Determining the impact of the Younger Dryas (YD) climate event on the unglaciated North American midcontinent has proved diffi cult due to a scarcity of suitable paleoclimate proxies. Here we present a well-dated carbon isotope (δ 13 C) record from a buried soil sequence in southwestern Missouri, which reveals a large isotopic excursion during the YD chronozone. In this region of the modern prairie-forest border, the δ 13 C signature of soil organic matter is a reliable indicator of past climatic change because δ 13 C values are controlled primarily by the relative abundance of C 3 and C 4 plants, which is tied to the environmental setting. Between ca. 13,200 and 11,900 yr ago, the abundance of C 4 grasses increased by upwards of 50% of the total biomass, indicating expansion of grassland most likely driven by increased aridity during this period. Environmental gradients in the midcontinent must have been very steep, because at the same time that a C 4 -rich prairie existed in southwestern Missouri, spruce forests grew in Iowa, Illinois, and Ohio.on June 5, 2015 geology.gsapubs.org Downloaded from
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