A perched erratic boulder tells a story of glacial thickening and thinning along the Lonewolf Nunataks, Upper Byrd Glacier - the ice surface was previously about 250m higher. Photo: Jamey Stutz
Rock Clocks: Using cosmogenic nuclides to reconstruct past ice sheet change
Researchers are about to return to Antarctica in search of rocks. Not just any rocks – rocks on mountain tops that have been dropped off by thinning ice since the last ice age.
Knowledge of when these rocks dropped off will improve our understanding of the pattern and rate of ice sheet retreat since the last glacial maximum, approximately 20,000 years ago. The rates and patterns of past ice sheet behaviour are used to improve models and projections for future change. This information is critical for preparing for the impacts of melting ice in a warming world, such as sea level rise.
The Lonewolf Nunataks stick up through the upper Byrd Glacier and serve as a glacial debris dumping ground. When analysed for cosmogenic nuclides, these samples will help scientists track past ice sheet thinning. Photo: Jamey Stutz
Using cosmogenic nuclides
The Earth is constantly bombarded by cosmic radiation in the form of high-energy particles sourced from our galaxy. Some of these particles penetrate the atmosphere and collide with the Earth’s surface. These collisions break apart common elements that make up the Earth’s crust (like oxygen and silica) and produce rare new ones, known as cosmogenic nuclides.
Accumulation of cosmogenic nuclides over time provides a useful tool for measuring how long rocks have sat at the Earth’s surface. We call this technique 'surface exposure dating'. In this project, we are applying this technique to rocks deposited alongside fast-moving parts of the Antarctic Ice Sheet, known as outlet glaciers. The team are working to determine the surface exposure ages down the sides of mountains and across multiple sites in a region. This information builds a picture of how ice sheets have behaved in the past.
Shaun Eaves and Jamey Stutz collecting rock samples for cosmogenic dating. Lonewolf Nunataks, Upper Byrd Glacier. Photo: Jamey Stutz
Dating rocks from the Ross Sea region
The plan is to extend the observational record of ice sheets by targeting strategic locations around the margins of the Ross Ice Shelf and Marie Byrd Land. In these places, glacial sediments deposited next to dynamic ice margins record the transient evolution of the ice surface elevation.
In the 2019/20 field season, the team visited the Byrd Glacier, Skelton Glacier and Mulock Glacier, in the southwestern part of the Ross Ice Shelf.
Location map identifying field locations for ASP Ice Dynamics Project objectives. Cosmogenic sampling is focused on the Byrd Glacier (far right) and the Rockefeller Mountains (bottom left).
During the COVID-19 pandemic, when field access was limited, the focus was on dating archived samples from the Rockefeller Mountains, collected during previous Antarctic expeditions and curated at the GNS Science National Petrology Reference Collection.
A recently published study presented some findings from the upper Byrd Glacier. The authors found that approximately 8,000 years ago the Byrd Glacier thinned rapidly, likely as the Ross Ice Shelf cavity formed.
This work provided useful constraints, but ice sheet likely overtopped these mountains, meaning only a small window of thinning was captured. This coming summer, the team are heading back into the field to Mt Tuatara at the Byrd Glacier in order to establish a complete chronology of glacial thinning from the ice age maximum to the modern-day position. Mt. Tuatara towers nearly 2 kilometers above the modern Byrd Glacier, allowing scientists to capture the entire range of glacial thinning since the last ice age.
This research is part of the Antarctic Science Platform’s Antarctic Ice Dynamics Project. It contributes to knowledge of Antarctica’s past dynamics by providing quantitative reconstructions of the past Antarctic Ice Sheet.