Mid-Holocene Glacier Peak and Mount St. Helens we tephra layers detected in lake sediments from southern British Columbia using high-resolution techniques

A Glacier Peak tephra has been found in the mid-Holocene sediment records o f two subalpine lakes, Frozen Lake in the southern Coast Mountains and Moun t Barr Cirque Lake in the North Cascade Mountains of British Columbia, Cana da. The age-depth relationship for each lake suggests an age of 5000-5080 C -14 yr B.P. (5500-5900 car yr B.P.) for the eruption which closely approxim ates the estimated age (5100-5500 C-14 yr B.P.) of the Dusty Creek tephra a ssemblage found near Glacier Peak. The tephra layer, which has not been rep orted previously from distal sites and was not readily visible in the sedim ents, was located using contiguous sampling, magnetic susceptibility measur ements, wet sieving, and light microscopy. The composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid-Holocene tephra layer. Using the s ame methods, the A.D. 1481-1482 Mount St. Helens We tephra layer was identi fied in sediments from Dog Lake in southeastern British Columbia, suggestin g the plume drifted further north than previously thought. This high-resolu tion method for identifying tephra layers in lake sediments, which has worl dwide application in tephrachronologic/paleoenvironmental studies, has furt hered our knowledge of the timing and airfall distribution of Holocene teph ras from two important Cascade volcanoes. (C) 2001 University of Washington .