Holocene glacier fluctuations on Mount Baker, Washington, USA

New electron microprobe data obtained from tephra found at Mount Baker, Washington, suggest an explanation of the Holocene glacial history of the region that differs from hypotheses previously published. Red scoria (Set SC tephra) from the Schreibers Meadow cinder cone (ca. 8800 14 C yr BP) was described by Thomas et al. (2000) as absent on a set of ridges located on the south flank of the mountain. This area consequently was interpreted by Thomas et al. (2000) and Kovanen and Slaymaker (2005) to have been ice-covered during the early Holocene. But these ridges are overlain by varying thicknesses of red tephra, in some places topped with coarse scoria shown here to be identical to that outside the previously mapped "scoria-free zone." In this study, tephra from within the "scoria-free zone" was compared to Set SC scoria that had been identified by Thomas et al. (2000). Electron microprobe analysis of individual glass shards shows that the unknown tephra resembles, and in any event cannot be differentiated from, the Set SC scoria. The apparent presence of Set SC scoria in the previously mapped "scoria-free zone" indicates a lack of extensive glacial ice at Mount Baker at 8800 14C yr BP. Three wood mats within lateral moraines at Easton and Coleman glaciers contain detrital wood separating individual tills. These wood mats record forested surfaces that were overridden by advancing glaciers at 5200, 700 and 400 14C yr BP. In addition, Set SC scoria and Mazama ash were found sandwiched between tills on a lateral moraine of Easton Glacier, and within a core obtained from Pocket Lake, further suggesting very limited ice in early Holocene time (10,000 to 6,000 yr BP). These results are significant because they support the hypothesis of a warm early Holocene in the Central North American Cordillera and contradict the notion of a significant early Holocene glacial advance.