RECESSION OF MODERN GLACIERS AND POST-LAST GLACIAL MAXIMUIM GLACIAL CHRONOLOGY IN THE TRINITY ALPS, CA

The presence of active cirque glaciers below 2743m (9000 feet) elevation in the Trinity Alps of northern California has been reported for over a century (Hershey, 1900; Sharp, 1960). We combine mapping and historical airphoto interpretation to show that only 2 of 3 glaciers that existed in 1900 remain, and that the remaining 2 glaciers have decreased 10-20% in area over the last 50 years. The remaining glaciers are located on the northern flanks of the highest peaks (Thompson and Caeser) in the range and cover ~5 hectares (1 hectare=10,000 m²) each. The active nature of the glaciers is reflected by extensive crevasses across the blue-ice. Each active glacier is associated with a series (3-4 each) of well-developed moraines. Relative dating techniques (e.g. soil development, stratigraphic position, geomorphic form) suggests that the youngest moraines record the timing and magnitude of multiple late Holocene glacial advances. The active cirque glaciers document a present-day equilibrium line altitude (ELA) of ≈2500m (8200 feet) for the Trinity Alps. A rise in present-day ELA in the Trinity Alps is in accord with similar observations in the Sierra Nevada but at odds with ELA depression reported for nearby Mt. Shasta, underscoring the complex local response to broader patterns of climate forcing.