Radiocarbon-dated pollen sites from the Allegheny Plateau (east) and the Till Plains (west) document significant vegetational and climatic change between 15,000 and 9000 14C BP south of the Great Lakes. Although limited by the problems of no-analog vegetation, pollen transfer functions (TF) and response surfaces (RS) quantify the climatic changes, including a cold oscillation chronologically equivalent to the European Younger Dryas. Phase I: before ca. 13,000 14C BP, conditions across the region were similar, (January -20 to -16°C; July 12°C (RS) or 19°C (TF); mean annual precipitation, 80 to 90 cm year-1). Phase IIa: between ca. 13,000 and 11,000 14C BP, N-S and E-W climate gradients were initiated. Till Plains temperatures increased by ca. 8°C in January and by 8°C (RS) or 2-3°C (TF) in July. Plateau increases for January were 3-6°C and for July were 8°C (RS) or 1-2°C (TF). Precipitation generally increased somewhat. Phase IIb: ca. 11,000 to 10,000 14C BP, gradients became less pronounced. The western sites show a climate reversal (January drop of ca. -5 to -2°C; July drop to ca. 1-2°C), becoming more like the Plateau sites which show a brief, small fluctuation. This is followed by an abrupt 10 to 20 cm year-1 reduction in precipitation, January warming of ca. 3°C and continued cooler Julys. Phase III: After 10,000 14C BP, gradients similar to those of today developed. The changes in Phases IIb and III are probably linked to the drainage history of the Great Lakes and via that history to the changes on the European continent.
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