New research documents rates of erosion in the Andes mountains.
Every second, Earth is bombarded by massive amounts of cosmic rays—invisible subatomic particles that come from things like the sun and supernova explosions. These high-energy cosmic rays, having traveled far, collide with atoms as they enter the Earth’s atmosphere and trigger cascades of secondary cosmic rays.
When secondary cosmic rays penetrate the upper reaches of Earth’s surface, they convert elements in minerals, such as oxygen, into rare radioisotopes (or “cosmogenic radionuclides”) such as beryllium-10 (Be-10) and carbon-14 (C -14).
Scientists can then study variations in the concentrations of these cosmogenic nuclides to estimate how long rocks have been exposed on Earth’s surface. This in turn allows researchers to gain a better understanding of planetary processes, such as erosion rates – from nothing more than a kilogram of river sand.
Gregory Hoke, professor and chair of the department of earth and environmental sciences at Syracuse University, JR Slosson, postdoctoral researcher at the University of Massachusetts Amherst, and Nat Lifton, associate professor of earth, atmospheric and planetary sciences at Purdue University , analyzed cosmogenic radionuclides in samples from the Argentine Andes. Their findings appear in the journal Geophysical Research Letters.
Their goal was to document the amount of time material remains on the slopes of the Andes relative to the overall erosion rate of the river basin. This information is critical to helping scientists identify landslide hazards and understand how climate change will affect the dynamics of material transport on slopes as areas become wetter or drier.
To determine erosion rates, the team took samples of river sand collected at the foot of the eastern side of the Andes mountains in the provinces of Mendoza and San Juan, located in west-central Argentina. The river sand should be a representative, well-mixed sample of the entire watershed (or watershed) upstream from the sample collection point.
In Hoke’s lab, the sand was processed to isolate the pure quartz from the other minerals in the sample. The researchers use pure quartz because it is the optimal source for Be-10 and C-14. The pure quartz sections were sent to Lifton’s laboratory where beryllium and carbon were extracted. Subsequent measurements of C-14 were performed at Purdue’s PRIME laboratory, and Be-10 was analyzed at Lawrence Livermore National Laboratory to estimate the concentrations of each radionuclide.
The highest non-volcanic peaks in the Andes are between Santiago, Chile and Mendoza, Argentina. The river basins that drain the high Andes extend to an elevation of 5,000 meters and their slopes are lined with accumulations of rock debris known as talus and stone.
Because Be-10 and C-14 are produced in proportion but decay at very different rates, concentrations of cosmogenic radionuclides in a sample reveal the rate at which sediment is produced from bare rock surfaces (Be-10) and the time it takes for it to travel down hills through landslides (C-14). As sediment is mobilized and buried by landslides, the rate of production of both isotopes decreases, but because C-14 decays 1,000 times faster than Be-10, their ratio changes rapidly. This proportional change allowed the authors to apply a statistical model to determine the average length of time it takes material to travel down the ankle slopes.
According to Hoke, this is one of the first studies to use the combination of Be-10 and C-14 to show the long-term average rate of sediment build-up and the time and process it takes to move to and through rivers. giving a broader picture of the factors involved.
“Previously, we relied almost exclusively on Be-10 and sediment concentration measurements made at river gauging stations to estimate average erosion rates,” notes Hoke. “What drew us to study these watersheds with C-14 was the agreement of the gauge and the Be-10 data. We expected to see the two isotopes and gauge data yield the same rates and demonstrate that the erosion of the mountain was occurring in a steady state.”
While the concentration of Be-10 rebounded as expected over a long period, they found that C-14 was much lower than expected, meaning that sediments eroded from high mountain catchments were shielded from cosmic rays for at least 7,000 to 15,0000 years. The authors explain that temporary storage on ankle slopes better explains the lower concentration of C-14 relative to Be-10.
“This study shows that it is possible to fill a significant gap in the observational timeline by using the C-14/Be-10 pair that brings to life what is really happening on the hillsides,” says Hoke.
With the risk that landslides pose to people and infrastructure, Slosson says their results show that C-14 may be important in revealing sediment transport dynamics in the future and potentially help predict where landslides may occur. future landslides occur. He explains, “Using C-14 together with Be-10 provides a new window into the complexity of sediment transport in mountainous regions and can provide a background for assessing modern changes in land surface processes.”
The project was funded by the National Science Foundation, the Geological Society of America and Syracuse University.
Source: Syracuse University