RESEARCH

Deglaciation of the Pacific coastal corridor

How did the first Americans migrate to the New World? The classical view states that early humans took an overland journey from Asia to the Americas, traversing an “ice-free corridor” between the Laurentide and Cordilleran Ice Sheets. However, recent studies suggest that human migration to the Americas occurred before the ice-free corridor existed, and attention has shifted to the Pacific coast as a possible entry route. We’re using cosmogenic beryllium-10 and chlorine-36 dating along the coast of Alaska to date the latest retreat of the Cordilleran Ice Sheet and determine when the “Pacific coastal corridor” opened. We’re also working with colleagues at the Tongass National Forest, US National Park Service, and US Geological Survey to reconstruct past sea levels, shorelines, and ecology to assess the viability of a coastal migration for early humans.

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Perched boulder and rôche moutonée on Suemez Island

Photo by Jason Briner

Glacial and tectonic history of the Teton Range

Wyoming's Teton Range is one of the most dramatic landscapes in the western United States. Bounded on its eastern edge by the ~70-km-long Teton fault, this mountain range hosted extensive alpine glaciers during the Last Glacial Maximum. Deposits left behind by these glaciers, including moraines and valley-fill sediments, as well as other features such as alluvial fans, have experienced tens of meters of vertical offset due to Teton fault motion. One of our project goals is to precisely date these features using cosmogenic nuclides, and use this information to constrain integrated Teton fault offset rates since the late Pleistocene. We are also exploring using remote sensing and triple-isotope cosmogenic dating to unravel complex bedrock exposure and erosion histories in formerly glaciated valleys in the eastern Teton Range.

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Boulder on the surface of an alluvial fan that has been offset by Pleistocene movement along the Teton Fault

Improving chlorine-36 dating techniques

Cosmogenic chlorine-36 dating techniques are widely used to determine surface exposure ages and erosion rates over the Quaternary period. In collaboration with scientists at the Center for AMS at Lawrence Livermore National Laboratory, we are updating the laboratory and analytical methods used to measure chlorine-36 concentrations in geologic materials. These new methods offer several advantages: (1) they require less isotopically-enriched chlorine spike solution, (2) AMS targets can be prepared with optimal total chlorine loads, and (3) AMS memory effects are substantially reduced, which improves measurement accuracy and precision.

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Dissolving whole-rock silicate samples for chlorine-36 analysis in the UNH Cosmogenic Isotope Clean Lab

Ice sheet response to abrupt climate change

Over the past 20,000 years, Earth’s climate has undergone a major transition from the cold Last Glacial Maximum to the warmer Holocene epoch. Superimposed on this longer-term trend are dramatic oscillations between warm and cold climate states. Some of these shifts, such as the 9.3 and 8.2 ka cooling events, likely occurred over fewer than 150 years. By investigating ice sheet response to these episodes of rapid climate change, we can gain insight into how ice sheets behave on human-relevant timescales. 

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Fjord Stade moraine, western Greenland

Photo by Jason Briner

Reconstructing Greenland's Last Deglaciation

Projections of future Greenland Ice Sheet mass loss, and by extension global sea level rise, are made by ice sheet models. These simulations benefit from recent advances in computing power and increased understanding of the physical principles of ice sheet mechanics. Yet assessing model performance can pose a challenge due to the short observational record of ice sheet change, which is largely limited to the satellite era. As part of the NSF-funded Snow on Ice project, we synthesized information from cosmogenic beryllium-10 dating of boulders, radiocarbon dating of lake sediments, and the newly available ArcticDEM to generate a detailed chronology of Greenland Ice Sheet extent over the past 12,000 years.