Davis, P.T., Osborn, G., Menounos, B., Ryane, C., Clague, J., Riedel, J., Koch, J., and Scott, K., 2005: New evidence for Holocene glacial fluctuations on Mt. Baker, Washington. American Geophysical Union, Fall Meeting 2005, abstract #U43A - 0827

New evidence for Holocene glacial fluctuations on Mt. Baker, Washington

Ongoing research on Mt. Baker, an active Cascades stratovolcano, provides new constraints on the timing of Holocene glacier fluctuations. Previously mapped deposits on the southwest flank of Mt. Baker suggested to some that glacial advances during the early to mid- Holocene were more extensive than those during the Little Ice Age (LIA). This interpretation was based on the presence and absence of Mazama (ca. 6800 14C yr BP) and Mt. Baker set OP (ca. 5800 14C yr BP) tephras, and a scoria deposited at ca. 8,800 14C yr BP. Our work indicates a more complex distribution of the scoria than previously thought, as well as its presence on deposits reported to be scoria-free. In addition, many of the landforms previously mapped as moraines are bedrock or bedrock-cored ridges. At Easton Glacier, we identified two tills separated by an abrupt unconformity in the east lateral moraine about 20 m below the moraine crest. The unconformity is marked by (1) a deformed mat of peat and detrital wood fragments and trunks up to 0.5 m in diameter, (2) two tephra layers, and (3) a thick red silt below the two tephra layers that may be a weathering product of the tephra(s) or, alternatively, a third, weathered tephra. The two tephras have field characteristics identical to those of Mazama and Baker Set OP present on the south flank of the volcano. Two samples of detrital wood yielded ages of 5260 ± 70 and 5240 ± 70 14C yr BP, which we interpret to indicate (1) construction of a moraine prior to 6800 14C yr by a glacier with an extent similar to that of the LIA, (2) retreat of the glacier, stabilization of the moraine, and establishment of a forest, and (3) advance of the glacier at ca. 5200 14C yr, overriding the vegetated moraine surface. This advance is correlative with the well-known `Garibaldi Advance' in the southern Coast Mountains of British Columbia. At Coleman Glacier, an unconformity about 12 m below the crest of the southwest lateral moraine is marked by a laterally-continuous wood mat. The wood mat dips into the proximal flank of the moraine and contains logs up to 0.4 m diameter, one of which dates to 940 ± 50 14C yr BP. This age is consistent with radiocarbon-dated glacier advances in the Canadian Rocky and Coast mountains. The data collectively suggest that Mt. Baker glaciers responded to Holocene climate change in a similar manner to other glaciers in western North America. We have found no evidence to support the suggestion that glaciers on Mt. Baker were more extensive during the early and middle Holocene than during the LIA.