The δ13C of soil carbonate in rhizoconcretions collected from a loess-paleosol sequence in the central United States indicates that growing-season C3/C4 plant ratio oscillated by 35% on a 900 ± 200 yr time scale during the late Wisconsinan glaciation. The pattern appears in phase with advance and retreat of the southern margin of the Laurentide ice sheet, suggesting influence by paleo–El Niño–Southern Oscillation cycles. The δ13C of soil organic matter indicates that the annual average C3/C4 plant ratio oscillated only by 18%, with a periodicity of 450 ± 100 yr, and closely matched the cyclic pattern of loess-paleosol layers. It suggests a periodic enhancement of the penetration of the Gulf of Mexico air over the region during this time.

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