A record of sustained prehistoric and historic land use from the Cahokia region, Illinois, USA

Abstract

In eastern North America, large prehistoric settlements were concentrated in and along the fl oodplains of the midcontinent, but few sedimentary records have been examined adjacent to these sites to evaluate the impacts of Native American land use on terrestrial ecosystems. Here we report a high-resolution and multiproxy paleoecological record from Horseshoe Lake, an oxbow lake in the central Mississippi River valley that is adjacent to the Cahokia site (Illinois, USA), the largest prehistoric settlement north of Mexico. Palynological and carbon isotope data document pronounced vegetation changes over the past 1700 yr driven primarily by land use, including 900 yr (450-1350 CE) of sustained prehistoric human impacts. Rapid forest clearance was followed closely by the proliferation of indigenous seed crops of the Eastern Agricultural Complex beginning ca. 450 CE, centuries before the emergence of Cahokia at 1050 CE. Agricultural intensifi cation that included the use of maize (Zea mays subsp. mays) followed this initial clearance, with peak land use intensity between 900 and 1200 CE. A large fl ood event ca. 1200 CE marks the onset of agricultural contraction and Cahokia's decline. Reforestation follows the abandonment of the Cahokia region at ca. 1350 CE. The Horseshoe Lake record thus indicates that regional agricultural activity began abruptly at 450 CE and intensifi ed over the following centuries, well before the formation of Cahokia and other large prehistoric settlements. The evidence that a major fl ood coincided with the onset of Cahokia's decline is noteworthy, but will require corroboration from additional records.

Cahokia’s emergence and decline coincided with shifts of flood frequency on the Mississippi River

“…3). Intensified cultivation of native domesticates in the central Mississippi River valley began during the early Late Woodland period, around A.D. 400-650 (11,12,35), when known settlements were concentrated on higher-elevation alluvial fans and terraces along the edge of the floodplain (16,36). The absence of large floods from A.D. 600 to A.D. 1200 corresponds to the expansion of settlements to lowerelevation floodplain ridges that are separated by swales, sloughs, and old meander scars (14,24,36,37), an increase in settlement numbers (6,36), and continued intensification in the cultivation of native domesticates, and, after ca.…”

“…The absence of large floods from A.D. 600 to A.D. 1200 corresponds to the expansion of settlements to lowerelevation floodplain ridges that are separated by swales, sloughs, and old meander scars (14,24,36,37), an increase in settlement numbers (6,36), and continued intensification in the cultivation of native domesticates, and, after ca. A.D. 900, maize (11,12). At A.D. 1050, toward the end of this multicentennial period of midcontinental aridity and infrequent large floods, Cahokia emerged as a hierarchically organized regional center that drew thousands of people from across the midcontinent (6,38).…”

“…Extensive inundation of the floodplain was unprecedented for the sociopolitical system established at Cahokia, and the return of large floods at ca. A.D. 1200 at the onset of regional depopulation (6), agricultural contraction (12), political decentralization (15), the construction of defensive palisades (42), destruction of outlying population centers (9,43), and decline of monumental construction at Cahokia (9) implicate flooding as a factor in the reorganization of Cahokia's sociopolitical structure that initiated its decline. In contrast to the large Mississippi River floods of the 19th century that fostered resilience and motivated legislation aimed at preventing damage from flooding (17), Cahokia's leaders appear to have been unable to maintain the impression of security and stability following the economic upheaval created by the return of large floods.…”

“…These composite cores measured 5.5 m and 2.2 m for Horseshoe Lake and Grassy Lake, respectively. Only the top 4.4 m from Horseshoe Lake were used in the present study, because the bottom section of core is characterized by interbedded sands and clays that were deposited before the main channel of the Mississippi River migrated to the low-sinuosity meander belt along the western edge of the floodplain (12,24). Sediment cores were sampled at 1-cm intervals and refrigerated in labeled Whirl-pak bags for future subsampling.…”

“…Cahokia's emergence as a regional center can be traced to the population growth and intensified cultivation of native domesticates that began around A.D. 400 in the floodplain of the central Mississippi River near modern-day Saint Louis, MO (6,11,12). By A.D. 1050, Cahokia emerged as a hierarchically organized cultural and political center in this region, which was inhabited by tens of thousands of individuals supported in part by the cultivation of native domesticates and maize (6,13).…”