MIDCONTINENTAL LAKE-BASED PALEOCLIMATE RECORDS OF LATE HOLOCENE HYDROCLIMATE VARIABILITY AND NATIVE AMERICAN POPULATION DYNAMICS

Late Holocene paleoclimate records from the midcontinental United States (US) are largely derived from sparsely distributed high-resolution records (mainly tree rings) and more abundant, but typically lower resolution lake and bog records (largely pollen). Pollen-derived temperature estimates indicate warmer conditions in the Midwest during the Medieval Climate Anomaly (MCA; 950-1250 CE), cooling during the Little Ice Age (LIA; 1350-1850 CE), and warming to the present. Hydroclimate variability during these events, however, is not well defined. Here, new and recently published lake sediment records are presented that detail late Holocene hydroclimate variability in the midcontinental US. These records are based on oxygen and carbon isotope analysis of authigenic and biogenic calcite as well as physical sedimentology, which is used to reconstruct landscape erosion related to warm-season rainstorm events. Proxy data from lakes located adjacent to Native American archaeological sites additionally reflect land-use and settlement histories that, when synthesized with existing archaeological data and the new climatic reconstructions, shed light on relationships between precipitation, abrupt climate events, and pre-Columbian (before 1492 CE) Native American population dynamics. We find that 1) northern hemisphere ocean-atmosphere dynamics that are expressed through variability in the Pacific North American (PNA) mode have been a fundamental driver of continental US hydroclimate during the last 2000 years and 2) that transitions between PNA mean states, which can persist for decades to centuries, can occur rapidly. Past PNA shifts during the MCA and LIA resulted in major reorganizations in the source, seasonality, and amount of precipitation. During the MCA, negative PNA conditions increased the delivery of warm-season moisture from the Gulf of Mexico and promoted longer, warmer and wetter summers. A transition to positive PNA conditions during the LIA, in contrast, promoted cooler and drier conditions with an increase in cold-season precipitation from Pacific and Arctic sources. Native American cultural changes in the central Mississippi and lower Ohio River watersheds are linked to these hydroclimate trends, with populations flourishing during the MCA, but later declining during the LIA.