PRE-COLUMBIAN POPULATION DYNAMICS AT CAHOKIA, IL (USA) AND THE ROLE OF LATE HOLOCENE FLOODING

Cahokia, located near present day East St. Louis, Illinois, emerged as the largest pre-Columbian urban center in the United States circa 1050 CE. During its occupation, Mississippian Native Americans adopted and relied heavily on maize agriculture. Following a steady population decline in the region, Cahokia and much of the central Mississippi Valley were abandoned by 1350 CE. While unambiguous regional flood records are lacking from this region, climate has generally been accepted as a driver of this societal reorganization. To investigate the role of flooding on Cahokia’s depopulation, we obtained a complete 2800-yr sediment record from three core sites at Horseshoe Lake, an oxbow lake three kilometers northwest of Cahokia. Sediment core age-depth relationships were geochronologically constrained with radiocarbon dating, and a multi-proxy suite of sediment analyses, such as X-Ray Fluorescence (XRF), grain size analysis, and nitrogen isotopes, were used to characterize flooding frequency and anthropogenic land-use. We compare these results with regional paleoclimate data to place flooding and land-use changes at Horseshoe Lake within the context of temperature and precipitation variability. Using evaporation (Ca/Zr) and weathering (Rb/Sr) indicators as evidence for effective moisture and clastic sediment input, the Horseshoe cores present distinct flooding regimes consistent with regional climatological changes during the Medieval Climate Anomaly (MCA; 950–1250 CE) and Little Ice Age (LIA; 1250–1800 CE). As populations grew at Cahokia during the late MCA, the Horseshoe Lake record suggests low flood frequency. However, anomalously high δ¹⁵N values ca. 1200 CE, which reflect peak Mississippian occupation near Horseshoe Lake, are coincident with a reduction in evaporation and increase in clastic sediment input characteristic of frequent flood recurrence. In the century following, a return to baseline δ¹⁵N values ca. 1300 CE places population decline at Cahokia within a period of sustained increased flooding during the LIA. Regional paleoclimate data and stream order dynamics suggest that increased flooding around the time of abandonment was driven by increased snowpack and was likely accompanied by an increase in summer aridity.