A LACUSTRINE-BASED MID- TO LATE-HOLOCENE HISTORY OF THE AGASSIZ GLACIER, GLACIER NATIONAL PARK, MONTANA, USA

Multiproxy study of sediment cores retrieved from Upper Kintla Lake provides the first detailed Neoglacial chronology for Glacier National Park. Core UKL-1, retrieved from 4.8 m of water near the inlet of Agassiz Creek, spans from AD 875 to 2007. Core UKL-3, retrieved from 35.7 m of water in the main body of Upper Kintla Lake ~900 m from the inlet, extends from the Mazama tephra (7600 BP) to ~1000 BP. Analysis focused on sedimentary properties sensitive to the extent of glacier ice up-valley, including: water, organic matter, carbonate, and biogenic silica content, bulk density, phosphorus fractionation, magnetic susceptibility, L*a*b* color values, and grain size distribution. Results from Core UKL-3 indicate that the Agassiz Glacier existed in an expanded state during four intervals before the last millennium: 6200-5700, 5100-4500, 3800-3400, and 2300-1900 BP. These fluctuations are notably consistent with Neoglacial moraine records for western Canada, and exhibit coherence with records from the western U.S. Quasi-periodic pacing of intervals of expansion may reflect variability in solar output and summer temperature, while significant centennial-scale cycles in many proxies could represent variability in the Pacific ocean-atmosphere system. Core UKL-1 provides a higher resolution record of the Agassiz Glacier during the past millennium. Values of carbonate content were low during the Little Ice Age maximum (constrained by tree-rings to ca. 1860), but reached an unprecedented maximum (10%) in the 1940s synchronous with a maximum in the measured rate of glacier retreat (~120 m/yr). This maximum was followed by an order-of-magnitude reduction in carbonate content during the 1960s that corresponds to a similar decrease in the rate of ice retreat. If major spikes in the abundance of carbonate rock flour reflect intervals of rapid ice retreat, then retreat from the Little Ice Age terminal moraine during the first half of the twentieth century was the most extreme in rate and magnitude over the past ~1100 years. Other major pulses of carbonate abundance occurred ca. A.D. 1800 and 1680.