UNRAVELING EARLY HOLOCENE GLACIOFLUVIAL ACTIVITY IN THE UPPER MISSISSIPPI VALLEY, USA: INSIGHTS FROM SLACKWATER DEPOSITS IN THE WHITEWATER RIVER REGION

Glacial meltwater floods raise river levels downstream, in turn elevating base level for their tributaries, whose lower valleys collect slackwater sediments in lacustrine-like depositional environments. These slackwater deposits—comprising fine sands, silts, and clays transported in floodwaters—record chemical and lithological signatures of their glacial provenance. Here, we decipher the late-stage deglacial history in the upper Mississippi Valley, USA, by analyzing such slackwater sediments. We examine four cores from a terrace 20 m above the Whitewater River, a tributary to the once-proglacial Mississippi River in southeastern Minnesota. The cores contain ~4.88 m of interbedded cm- to mm-thick red, gray, and brown silty clay and sand beds, distinct from the sandy fluvial terrace deposits below. Single-grain Optically Stimulated Luminescence dates indicate the laminated slackwater sediments were deposited at 11.67±1.51 ka (1.16–1.64 m depth) and 11.56±1.44 ka (2.36–2.59 m depth). These dates overlap with the ~11.6–10.6 ka Marquette Advance of the Laurentide Ice Sheet, in which glacial ice reoccupied the Lake Superior basin. This readvance may have driven the dated reoccupations of the southern outlets of both Glacial Lake Agassiz and Glacial Lake Duluth in the modern Lake Superior basin. Deposit colors suggest alternating Agassiz and Superior sources. To test for possible western (Agassiz) and eastern (Superior) meltwater sources, we turn to both classic lithologic descriptions and X-ray Fluorescence (XRF), with the latter providing elemental compositions. Previously studied slackwater deposits from the Superior Lobe were rich in Fe, Cu, Zr, Ti, Mn, Ni, Cr, and Co, whereas Des Moines lobe (Agassiz Basin) deposits were rich in Se, Ca, Ar, Cd, Mo, and Zn. Preliminary analysis of the XRF results confirms that the slackwater deposits are distinct from the locally derived sandy terrace materials in the bottom ∼10cm of the core. Going forward, we will fingerprint these slackwater sediments to source regions throughout the analyzed core. The results from this study will contribute valuable insights into the final meltwater pulses to enter the Mississippi River, including their sources and constraints on paleodischarge, and expand our knowledge of early-Holocene ice-sheet evolution in North America.