Al and 10Be atoms, so such accuracy is essential.
In the Field
Those tiny radioactive atoms whirling around inside the particle accelerators have big consequences for the world of erosion. By analyzing riverbed and hillside samples, geomorphologists get a detailed understanding of the local erosion history. Many aspects of erosion previously open to speculation are now quantitatively understood, often with surprising results. Analysis of 26Al and 10Be concentrations allows us to understand Earth’s surface as we never have before.
Because cosmic rays bombard a surface at a constant rate and have done so for billions of years, a few good measurements give remarkable insight into past erosion patterns. The higher the concentration of cosmogenic nuclides, the longer the rock was exposed before eroding. Similarly, low nuclide concentrations tell us that the rock eroded quickly and was buried by sediment.
With this in mind, it is now possible to understand the erosion patterns of entire watersheds. By analyzing quartz samples from streambeds, one can deduce how quickly the hill eroded. Surprisingly, Jim Kirchner, Ph.D., from the University of California, Berkeley, performed such a measurement on Idaho’s streams and found erosion rates over the past 27,000 years to be 17 times higher than present rates. The higher rates are probably due to rare major events, such as floods and fires. If the window we looked at for “present” rates were wide enough to include more catastrophic events, past and present erosion rates would be closer.
Another application for cosmogenic nuclide analysis is to study soil production rates. Dartmouth’s Arjun Heimsath, Ph.D., recently used cosmogenic nuclides to quantify bedrock conversion to soil. By analyzing soil samples from the base of an escarpment, Heimsath discovered that soil production rates decline exponentially with increasing soil depth. In other words, the more exposed a rock is, the more soil it makes.
Another application is to study runoff and erosion from mine tailings. Ranger Uranium Mine is situated within Kakadu National Park World Heritage Area in northern Australia. Since the mine’s commencement in 1980, it has produced more than 16 million tons of radioactive tailings. The tailings have been mismanaged and are currently eroding into the surrounding park.
Heimsath used cosmogenic nuclides to determine erosion rates for the surrounding areas. The data were then combined with numerical models by other scientists. The result is an accurate mine runoff model that identifies each potentially toxic step.
Erosion in Sri Lanka As Told by Radioactive Beryllium
Sri Lanka is a tear-shaped island about the size of West Virginia off the coast of India. Although beautiful, Sri Lanka is plagued by more erosion problems than a sandbox next to a sprinkler. Two hundred years ago, Sri Lanka was covered entirely by dense rainforests. Since then, almost all the forests have been cleared and the land used for agriculture, especially tea farms. Very few unmodified forest patches remain, but the ones that still stand make good control groups for measuring 10Be concentrations and determining human influence on island erosion. The University of Bern’s Friedhelm von Blanckenburg, Ph.D., and his colleagues took the measurements.
For several reasons, Sri Lanka is ideal for cosmogenic nuclide analysis. In addition to still having a few rainforests, Sri Lanka has rocks under the sample area that are uniformly crystalline. Also, uplift rates are minimal, resulting in expectations of a very low natural erosion rate. Geomorphological parameters, lithography, and climate are uniform, so any erosion differences are from human influence.
The group took quartz samples from creek sediments below two of the remaining natural forest patches and compared them with sediment samples from below farmland. The quartz below the rainforest had far more cosmogenic nuclides, implying more surface exposure and thus slower erosion. But no one expected the magnitude of difference between the two. Different samples gave different results, but the farmland had an erosion rate somewhere between 20 and 100 times that of the natural forest land, highlighting just how much farming has damaged the landscape.
Findings in Himalayan Nepal
The Himalayan region of Nepal is another prime area for using cosmogenic nuclides to study differences between long-term and short-term erosion. Terrace farms dot the expansive hills and mountainsides. Devastating landslides plague Nepal’s farmers throughout the region. “Large landslides take everything–the field, the village–and dump it in the river,” says Heimsath, who did extensive research in the region. He and his colleagues went to Nepal to compare catastrophic erosion events with long-term erosion using nuclide analysis. “Nepal is very exciting because we are pushing the limits of this technique,” Heimsath says. It was once thought that hundreds of years of farming caused frequent landslides, but the geomorphologists’ recent results tell a different story. Unlike Sri Lanka, erosion has remained about the same in Nepal. Humans do not play the primary role in the landslides that wipe out their farms and villages. In fact, terracing by farmers helps the land stay in place.
To obtain an understanding of Nepalese erosion, the research group needed enough quartz to get accurate readings for at least the past 2,000 years. This corresponded to about 100-200 g of quartz. Because the rock is only 20% quartz at best, the crew had to take substantial rock samples. Heimsath explains, “When we’re working at 5,500 meters [18,000 feet] carrying out hundreds of kilograms of rock samples on mules and our backs, things start to get interesting.”
The Himalayas have some tricks to throw scientists off. For instance, frequent landslides do not allow materials to stay exposed to cosmic rays for long. If sediment results primarily from a recent major landslide, its cosmogenic nuclides will not give an accurate reading of long-term erosion. River sediment is the best sample source because of the general tendency for it to be a spatial average of materials above, but one must keep in mind accuracy limitations in areas such as Nepal. A misrepresentative batch of quartz can end up wasting lots of money.
The fact that catastrophic landslides occur frequently in Nepal is nothing new to the Nepalese people or government. What’s new is that for the first time scientists have quantified the landslides. This is an important fact for the terracing farmers because having hard data will help them work with government agencies to figure out what can be done to help.
The Himalayan landslide dilemma is as much a social problem as a geographical one. People would not be farming land in such dangerous locations if there were not so many people already farming the flat land. “The classic marginalization of people leads to a situation where the poorest people get nailed by the biggest disasters,” laments Heimsath. “As a scientist, I can only quantify something that they are intuitively aware of.” In Nepal, poverty increases with altitude. The poor not only have to carve out livings on terrace farms, but they also have to live in constant exposure to potential landslides. Heimsath does not pretend that overpopulation is not a problem in the region. “I am one of those people who believes that the root of many environmental problems is from too many people trying to live on too little land.” This is another fact of which the Nepalese are already vividly aware.
