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North-Central Section - 54th Annual Meeting - 2020

Paper No. 31-11
Presentation Time: 8:30 AM-5:30 PM

GEOCHEMICAL AND ISOTOPIC TOOLS FOR PLEISTOCENE SEDIMENT PROVENANCE IN THE NORTH AMERICAN MIDCONTINENT


WITTKOP, Chad, Department of Chemistry and Geology, Minnesota State University, Mankato, MN 56001, BARTLEY, Julie K., Geology Department, Gustavus Adolphus College, 800 W. College Ave, St. Peter, MN 56082, KRUEGER, Russell, Geological Engineering, Univ. of Wisconsin-Madison, Madison, WI, BOUVIER, Audrey, Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, 95440, Germany, GEORG, R. Bastian, Agilent Technologies, Mississauga, ON L5N 8B3, Canada, KNAEBLE, Alan R., Minnesota Geological Survey, University of Minnesota, 2609 W Territorial Road, Saint Paul, MN 55114, PIPER, Christian, Department of Chemistry and Geology, Minnesota State University, Ford Hall 241, Mankato, MN 56001, ST. CLAIR, Katherine, Department of Mathematics and Statistics, Carleton College, Northfield, MN 55057 and BRECKENRIDGE, Andy J., Department of Natural Sciences, University of Wisconsin - Superior, Belknap and Catlin, P.O. Box 2000, Superior, WI 54880

Thick sequences of pre-LGM Pleistocene glacial tills are preserved throughout the North American midcontinent, but their discontinuous nature and lack of geomorphic expression makes correlating these sediments challenging. A better understanding of the provenance of these sediments can help inform our view of the relationships between glaciation and source rock weathering, which in turn has important implications for our understanding of carbon cycling and climate change. To address these challenges, we examined the geochemical relationships among Pleistocene tills representing four sediment provenances collected from across Minnesota, USA. The sources for these tills range from crystalline rocks of the Canadian Shield, to Phanerozoic sediments of the high plains. Changes in till provenance explain the primary variations in major element geochemistry, trace element geochemistry, and radiogenic isotopic compositions (Sr, Nd, and Hf) in our samples. Sample Na2O+K2O versus Fe2O3 or CaO tracks with changes between northeast- versus northwest-sourced tills, and also follows source mixing on a 100-km glacial transport scale. Trace element proxies including Zr/Sc, Cr+Ni, Rb+Sr, and V+Zn also reflect changes in the relative proportion of crystalline versus sedimentary rock till sources. The Hf- and Nd-isotopic composition of till matrix sediments reliably discriminates between tills derived from erosion of crystalline rocks versus tills derived from erosion of sedimentary rocks. The Hf-isotopic composition of glacial sediments also responds to sediment reworking in lacustrine environments where heavy minerals are preferentially removed. The Sr-isotopic composition of tills appears to record a signal influenced by combination of provenance, source mineralogy, and weathering. Together, these geochemical and isotopic proxies provide a rigorous framework for assessing sediment provenance in buried tills. The additional capability of these tools to identify artifacts of sediment transport, source mixing, and weathering influences presents an opportunity to significantly enrich our understanding of the record of Pleistocene ice sheet dynamics archived in the buried tills of the North American midcontinent.

Session No. 31--Booth# 40

T11. Glaciation and Deglaciation along the Southern Margin of the Laurentide Ice Sheet: Bringing Multiple Tools to Bear (Posters)
Tuesday, 19 May 2020: 8:30 AM-5:30 PM
Lake Superior Ballroom KJ (Duluth Entertainment Convention Center)
Geological Society of America Abstracts with Programs. Vol. 52, No. 5
doi: 10.1130/abs/2020NC-348037
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