Evidence of Younger Dryas aridity in dune-paleosol successions in the Midwest of U.S.A.

Two dune-paleosol successions were found in the Illinois River Valley in U.S.A. The most significant observation is that a paleosol/gyttja complex separates two late-glacial dune sequences at both sites. The optically stimulated luminescence (OSL) dates indicate that upper dune sand is the Younger Dryas (YD) interval and 14C dates indicate that the gyttja and underlying paleosol is a Bølling/Allerød (B/A) equivalent unit. The paleosol reflects a warmer and more humid climatic condition and the gyttja accumulated in either a swamp or nearshore position of a lake, which fundamentally differs from dry climate associated late-glacial dune formation. Detailed 14C and OSL dating analyses were applied to both successions and reflectance (L*a*b*), magnetic susceptibility, total carbonate content, trace and rare earth element analyses were applied over 5-cm increments at one site. These results confirm that dry YD and wet B/A climates prevailed in the Illinois River Valley. We propose that during the Northern Hemisphere (NH) warming, the jet streams shifted northward, allowing warm/moist airmasses from the Gulf of Mexico persisted in the Midwest, which increased summer precipitation supporting a healthy habitat and stabilizing the landscape. During the NH cooling, the jet stream shifted southward forcing the dry airmasses from the Arctic and Northern Pacific into the Midwest, which ultimately reduced summer precipitation, destabilized the landscape, and caused the YD dune field to form.