QUANTIFYING PLEISTOCENE LOESS PROVENANCE IN MIDCONTINENTAL NORTH AMERICA USING MULTIPLE SEDIMENT SOURCES AND A MIXING MODEL: IMPLICATIONS FOR GLACIAL LOBE EVOLUTION ALONG THE SOUTHERN LAURENTIDE ICE SHEET

Pleistocene glaciations have significantly shaped the midcontinental North American landscape and resulted in a proglacial terrestrial sediment record comprising glacial lake deposits, glacio-fluvial sand and gravel, and wind-blown loess. This study expands on previous research by employing detrital zircon (DZ) provenance analysis to explore the origins of midcontinental North American loess in the context of the glacial history of the southern Laurentide Ice Sheet.

We incorporate previously published DZ data from last glacial tills (derived from the Huron-Erie, Lake Michigan, Green Bay, Superior, Des Moines, and James Lobes), as well as middle to late Pleistocene loess from multiple sites along the Illinois and Mississippi River valleys, and suspended sediment load inputs from the modern Missouri and Arkansas Rivers. Using a statistical mixing model (DzMix V. 2.2), we estimate the relative proportions of these glacial and nonglacial sources to Wisconsin Episode, Illinois Episode, and pre-Illinois Episode age loess.

Our findings reveal that mixing models incorporating the modern Missouri and Arkansas Rivers as sources significantly enhance (up to 23% increase in cross-correlation value) the source characterization of Illinois Episode (penultimate glacial) and Wisconsin Episode (last glacial) loess deposits within the Mississippi River drainage basin. These river sources, which approximate Pleistocene nonglacial sources, are dominant inputs for certain loess sites (up to 51%), though their contributions vary over time and space. This variability has implications for understanding differences in sediment source estimates and the evolution of Pleistocene glacial and fluvial sediment transport.

Differences in relative sediment proportion estimates among Wisconsin, Illinois, and pre-Illinois Episode loess support previous evidence of a persistent Quebec-Labrador Ice Dome source through multiple glacial cycles, even with the inclusion of river sources in updated models. Additionally, Illinois and pre-Illinois Episode loess in the southern study area received approximately 20% DZ input from sources akin to the present-day Missouri and Arkansas Rivers, indicating significant sediment contributions to the lower Mississippi River valley region during the middle Pleistocene.