Regional paleoclimate inferences from Succineidae gastropod stable isotope analysis in last glacial maximum loess; Ward Quarry, Plainfield and Clayton sections

Spatial gradients of oxygen isotope ratios (delta (super 18) O) from aragonitic, terrestrial gastropod shells provide a means to evaluate regional variations in climate proximal to the southernmost extent of the Laurentide Ice Sheet during the Last Glacial Maximum (LGM). Here we present new delta (super 18) O data from Succineidae shells (2.5-6 mm in length) from resedimented loess below the Cartersburg Till Member at the Clayton Section, Plainfield Section and Ward Quarry in central Indiana. The Plainfield and Clayton sections are approximately 20 km north of the Wisconsin Episode glacial limit. The Ward Quarry section is located approximately 70 km southeast from these sites, proximal to the Wisconsin Episode glacial limit. The fossiliferous silt at the Clayton Section is stratigraphically bounded by two till units representing the maximum extent of the Laurentide Ice Sheet during the LGM ( approximately 24 cal ka) and a subsequent re-advance ( approximately 21.7 cal ka). The fossiliferous silt also occurs below the upper till unit at the Plainfield and Ward Quarry sites. Radiocarbon ages on plant macrofossils and terrestrial gastropod shells in the silt unit range from 22.0-21.5 cal ka BP. To avoid the influence of different vital effects between gastropod families, stable isotope analysis was performed solely on Succineidae, which were present at all 3 sites. We find that median delta (super 18) O values are highest at the Ward and Plainfield sites (2.0+ or -0.7 ppm and 1.5+ or -1.3 ppm), and lower at the Clayton site (0.9+ or -0.9 ppm). However, all sites show substantial variance, and the median values are not statistically different between sites at the 95% confidence level. Yet delta (super 18) O values at the Indiana sites are significantly different, and higher than the median, 22-21 cal ka BP Succineidae delta (super 18) O values from the New Cottonwood School loess section, located just east of the Illinois Valley (-0.1+ or -0.7 ppm). We hypothesize this zonal gradient in delta (super 18) O values is primarily a result of colder temperatures at the Indiana sites, which is supported by differences in the gastropod assemblages.