TIMING OF DEGLACIATION FROM MULTIPLE COSMOGENIC NUCLIDES IN BEDROCK AT MCCULLOUGH GULCH, SOUTHERN ROCKY MOUNTAINS, USA

Documenting the spatial and temporal patterns of glacier retreat in the Western United States is important for understanding how alpine glaciers in this region respond to global and regional forcings. In the southern Rocky Mountains, there is evidence that deglaciation following the Last Glacial Maximum (LGM, ca. 20 ka) was delayed by several thousand years and then proceeded rapidly. This is hypothesized to be driven either by separation of the Laurentide and Cordilleran ice sheets or by abrupt warming in the North Atlantic (Tulenko et al., 2020). We documented the timing of glacial retreat to the cirque in McCullough Gulch, located in the Tenmile Range of the Southern Rocky Mountains in Colorado, USA, using measurements of multiple cosmogenic nuclides in three bedrock samples. Exposure ages calculated from in situ cosmogenic ¹⁰Be in quartz and ³⁶Cl in whole rock agree within uncertainty and are consistent with simple exposure with negligible erosion or snow cover since 14 ka. Glacier retreat to the cirque at McCullough Gulch is consistent with the timing of ice retreat to cirques in the nearby Sawatch Range (Tulenko et al., 2020) and consistent with delayed deglaciation in the southern Rocky Mountains. Measurements of in situ cosmogenic ¹⁴C (in situ ¹⁴C) in quartz yield significantly older ages than ¹⁰Be or ³⁶Cl – the potential source of this apparent excess in situ ¹⁴C inventory is still under investigation. Additional bedrock samples collected at lower elevations in McCullough Gulch and elsewhere in the Tenmile Range could be used to document the pace of glacial retreat following the LGM.

Reference: Tulenko, J. P., Caffee, W., Schweinsberg, A. D., Briner, J. P., and Leonard, E. M.: Delayed and rapid deglaciation of alpine valleys in the Sawatch Range, southern Rocky Mountains, USA, Geochronology, 2, 245–255, https://doi.org/10.5194/gchron-2-245-2020, 2020.