IDENTIFYING TRACE ELEMENT CHRONOLOGICAL MARKERS WITHIN UPPER MISSISSIPPI RIVER FLOODPLAIN-SEDIMENTS

During the 19^th and 20^th Centuries, efforts to control the upper Mississippi River resulted in significant geomorphological changes to the channel and floodplains, leading to rapid sedimentation in parts of the floodplain. Some areas along the Mississippi River have experienced up to 3 meters of alluvial sediments during post-settlement times. Previous studies suggest that this rapid sedimentation was caused largely by anthropogenic manipulation of the river and its valley (e.g. levee construction, wing dams, and locks and dams). The original goal of this study was to identify anthropogenic elements in the sediment that could separate pre-settlement from post-settlement times, thus providing a chronologic marker. However, high water conditions impeded deep sampling, and these results suggest that samples are limited to post-settlement times. The study site is located on the east bank of the Mississippi River near river mile 425 and downstream of L&D No. 17. Historical data indicate rapid sedimentation rates adjacent to the main channel in this area in the past 100 years. Samples were collected on three separate occasions over a timespan of a year and a half, using soil pits, hand-driven soil cores, and a Giddings drill rig. In total, 28 samples were taken, with a maximum subsurface depth of approximately 2 meters. Bulk sediment samples were analyzed by XRF spectroscopy for Y, Zr, Nb, Mo, Ce, Ba, Pb, Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃.Trace elements Zr, Mo, Fe₂O₃, and MnO exhibited trends in the data that suggest that fluctuations in their concentrations could be linked to flood events. Further testing is required to verify that relationship; however there is an apparent upward trend in Pb concentrations with depth, possibly caused by an influx of detrital Pb during the 19^th C, when upstream regions experienced significant mining and deforestation. These data suggest that Pb could be a viable element to distinguish pre-settlement and post-settlement sediments in this area. Further testing at greater depths are required to corroborate these findings.