COSMOGENIC NUCLIDE EXPOSURE AGE LIMITS ON THE PENULTIMATE GLACIATION IN THE CONTERMINOUS WESTERN U.S

The timing of the penultimate glaciation in the conterminous western U.S. has been challenging to determine precisely and accurately through numerical dating of glacial deposits and landforms. Applications of cosmogenic nuclide surface-exposure dating to moraines and associated outwash in several glaciated mountains have provided numerous age limits, but the reliability and resolution of these age estimates have been hindered by uncertainties related to moraine degradation, boulder-surface erosion, and in-situ production of cosmogenic nuclides. Moreover, relative to the more recent Last Glacial Maximum, far fewer details are known about the penultimate glacial history because features from this earlier period were largely obliterated by subsequent overriding ice advances in many glaciated regions. New and existing cosmogenic ¹⁰Be, ³⁶Cl, and ³He exposure ages of moraines and outwash of the penultimate glaciation are assessed here using updated production rates, scaling models, and commonly applied statistics, to (1) determine whether the timing of the penultimate glaciation can be constrained from a set of widely ranging terrestrial cosmogenic nuclide (TCN) exposure ages, and (2) identify a signal of temporal correspondence among the age limits. TCN exposure ages of moraines from the North Cascade Range, greater Yellowstone, Middle and Southern Rocky Mountains, Colorado Plateau, Great Basin, and of moraines and outwash in the Sierra Nevada are evaluated here with assumed rock surface erosion rates of zero to 1 mm/kyr. Our assessment reveals that the coefficient of variation of TCN exposure ages of moraines from the penultimate glaciation exceeds 20% in most study areas, most likely reflecting inconsistent exposure history among erratic boulders on moraine crests. Additionally, the distribution of TCN exposure ages for most moraines is negatively skewed, suggesting that moraine degradation is the most probable cause of the variable boulder-exposure histories. In such cases, the oldest TCN exposure age of each moraine provides the most accurate age limit. Across the region, the oldest TCN exposure ages (or age modes) fall within the later part of marine isotope stage 6, ca. 150-130 ka, suggesting that mountain glaciation in the western U.S. culminated in broad synchrony and in step with global ice volume.