Late Pleistocene Environments Of The Bighorn Basin, Wyoming-Montana, Usa

Limited numbers of high-resolution records predate the Last Glacial Maximum (LGM) making it difficult to quantify the impacts of environmental changes prior to peak glaciation. We examined sediments from Last Canyon Cave in the Pryor Mountains of Montana and Wyoming to construct a >45 ka environmental record from pollen and stable isotope analysis. Artemisia pollen was hyper-abundant at the beginning of the record. Carbon isotope values of bulk organic matter (>40 ka) showed little variation (-25.3 +/- 0.4 parts per thousand) and were consistent with a arid C-3 environment, similar to today. After 40 cal ka BP, Artemisia pollen decreased as herbaceous taxa increased toward the LGM. A significant decrease in delta C-13 values from 40-30 cal ka BP (similar to 1.0 parts per thousand) established a new baseline (-26.6 +/- 0.2 parts per thousand), suggesting cooler, seasonally wetter conditions prior to the LGM. These conditions persisted until variation in delta C-13 values increased significantly with post-glacial warming, marked by two spikes in values at 14.4 (-25.2 parts per thousand) and 13.5 cal ka BP (-25.4 parts per thousand) before delta C-13 values dropped to their lowest values (-26.9 +/- 0.2 parts per thousand) at the onset of the Younger Dryas (12.8 ka). These results provide insights into late Pleistocene conditions and ecological change in arid intermontane basins of the Rocky Mountains.