Meltwater, mud, and the Mississippi: Upper Mississippi River Valley slackwater sediments reveal shifting deglacial meltwater sources associated with the Marquette Readvance of the Laurentide Ice Sheet

Abstract. Meltwater from the Laurentide Ice Sheet changed course frequently over the last deglaciation, with impacts on local landscapes and global climate. However, a scale gap has confounded efforts to trace the sources and pathways of this meltwater: well-dated paleoceanographic records lack the precision of regional geological and geomorphic evidence, but local features are often dispersed and difficult to precisely date, making it challenging to construct a spatially distributed source-to-sink meltwater timeline. Here, we bisect this scale gap by using the upper Mississippi River network as a local integrator of upstream meltwater routing. We analyze a ~400 cm core retrieved from a fluvial terrace near the mouth of the Whitewater River, a tributary to the Upper Mississippi River in southeastern Minnesota, where high water from meltwater floods deposited slackwater sediments. Optically stimulated luminescence (OSL) dates of 11.67±1.51 ka and 11.56±1.44 ka indicate that these slackwater sediments likely coincide with the Marquette Readvance of the Superior Lobe, a period when Laurentide Ice Sheet ice briefly reoccupied the Superior Basin, rerouting deglacial meltwater to the south. We geochemically fingerprint these sediments by applying Principal Component Analysis (PCA) and k-means clustering to X-Ray Fluorescence data, and find that Marquette-age meltwater flowed to the Mississippi first from the Glacial Lake Agassiz basin (rich in Ca) via the modern Minnesota River valley, and then from the Superior basin (rich in Fe, Cu, and Ni) via the glacial-stage St. Croix River. Weathering and post-depositional alteration obscure geochemical indicators of provenance in the top 200 cm of the core, which records the remainder of Marquette-age flood inundation. This study brings the pattern of ice-sheet retreat and associated meltwater routing into sharper focus and highlights the power of alluvial stratigraphy as an intermediary between local glacial geology and the marine sedimentary record.