THE ENVIRONMENTAL CONTEXT OF CAHOKIA’S EMERGENCE AND DECLINE: NEW PALEOECOLOGICAL RECORDS FROM THE CENTRAL MISSISSIPPI RIVER VALLEY

Cahokia emerged as a major Mississippian population center in the central Mississippi River valley at AD 1050, but the entire region was almost completely abandoned by AD 1350. Environmental changes, namely resource depletion, flooding, and/or drought, have previously been invoked as important factors in Cahokia’s emergence and decline, although few paleoenvironmental records exist from this region to test these hypotheses. Here, we present multi-proxy paleoenvironmental records (pollen, charcoal, δ¹³C_org, particle-size) from three oxbow lakes along the central Mississippi River valley that provide an ecological and hydro-climatic history for Cahokia and its surrounding region.

At Horseshoe Lake in Madison Co., IL (HORM12), situated adjacent to Cahokia, pollen assemblages track the removal of floodplain and upland trees and the expansion of indigenous seed crop production beginning at AD 450, during the Late Woodland period, followed by the gradual intensification of maize agriculture through the Mississippian period to the abandonment of Cahokia at AD 1350. At Horseshoe Lake in Alexander Co., IL (HORX13), situated 200 km downstream, the abrupt replacement of trees with stands of indigenous seed crops also occurs during the Woodland period, although there is little agricultural activity during the ensuing Mississippian period. Together, these records demonstrate that humanized landscapes emerged in this region centuries before the emergence of Cahokia.

At Horseshoe Lake (HORM12) and Grassy Lake (GRAS13), particle-size analyses reveal pronounced shifts in grain-size distributions that result from sediment deposition during high-magnitude floods of the Mississippi River. The absence of large floods from AD 600–1200 corresponds to agricultural intensification, population growth, and settlement expansion across the floodplain associated with the emergence of Cahokia as a regional center. The return of large floods after AD 1200 marks the onset of depopulation and sociopolitical upheaval that culminate in the abandonment of Cahokia. Our data show that shifts in the frequency of high-magnitude floods are linked to Cahokia’s emergence and decline, and that hydroclimatic variability played an important role in the late Prehistoric abandonment of the central Mississippi River valley.