Expression of Millennial Climate Events in the Central United States during the Last Glacial Period

The nature of Late Glacial millennial-scale climate variability in the central United States is not well understood owing to a paucity of high-resolution, precisely-dated records in this region. Here we present a discontinuous, composite stalagmite record from Cosmic Cavern (36.4˚N, 93.5˚W), located on the southwestern margin of the Ozark Plateau in northwestern Arkansas, that preserves evidence of environmental change coincident with Greenland (inter)stadials and Heinrich stadials from 54-14 ka (marine isotope stages 3-2). Among the four stalagmites that comprise this record, carbon isotopes replicate well. Oxygen isotope ratios in three of the four stalagmites also exhibit robust replication, but in the fourth, oxygen isotope ratios are offset from coeval stalagmites by 1.5 permil, and thus this stalagmite was excluded from the composite record. Variability in stalagmite carbon and oxygen isotopes are broadly coincident with, and characterized by the same asymmetry as, the Greenland Ice Sheet Project Two (GISP2) oxygen isotope record; pronounced positive carbon and oxygen isotope anomalies also mark Heinrich stadials. Elevated carbon isotope values likely reflect reductions in soil respiration rates and increased CO2 out-gassing in voids above the cave during colder, and perhaps drier, stadials. Shifts in stalagmite oxygen isotope ratios are interpreted as reflecting variations in air temperature, consistent with modern temperature effects on precipitation oxygen isotope values documented over the course of a multi-year (and still on-going) precipitation monitoring program. We investigate the impacts of precipitation seasonality and moisture source on oxygen isotopic variability using Coupled Model Intercomparison Project phase 5 (CMIP5) simulations, namely the historical, pre-industrial, and Last Glacial Maximum simulations of the MIROC Earth System Model. Particular focus is placed on quantifying changes in moisture source and transport in the model simulations and the roles played by the North Atlantic subtropical high pressure system to the south and east, and precipitation derived from a Pacific moisture source to the west.