An updated radiocarbon-based ice margin chronology for the last deglaciation of the North American Ice Sheet Complex

The North American Ice Sheet Complex (NAISC; consisting of the Laurentide, Cordilleran and Innuitian ice sheets) was the largest ice mass to repeatedly grow and decay in the Northern Hemisphere during the Quaternary. Understanding its pattern of retreat following the Last Glacial Maximum is critical for studying many facets of the Late Quaternary, including ice sheet behaviour, the evolution of Holocene landscapes, sea level, atmospheric circulation, and the peopling of the Americas. Currently, the most up-to-date and authoritative margin chronology for the entire ice sheet complex is featured in two publications (Geological Survey of Canada Open File 1574 [Dyke et al., 2003]; ‘Quaternary Glaciations – Extent and Chronology, Part II’ [Dyke, 2004]). These often-cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka (all ages in uncalibrated radiocarbon years) using a combination of geomorphology, stratigraphy and radiocarbon dating. However, by virtue of being over 15 years old, the ice margin chronology requires updating to reflect new work and important revisions. This paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies. We also update the original radiocarbon dataset from the 2003/2004 papers with 1541 new ages to reflect work up to and including 2018. A major revision is made to the 18 ka ice margin, where Banks and Eglinton islands (once considered to be glacial refugia) are now shown to be fully glaciated. Our updated 18 ka ice sheet increased in areal extent from 17.81 to 18.37 million km2, which is an increase of 3.1% in spatial coverage of the NAISC at that time. Elsewhere, we also summarize, region-by-region, significant changes to the deglaciation sequence. This paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of Dyke et al. (2003) and Dyke (2004) where new information is lacking; this is a pragmatic solution to satisfy the needs of a Quaternary research community that requires up-to-date knowledge of the pattern of ice margin recession of what was once the world’s largest ice mass. The 36 updated isochrones are available in PDF and shapefile format, together with a spreadsheet of the expanded radiocarbon dataset (n = 5195 ages) and estimates of uncertainty for each interval

RI-68 Quaternary Lithostratigraphic Units of Minnesota

Much of Minnesota is covered by sediment of Quaternary age that was deposited during numerous glaciations by ice, wind, and water. In this report, we follow guidelines of the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2005) to create a framework for establishing formal lithostratigraphic units in Minnesota. We evaluate over 100 lithostratigraphic units that have been identified in Minnesota. Eighty (80) units are considered to be useful lithostratigraphic units of formation and member rank, and these are formally accepted in this report or are recommended to be so in future publications. These 80 units include previously named formal lithostratigraphic units that are recognized and accepted as originally defined, but also formally defined units that we have revised or redefined to better fit into our stratigraphic framework. The remaining lithostratigraphic units have been used informally in earlier reports or are newly named in this report. Additional units that are no longer considered necessary as lithostratigraphic units are abandoned in this report. These units include previously used units of both formal and informal status. Of the 80 lithostratigraphic units recommended to be retained, 47 are formally defined, revised, or redefined in this report. The remaining 33 units are recommended to be formally named in a future Minnesota Geological Survey Report of Investigations.Johnson, Mark D.; Adams, Roberta S.; Gowan, Angela S.; Harris, Kenneth L.; Hobbs, Howard C.; Jennings, Carrie E.; Knaeble, Alan R.; Lusardi, Barbara A.; Meyer, Gary N.. (2016). RI-68 Quaternary Lithostratigraphic Units of Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/177675