Towering mountains, rolling hills, and winding river beds may appear like permanent features of a landscape, but they are actually changing all the time. With the help of water, wind, and gravity, the process of erosion continuously alters Earth’s surface by moving soil and rocks from steep mountaintops to flat, sandy beaches.
While shaping the landscape, erosion can strip fertile soils away from farms, overload streams and reservoirs with sediment, and undermine the foundations of infrastructure. Because this powerful process has the potential to damage ecosystems and communities, it is important for us to understand it better. We want to understand how quickly places are eroding and how that might increase or decrease with environmental and anthropogenic influences.
One way we are able to measure the rates of erosion in diverse landscapes across the world is by using cosmogenic nuclides (Kirchner et al., 2001). Cosmogenic nuclides are chemical signatures produced in soil and rocks by high energy cosmic rays from outer space activity, such as supernovae. These high energy rays constantly bombard the Earth and trigger chemical reactions that alter the composition of soil and rock as deep as the first few meters of the surface. For example, cosmic rays react with quartz grains and change a form of the element oxygen, 16O, into the cosmogenic nuclide of beryllium, 10Be (Fig. 1). We are able to measure the concentration of the cosmogenic nuclides in quartz grains and figure out how fast a given surface is eroding. For example, somewhere with a high concentration of cosmogenic nuclides is eroding slower than somewhere with a low concentration of nuclides because there has been more time for the surface material to be impacted by cosmic rays and accumulate nuclides.
Figure 1. Diagram showing cosmic rays (red lines) entering Earth’s atmosphere, reacting with the atmosphere and reacting with a dark mineral on the surface. The cosmogenic nuclide 10Be is produced from 10O. Image acquired from Cerege.fr.
Knowing how fast cosmic rays accumulate in different places, such as high altitude mountains vs. sea level, we are able to quantify the rate at which a location is eroding. For example, the tall and steep Himalayan Mountains are eroding as fast at ~3,000 meters per million years (m/Myr), whereas the low slopes of the South Carolina Piedmont are eroding as slowly as ~3 m/Myr. Because cosmogenic nuclides tell us about erosion rates over long timescales, we can use these long term background rates to detect changes in the recent past (Kirchner et al., 2001).
As a part of the Bedrock Critical Zone team, I am working to understand how the rate of erosion in the South Carolina Piedmont has changed over time. To do this, I will be analyzing concentrations of cosmogenic nuclides in soil depth profiles. My findings should help us answer questions such as how transient is this landscape and how has the climate and agricultural practices impacted erosion throughout time.
Please reach out if you are interested in learning more or discussing this topic!
Nancy Weinheimer
graduate student, University of Wyoming
Image
Diagram showing cosmic rays entering Earth’s atmosphere, reacting with the atmosphere and reacting with a dark mineral on the surface. The cosmogenic nuclide 10Be is produced from 10O. Image acquired from Cerege.fr.
Works Cited
Kirchner, J. W., Finkel, R. C., Riebe, C. S., Granger, D. E., Clayton, J. L., King, J. G., & Megahan, W. F. (2001). Mountain erosion over 10 yr, 10 k.y., and 10 m.y. Time scales. Geology, 29(7), 591– 594. https://doi.org/10.1130/0091-7...;0591:MEOYKY>2.0.CO;2