Abstract of project: This page describes an NSF-funded project directed at learning about the many advances and retreats of the Laurentide Ice Sheet into North America over the past 2-3 million years by studying the stratigraphic record of these glaciations in the north-central U.S. The links at right lead to more information about the project, the people involved, and the results of the work -- for a summary of the purpose and importance of the project, please look at the original proposal. For a summary of the important results so far, please look at the linked publications. The panels below provide access to all of the analytical data and supporting information generated during the project. For
The regular advance and retreat of continental ice sheets is the defining feature of the last several million years of Earth history, but, paradoxically, most of what we know about these ice sheets comes from indirect evidence preserved in marine sediments. The terrestrial deposits which hold the direct record of which ice sheets were where at what time are very difficult to date and correlate. It's very hard to answer even some very basic questions about the Plio-Pleistocene ice ages -- for example, when did the first Northern Hemisphere ice sheets form, and how big were they? During the mid-Pleistocene transition 900,000 years ago, when marine paleoclimate records show a large increase in continental ice volume, did existing ice sheets get bigger or did new ice sheets form? We can't answer these questions unless we can directly date and correlate the terrestrial records of Plio-Pleistocene ice sheet advances.
Thus, the main point of this project is to develop new means of dating glacial deposits that are too old for existing methods such as radiocarbon or optical dating. We've done this by exploiting the idea of 'burial dating' using pairs of cosmic-ray-produced radionuclides that are produced at the surface at a fixed ratio, but have different half-lives. If a target mineral is exposed at the surface for a time, then buried and removed from the cosmic-ray flux, the divergence of the measured ratio of two such radionuclides from the production ratio reflects the burial age of the sample. As the glacial stratigraphy of the north-central U.S. reflects repeated alteration between land surface exposure during interglaciations and burial of these surfaces by later glacial deposits, we can use this idea to date these stratigraphic sequences.
Besides this main goal of dating glacial deposits that couldn't be dated before, a secondary goal of this project is to explore the idea of using beryllium-10, a cosmic-ray-produced radionuclide that is produced in the atmosphere and accumulates in soils, to determine the sources of sediment eroded and transported by the Laurentide Ice Sheet during its evolution. This allows us to distinguish tills derived from soils and weathered material that predated continental glaciation from later tills derived from erosion of fresh bedrock, which in turn tells us something about the importance of the subglacial sediment budget to ice sheet dynamics.
Univ. of Washington
Minnesota Geol. Survey
Original proposal to NSF (1.3 mb PDF)
Summary paper on dating pre-late-Pleistocene tills (American Journal of Science, 2005 -- 6 mb PDF)
More detailed description of dating till-paleosol sequences (EPSL, 2005)
Greg Balco's Ph.D. dissertation (1.5 mb PDF)