Paleoenvironments of the Pacific coastal corridor
How did the first humans 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 and the State of Alaska to reconstruct past sea levels, shorelines, and ecology to assess how humans living on the Pacific coast responded to rapid Earth surface change. These projects involve cosmogenic dating, lake sediment coring, and GIS.
Perched boulder and rôche moutonée on Suemez Island
Photo by Jason Briner
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. Past projects have explored the last deglaciation of the Greenland Ice Sheet, and we are currently investigating the processes and timing of Cordilleran and Laurentide Ice Sheet response to abrupt climate change at the end of the Pleistocene.
Fjord Stade moraine, western Greenland
Photo by Jason Briner
Glacial and tectonic history of the western USA
During the Last Glacial Maximum, mountain ranges in the western USA hosted extensive alpine glaciers. In some cases, 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 offset due to fault motion. One of our goals is to precisely date these features using cosmogenic nuclides and constrain open-ended fault offset rates since the late Pleistocene. We are also assessing the roles of climatic and non-climatic factors as drivers of North American alpine glacier retreat since the Last Glacial Maximum, as this will help to better predict the future behavior of other glacier systems worldwide.
Boulder on the surface of an alluvial fan that has been offset by Pleistocene movement along the Teton Fault
Improving cosmogenic nuclide dating techniques
Cosmogenic nuclide 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 efficiency. We are also using landslides in the western USA to constrain cosmogenic nuclide production rates in high-elevation settings.
Dissolving whole-rock silicate samples for chlorine-36 analysis in the UNH Cosmogenic Isotope Clean Lab