A study led by UMass Amherst researchers found that weak, shale-rich mountain watersheds in Colorado wear away about three times as fast as nearby hard-rock terrain, even where slopes are gentler, a difference that could affect estimates of the mountains’ role in releasing carbon dioxide.
The researchers studied the East River watershed, where soft shale, a sedimentary rock, lies beside more durable igneous rock formed beneath the ground. To compare erosion in the two rock types, the team developed a way to measure beryllium-10, a rare form of the element beryllium, in shale, where fine grains have made standard measurements difficult. Rays from space, called cosmic rays, create the isotope in quartz exposed at the surface, leaving a signal scientists can use to estimate how quickly land wears away.
The team combined these measurements with analysis of land shape, water chemistry and models of the landscape. The results indicate that rock type can help explain erosion patterns that slope steepness alone might not reveal. The results also indicate that erosion can be rapid in relatively gentle terrain.
The researchers say faster shale erosion can increase the amount of material dissolved in water. They say it can also contribute to a net release of carbon dioxide through weathering of rock that is not caused by living things, as the landscape changes over millions of years. The findings suggest that estimates of the carbon cycle may need to account more carefully for differences in bedrock.
However, the study focuses on one Colorado watershed.