April 12, 2024
Cosmic Rays Reveal Watershed History in Colorado Rockies
Dating boulders with cosmic rays expands understanding of paleoclimate and glacial history in Colorado River headwaters.

Glacier extents and time for the East River watershed in Colorado.
[Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Quirk, B. J., et al. "Latest Pleistocene Glacial Chronology and Paleoclimate Reconstruction for the East River Watershed, Colorado, USA." Quaternary Research 119 86–98 (2024). DOI:10.1017/qua.2024.5.]
The Science
The Rocky Mountains were covered by glaciers during the ice age. Glaciers eroded deep valleys and left huge piles of sediment and bedrock boulders behind when the climate warmed. Since the ice melted, boulders have been continuously bombarded by cosmic rays, which are produced by exploding stars and that travel through space before colliding with Earth. Cosmic rays have such high energy that they break atoms apart and form new ones when they crash into rock. By measuring the concentration of these new atoms in boulder samples from the Rocky Mountains, scientists can determine when glaciers last filled valleys with ice.
The Impact
Glacier erosion strips away soil, leaving only bedrock behind. Understanding when bedrock was first exposed by melting ice is needed to determine how quickly bedrock is broken down into soil. In the Colorado River headwaters, glaciers were largest about 18,000 years ago and had mostly melted by 13,000 years ago, indicating all the valley soil has formed since then. Based on how far the ice extended down the valley, computer models indicate temperatures were around 7°F (4°C) cooler 15,000 years ago than today.
Summary
By measuring tiny quantities of rare atoms, a multi-institutional team of researchers determined the timing of glaciation in the East River watershed near Crested Butte, Colorado. The glacial history is like other valleys in the Rocky Mountains. This research indicates only several degrees of temperature change caused glacier melting as Earth warmed during the last ice age’s transition to a warmer Holocene climate.
The East River watershed is a site of intensive research focused on how water changes as precipitation moves through soil before becoming streamflow. Chemical reactions change over time as the rock left behind by melting glaciers is weathered, changing the soil and bedrock chemical composition. With knowledge of when the glaciers melted, scientists can now determine the rates at which chemical reactions occur and build better models to predict how rock weathering influences water quality.
Principal Investigator
Isaac Larsen
University of Massachusetts–Amherst
[email protected]
Program Manager
Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]
Funding
This material was based on work supported by the Biological and Environmental Research program within the U.S. Department of Energy (DOE) Office of Science and the DOE Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory.
References
Quirk, B. J., et al. "Latest Pleistocene Glacial Chronology and Paleoclimate Reconstruction for the East River Watershed, Colorado, USA." Quaternary Research 119 86–98 (2024). https://doi.org/10.1017/qua.2024.5.