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 the human colonization of 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 Be-10 dating along the coastlines 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 to reconstruct past sea levels, shorelines, and ecology to assess the viability of a coastal migration for early humans. Our first manuscript on this topic was published in Science Advances in May 2018.

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 triple-isotope cosmogenic dating to unravel complex bedrock exposure histories in formerly glaciated valleys in the eastern Teton Range.

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

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, 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. Our results show that ice sheets in Greenland and Alaska were more sensitive than previously thought to short-term changes in atmospheric and sea surface temperatures. We published one manuscript on this topic in January 2018, and another manuscript is currently under review at Quaternary Research.

Fjord Stade moraine, western Greenland

Photo by Jason Briner

Reconstructing Greenland's Last Deglaciation

Projections of 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’re synthesizing information from cosmogenic Be-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. A paper on this topic was recently accepted for publication in Geophysical Research Letters, so stay tuned!

Coring Baby Loon Lake, western Greenland, 2016. I use lake sediments from proglacial lakes such as this to reconstruct the history of the Greenland Ice Sheet when it was smaller than it is today.
Coring Baby Loon Lake

© 2021 by Alia Lesnek

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