Late Holocene glacial advances in the Klamath Mountains, northern California, determined from 10Be cosmogenic exposure dating and dendrochronology

The well‐preserved moraines in the cirque at the head of Grizzly Creek, Klamath Mountains, California, provide the most complete record of late‐Holocene glacier fluctuations yet documented in this region. Two separate moraine complexes lie below the modern glacier, within the cirque, only one of which supports substantial tree growth. 10Be cosmogenic ages of scoured bedrock surfaces and moraine boulders, as well as tree‐ring ages indicate the approximate timing of glacial maxima in the Grizzly Valley cirque. The combination of the detailed climate record, provided by tree‐ring widths, and the estimated moraine ages, determined from dendrochronology and cosmogenic dating, allows for an accurate reconstruction of the Grizzly Valley Glacier fluctuations over the last 1,000 years. Nine new cosmogenic exposure ages, combined with dendrochronology, constrain the timing of glacier maxima to ~690, ~150 and ~130 years before present (ybp). Around 690 ybp, the equilibrium‐line altitude (ELA) was depressed ~160 meters relative to the ELA of the modern glacier. Using local temperature and precipitation lapse rates and the elevation of the contemporary glacier, we found that in comparison with modern climate conditions, the mean summer temperature during the ~690 ybp glacier maximum was ~0.9°C less and winter precipitation was ~95 cm in snow water equivalent (SWE) greater. During the -150 and ~130 ybp glacier maxima, the ELA was ~67 meters lower than the modern ELA. The mean summer temperature corresponding to this glacier maximum was ~0.4°C cooler and winter precipitation was ~44 cm greater in comparison with modern climate. The climate regime over the last 1,000 years in the Klamath Mountain region seems to be cool and exceptionally wet. The ~690 ybp glacier maximum in the Klamath Mountains is not apparent in the Sierra Nevada or Cascade Ranges. Also, glaciers in the Sierra Nevada and Cascade Ranges retreated from their most recent LIA maxima ~20-30 years before the glaciers of the equivalent advance in the Klamath Mountains. The climate in the Klamath Mountains likely varies from the Sierra Nevada and Cascade ranges due to the proximity to the Pacific Ocean. The chronology of Grizzly Valley Glacier fluctuations, determined in this study, suggests that in California the regional response to large‐scale climate regimes can vary over relatively short lateral distances.