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.