This study will improve the retreat chronology of the southern Laurentide Ice Sheet (LIS) in Wisconsin by measuring in-situ cosmogenic nuclides on glacially deposited boulders from terminal and recessional ice marginal deposits. Sample locations will span a range of climate settings and glacial bed conditions. The proposed research will address two hypotheses. (1) Initial ice margin retreat in Wisconsin was synchronous with the rest of the southern LIS. (2) Increasing boreal summer insolation forced LIS deglaciation. Determining the phasing of ice-sheet retreat relative to changes in insolation, atmospheric greenhouse gases, and global climate is important to understanding the mechanisms that give rise to deglaciations. The leading hypothesis for the forcing of deglaciations is that increasing Northern Hemisphere summer insolation initiates ice-sheet retreat and deglaciation. Alternatively, tropical sea surface temperatures and atmospheric carbon dioxide may lead portions of ice-sheet retreat, implicating them as the driver of deglaciation. Key to discerning between these deglacial-forcing hypotheses is a precise Northern Hemisphere ice-sheet margin chronology. Of the Northern Hemisphere ice sheets, the southern LIS descended to the lowest latitude and should have had the highest climate sensitivity. It is thus an ideal location to study the mechanisms driving initial ice retreat. Despite over 40 years of chronologic research, however, the southern LIS chronology remains incomplete. While the LIS retreat chronology is fairly well constrained in Iowa, Illinois, Indiana, Ohio, and New England, a substantial gap in the chronology of initial deglaciation exists in Wisconsin where permafrost conditions at the Last Glacial Maximum preclude the use of traditional radiocarbon dating methods.
This new dataset will identify the timing of LIS retreat from terminal moraines in Wisconsin and fill a gap in the southern LIS deglacial chronology. Dates will be compared with other parts of the southern LIS margin chronology, deglacial climate forcings and climate model simulations to determine the major triggers of Northern Hemisphere deglaciation. Broader impacts include support of the doctoral research and education of Ph.D. candidate David Ullman at the University of Wisconsin-Madison along with the education of undergraduate students in the field and the laboratory, introducing them to the fields of glacial geomorphology, paleoclimatology, geochronology, paleogeography and geochemistry. Results and overall concepts from this research will be presented to the public via the Ice Age Trail Alliance (IATA) in Wisconsin to improve the educational focus of the organization with updated signs, placards and posters along the IATA trails and at various visitor centers. The University of Wisconsin-Madison Geology Museum exhibit on the deglacial history of Wisconsin will also be expanded.
This project investigated the timing of Laurentide Ice Sheet (LIS) margin retreat in Wisconsin, USA. To do this, we sampled boulders left by the LIS along its terminal end moraine that it deposited at its maximum extent. We measured the cosmogenic nuclide 10-Be in these samples to date the timing of ice retreat. This technique determines how long a rock has been exposed to cosmic ray bombardment since it was left by ice. We produced in total 50 dates from multiple sites in Wisconsin (see Figure). We found that ice began retreat at ~23 ka and accelerated retreat at ~19 ka. This onset of retreat is synchronous with the initial rise in incoming summer solar radiation, which we conclude was the forcing of initial retreat. We also found that LIS retreat in Wisconsin was synchronous with retreat elsewhere along its southern margin and that all of this retreat occurred before the initial rise in atmospheric carbon dioxide concentration. The broader impacts of the award were the support of Dr. David Ullman’s PhD. The results are also being incorporated into Wisconsin Geological and Natural History Survey bulletins on the glacial history of Wisconsin to educate the general public. One publication is in review at present in the journal Geology, and were presented at five conferences.
