MULTIPLE GLACIER ADVANCES AFTER 15.5 KA IN THE KLAMATH MOUNTAINS, NORTHERN CALIFORNIA DETERMINED FROM 10BE EXPOSURE DATING AND DENDROCHRONOLOGY

The west coast of the United States contains a variable history of glaciation since the last glacial maximum (LGM). Records from the Sierra Nevada show the LGM termination at ~18 ka and only sparse evidence for an ~150 years before present (y.b.p.) advance during the Little Ice Age (LIA). In contrast, there is evidence from Oregon and Washington for multiple post-LGM advances. The well-preserved moraines in the Sugar Pine Creek, Upper Coffee Creek, and Grizzly Creek drainages of the Klamath Mountains in northern California provide evidence for at least three post LGM advances. We provide 32 new cosmogenic exposure ages combined with dendrochronology to constrain the extent of glaciation at ~15,500, ~11,800, ~650, and ~130 y.b.p.. These new ages provide one of the most complete records of glacier fluctuations from the west coast of the US. Boulder exposure ages from terminal and lateral moraine boulders provide evidence for widespread glaciers that extend over 10 km down valley at ~15,500 y.b.p. and represent the end of the last glacial maximum. Inset within these are distinctive, boulder-rich moraines that extend to 2-3 km from the LGM terminal moraines and have an age of ~11.800 y.b.p. based on boulder and scoured bedrock exposure ages. Two separate LIA moraine complexes lie high within the cirque above Grizzly Creek cirque. 10Be cosmogenic ages from moraine boulders, as well as tree-ring ages, provide ages for two distinct Little Ice Age advances at ~650 and ~130 y.b.p., when the climate was likely wetter and cooler than today. In the Sierra Nevada, no significant glacier advance was documented ~650 years ago and glaciers retreated from the Recess Peak advance ~30 years earlier than the equivalent advance in the Klamath Mountains. The significant difference in chronology of the LGM termination between the Sierra Nevada (~18 ka) and the Klamath Mountains (~15.5 ka), as well as the unique glacial maxima ~650 y.b.p. in the Klamath Mountains, suggests regional response to large-scale climate regimes can vary over relatively short lateral distances. Addressing the variation in the LGM chronology between the Klamath Mountains and the Sierra Nevada is critical to better understand the Pleistocene-Holocene transition.