DEVELOPING A LACUSTRINE-BASED, HIGH-RESOLUTION NEOGLACIAL CHRONOLOGY FOR SIYEH GLACIER USING A SEDIMENT CORE FROM CRACKER LAKE, GLACIER NATIONAL PARK, MONTANA, USA

The alpine glaciers of Glacier National Park (GNP) have rapidly retreated over the past 150 years from their maximum extents at the end of the Little Ice Age (LIA). Some models predict the glaciers will disappear entirely over the next few decades. To better understand the changes that are occurring in GNP now, it is necessary to reconstruct how these glaciers behaved before the LIA. A 330-cm sediment core was retrieved from Cracker Lake (1801 m), below Siyeh Glacier, on the eastern side of the Continental Divide in GNP during July, 2010. AMS radiocarbon dating of 5 terrestrial macrofossils reveals that the core spans 2000 years with every centimeter representing ~7 years of sedimentation. A multi-proxy laboratory analysis concentrated on sediment indicators of Siyeh Glacier’s past extent and activity at the head of Cracker Lake. Analyses included X-radiography, magnetic susceptibility, color spectrophotometry, loss on ignition, bulk density, grain size distribution, and biogenic silica content. The high-resolution core records sub-decadal fluctuations of Siyeh Glacier with large pulses of sediment assumed to represent times of glacial retreat. Levels of bedrock-derived carbonate in the core reveal unprecedented retreat rates of Siyeh Glacier in the early 20^th century, synchronous with historical records of other glaciers and carbonate levels from other lake cores. This is also consistent with observations by the USGS, which indicate that Siyeh Glacier lost ~75% of its area between 1966 and 2005. Spikes in mean grain size, magnetic susceptibility, carbonate content, and sharp decreases in organic and biologic productivity could represent other advance-retreat cycles occurring between A.D. 300-400, 600-750, and 1400-1550. Spectral analysis of carbonate concentrations reveals a significant 40-year cycle that may correlate with the Pacific Decadal Oscillation (PDO), which has been closely linked to Snow Water Equivalent levels in GNP over the last century. Extremely low levels of biogenic silica throughout the core suggests that Siyeh Glacier has been present above Cracker Lake, creating consistently turbid and cold water, for the past 2000 years. Complete disappearance of Siyeh Glacier would significantly alter the hydrology and ecology of the Cracker Lake watershed.