Climatic and anthropogenic influences on sediment mixing in the Mississippi source-to-sink system using detrital zircons: Late Pleistocene to recent

U-Pb geochronology of detrital zircons (DZ) is a robust tool used to elucidate linkages between tectonics, climate, and drainage configurations. However, timescales of sedimentary system response to modulation of up-system boundary conditions are rarely investigated using detrital geochronology. Here we present results of mixture modeling using modern and Late Pleistocene DZ samples from each of the Mississippi system segments, and show that high-frequency changes in up-system boundary conditions-anthropogenic sediment impoundment and late Pleistocene ice sheet dynamics-have measurable effects on detrital compositions. Results of DZ mixing models show a high positive correlation to measured suspended sediment loads for each major tributary (ca. 1970s-2000s). Differences between model results and historical records are explained by recent anthropogenic sediment impoundment. Results of DZ mixing models using late Wisconsin deep-sea samples indicate major increases from the Missouri and Upper Mississippi rivers. Boundary conditions responsible for increased sediment loads from these catchments include ice stream activity, increased transport capacity during deglacial melt-water floods, and an increased gradient of a glacial lower Mississippi Valley. These results suggest sediment mixtures in large rivers respond to icehouse climate change at timescales of 103-4 yrs, in contrast to calculated equilibrium response times of ca. 250-25 ka for the Mississippi system, and indications that anthropogenic river modifications alter relative sediment loads instantaneously. Adjustments in detrital mixtures occur at timescales an order-of-magnitude less than Milankovitch-timescale climate change, indicating rapid environmental signal propagation and preservation within transcontinental source-to-sink systems influenced by continental ice sheets.