ASSESSING CLIMATIC AND NON-CLIMATIC FORCING OF PINEDALE GLACIATION AND DEGLACIATION IN THE WESTERN US

Moraine chronologies often reveal significant age differences among moraines deposited during a single glacial cycle, differences which commonly are used to infer spatio-temporal patterns of regional climate. Within the western US, ages of Pinedale (and Pinedale equivalent; marine isotope stage 2) terminal moraines of ~18 to ~16 ka in the northern Rocky Mountains are ~5000 yr younger than terminal Pinedale moraines in the Wind River Range and at some locations in Colorado. While most authors focus almost solely on climatic factors to explain these age differences, non-climatic factors intrinsic to individual glacier systems (e.g. hypsometry) may also influence moraine ages. To test whether non-climatic factors can influence Pinedale terminal moraine ages, we obtained ¹⁰Be ages from glacial landforms in three adjacent valleys in the upper Arkansas River basin, Colorado. Based on these new ages and a compilation of other moraine chronologies throughout the western US, we hypothesize that non-climatic factors are an important source of age differences of Pinedale terminal moraines within, and between western US mountain ranges. When incorporating cosmogenic exposure ages of Pinedale recessional moraines, this compilation indicates deglaciation began ~16 ka, and that the most significant change of western US glaciers since the Last Glacial Maximum is their near synchronous demise between ~15 and ~13 ka. Furthermore, we wonder if the near synchronous retreat was driven by the first major warming in the Northern Hemisphere following the Last Glacial Maximum, the Bølling-Allerød period, recorded in Greenland ice cores. These results suggest the need for caution in focusing exclusively on climate forcing to explain apparent asynchrony in terminal moraine ages.