IMPROVED TEPHROCHRONOLOGY FOR THE 125,000 YEAR RECORD FROM CARP LAKE, WASHINGTON, USA: NEW GEOCHEMISTRY, MORE TEPHRA BEDS, AND A NEW AGE MODEL

Two cores from Carp Lake, Washington, reaching 23 m depth (bottom of sediment not reached), provide a 125,000 year paleoenvironmental and tephra record. The tephra beds both constrain the chronology of the cores and contribute information about regional volcanic activity, especially eruptions of Mt. St. Helens (MSH). Previous work reported 15 tephra layers, but several major eruptions from MSH were not identified in the cores (Whitlock et al., 2000). We have re-examined and re-sampled the two Carp Lake cores and have recognized additional tephra beds. The tephra beds range in thickness from about 1 mm to 40 cm, and the grain sizes range from fine ash to coarse sandy ash.

Tephra samples were wet sieved, mounted, polished, and then analyzed for SiO₂, TiO₂, Al₂O₃, FeO, MnO, MgO, CaO, Na₂O, K₂O, P₂O₅, and Cl on the ARL-SEMQ electron microprobe at Concord University. Newly characterized tephra layers include a MSH set S, a MSH set M, an additional MSH set C, two unidentified rhyolitic tephras, and a new mafic tephra. We anticipate identifying additional new tephra beds once all samples have been analyzed.