Isotopic Expression of Millennial‐Scale Climate Events in the Central United States During the Last Glacial Period

Abstract Millennial‐scale climate variability punctuated the last glacial period around the globe, but its expression in the North American midcontinent is not well understood. This is due, in part, to the fact that only a small number of published speleothem time series document these events from this region, and that they surprisingly exhibit isotopically higher oxygen isotope values during North Atlantic cold periods (Greenland stadials (GS) and Heinrich stadials) than during time‐adjacent interstadials, opposite the modern positive correlation with air temperature. Here we present a composite stalagmite record from Cosmic Cavern (CC), central United States, that preserves evidence of environmental change from 54‐14 ka. Stalagmite stable carbon (δ 13 C) and oxygen (δ 18 O) isotope variations are coincident with millennial‐scale events in the North Greenland Ice Core Project (NGRIP) δ 18 O record. During North Atlantic warm periods, stalagmite δ 13 C values decreased by 2–3‰, interpreted here as predominantly reflecting elevated soil respiration rates driven by increased temperature. Stalagmite δ 18 O values also vary broadly in concert with the NGRIP record, and exhibit the previously identified inverse correlation to air temperature, shifting by 0.5–1.5‰ between GS and interstadials. The consistency of this relationship across the central United States suggests that it reflects the δ 18 O of precipitation rather than site‐specific secondary effects. Interstadial precipitation δ 18 O values were lower due to regional enhancement of mesoscale convective storm activity and/or enhanced glacial meltwater flux to the Gulf of Mexico, the region's primary moisture source. Changes in precipitation seasonality, identified through analysis of climate model simulations, may have also played a role.