HYDROCLIMATE VARIABILITY OF THE MISSISSIPPI RIVER BASIN IN THE COMMON ERA: IMPLICATIONS FOR MODERN AND ANCIENT HAZARDS (Invited Presentation)

Floods and droughts in the Mississippi River basin are perennial hazards that cause severe economic disruption today and in ancient times. Here I describe a new lipid biomarker record from Horseshoe Lake (Illinois, USA) to evaluate the climatic conditions associated with hydroclimatic extremes that occurred in this region over the last 1,800 years. Geochemical proxy evidence of temperature from branched glycerol dialkyl glycerol tetraethers (brGDGTs) show pronounced warming during the Medieval era (CE 1000–1,600) that corresponds to an increase in the frequency of midcontinental megadroughts and reduced discharge of the lower Mississippi River. Isotopic composition of leaf waxes (δ²H_wax) in this same record reflect variations in warm-season moisture, and correspond to enhanced flood frequencies on the upper Mississippi and Missouri River basins. These data support hypotheses that invoke enhanced midcontinental warmth, drought, and flooding during the Medieval era in the growth and/or decline of Mississippian polities, including Cahokia – though casual mechanisms supporting these claims remain difficult to test. The available paleohydrologic data are generally consistent with ensemble model simulations (CESM) of the last millennium, providing a valuable means of testing the response of the Mississippi River basin to regional warming.