DEVELOPING A HOLOCENE NEOGLACIAL CHRONOLOGY FOR GLACIER NATIONAL PARK, MONTANA

Alpine glaciers in Glacier National Park have retreated dramatically from their Little Ice Age (LIA) maximum positions over the past 150 years and are projected to disappear entirely within the next few decades. To develop a chronology of pre-LIA glacier activity, sediment cores were retrieved from four lakes, two on each side of the Continental Divide, located below some of the largest remaining glaciers in Glacier National Park: Upper Kintla Lake below the Agassiz Glacier, Cracker Lake below the Siyeh Glacier, Harrison Lake below the Harrison Glacier, and Cosley Lake in the headwaters of the Mokowanis River. Radiocarbon dating, 210-Pb analysis, and tephrachronology reveal that the cores span 1000 to >10,000 years with sedimentation rates ranging from 0.36 to 3.38 mm/yr. Laboratory analysis focused on sediment properties sensitive to the extent and activity of glacier ice upvalley. High-resolution (HR) records appear sensitive to pulses of sediment released by glacier retreat, while low-resolution (LR) records track episodes of overall enhanced glacier activity and/or extent. The LR core from Cosley Lake includes over 1.5 meters of rock flour above till from the latest Pleistocene deglaciation, and records a dramatic shift in sedimentary properties corresponding to the separation of Cosley Lake from Glenns Lake upvalley. A LR core from Upper Kintla Lake records an expanded and/or more active Agassiz Glacier during four intervals: 6200-5700, 5100-4500, 3800-3400, and 2300-1900 cal. yr BP. The HR core from Harrison Lake records a strong signal of cold, turbid meltwater from the Harrison Glacier during the past millennium, and at ca. 2400, 3000, and 3700 cal. yr BP. Carbonate flux in a HR core from Upper Kintla Lake reaches peak values in the early 20^th Century synchronous with maximum measured retreat rates of the Agassiz Glacier from its LIA moraine. A similar spike in carbonate flux is seen in the HR record from Cracker Lake, but ca. 50 years earlier, indicating that the smaller Siyeh Glacier responded faster to the end of the LIA, or that conditions driving glacier retreat were out of phase on opposite sides of the Continental Divide. Overall, the records exhibit broad synchroneity with each other, and with Neoglacial chronologies from elsewhere in western North America.