EXPANDED AND REVISED TEPHROCHRONOLOGY FOR THE 220,000-YEAR RECORD FROM CARP LAKE, WASHINGTON, USA

Multiple cores taken in the early 1990s from Carp Lake, Washington, reaching 23 m depth, provide a 220,000 year paleoenvironmental and tephra record. The tephra beds help constrain the chronology of the cores and also contribute information about regional volcanic activity, especially eruptions of Mt. Saint Helens (MSH). Tephra from nearby Mt. Hood and Mt. Adams might also be present. Previous work (Whitlock et al., 2000) described 15 tephra layers.

Re-examination of 24.5 m of previously opened core (Kalteyer and Kuehn, 2013; Kuehn et al 2014) and 15.5 m of newly opened core yielded 85 samples of tephra beds ranging in thickness from about 1 mm to 35 cm with grain sizes ranging from fine ash to coarse sandy ash as well as multiple samples of tephra-bearing sediments. Analysis of major- and minor-element glass compositions using the ARL-SEMQ electron microprobe at Concord University resulted in the identification of 42 tephra beds with well-defined geochemistries. Additional geochemical populations are present in samples that contain mixtures of probably reworked tephra grains. Most tephras contain rhyolitic glass, but andesitic and basaltic tephras are also present. The majority of the tephra layers appear to originate from MSH with tephra sets Y, S, M, C, and probably J represented. Also present are at least 10 tephras pre-dating MSH set C with C-, Y-, and S-like chemistries. Together, these older samples provide the best known single record of the early 50 to 200+ ka pyroclastic eruptive history of MSH. Three samples contain tephra that probably originates from the Three Sisters volcanoes in central Oregon. The lowermost tephra bed in the cores is an excellent geochemical match to the ~215 ka Antelope Well tuff from Medicine Lake volcano in northern California which is also known at Summer Lake, Oregon as bed KK (Kuehn et al., 2010).

The state of preservation of the cores at the time of this study and the limitations of the original core recovery, leave some uncertainty about the original thicknesses of several of the tephra layers and about the actual number of tephras deposited into the lake. A deeper re-coring of Carp Lake could help reduce uncertainties regarding the known eruptions and would likely yield additional tephra beds. New cores could also yield an improved paleoenvironmental record and a new paleomagnetic record.