LGM ISOSCAPE OF CENTRAL NORTH AMERICA: IMPLICATIONS FOR AN ICE-PROXIMITY CLIMATE GRADIENT
Widespread, abundant, terrestrial paleoclimate proxies dating to the last glacial maximum (LGM) are rare. Land snails are preserved in Quaternary loess and palustrine deposits worldwide. The stable oxygen isotope (δ18O) values of land snail shells provide a record of past climate variables, including precipitation, temperature, and the δ18O value of precipitation. Modern land snail δ18O values throughout North America are largely driven by average annual precipitation δ18O values. Here we investigate land snail δ18O values from deposits dated to the LGM in the upper Midwest USA and Mississippi River Valley. We present 387 measurements of δ18O and δ13C values from 12 gastropod taxa (including Succineidae, Vertigo, Discus, Carychium and Stenotrema). Gastropod shells were collected from 7 sites distributed along a north-south transect, following the Mississippi River from northern Illinois to the Gulf of Mexico, and from 5 sites along a west-east transect from Iowa to Indiana. The δ18O and δ13C values range from 2.9‰ to -3.8‰ and from -0.5‰ to -10.9‰, respectively. We hypothesize that spatial variability in shell δ18O values is influenced by the presence of the Laurentide Ice Sheet due to its effect on temperature and atmospheric circulation. We observe a gradient in mean Succineidae δ18O values from Iowa to southern Illinois, with lower δ18O values from higher latitude samples closer to the ice margin. Lower δ18O values in samples closer to the ice sheet margin may be caused by increased warm-season storminess and lower precipitation δ18O values, due to the steepened land-ice meridional temperature gradient during the LGM. There is no observed north-south trend in δ13C values. However, a trend with proximity to the ice margin may provide more insight.