2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 12
Presentation Time: 1:30 PM-5:30 PM


STARRATT, Scott W.1, BARRON, John A.1, KNEESHAW, Tara1, PHILLIPS, R. Larry, LOWENSTERN, Jake3 and WANKET, James4, (1)US Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025-3591, (2)U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025-3591, (3)Dept. of Geography, University of California, Berkeley, CA 96720, sstarrat@usgs.gov

Medicine Lake is a small (1.6 km2), relatively shallow (average 7.3 m), medium- altitude (2,036 m) lake located within the summit caldera of Medicine Lake volcano, a dormant Quaternary shield volcano located in the southern Cascade Range. During the fall of 1999 and 2000, high-resolution bathymetry, seismic-reflection profiles, and sediment cores were collected from the lake. Twenty six samples from core B100NC-1 (water depth 12.6 m; length 226 cm) were analyzed for physical properties, sediment grain size, diatoms, pollen, and total organic carbon. Using both 14C (AMS) dating and tephrochronology, the sediments at the bottom of the core are estimated to be 11,000 cal yr B.P., thus yielding an estimated average sedimentation rate of about 21 cm/1,000 yr. The lowermost part of the core (226 cm - ~200 cm) records the transition from glacial to interglacial conditions. During the period from about 11,000-7,200 cal yr B.P., lake level fluctuated between deeper oligotrophic conditions with a diatom flora dominated by Cyclotella spp. and shallower intervals with a diverse benthic flora . The earliest part of this interval (226 cm - 210 cm) is almost devoid of Cyclotella and may represent a perpetually ice-covered lake in which only a small benthic flora can exist. The relative low abundance (10-15%) of Abies (fir) pollen and relative high abundance (30-40%) of Artemesia (sagebrush) suggest drier than present-day conditions. Sediments are dominated by glacial mud. From about 7,200 cal yr B.P. to the present, conditions have fluctuated between higher lake levels (three intervals) that are dominated by Cyclotella with a reduced number and diversity of benthic taxa, and lower lake levels (two intervals) during which the abundances of Cyclotella decrease to less than 10%. Relative values of Abies and Pinus (pine) are higher during high lake levels whereas aquatic taxa (primarily Isoetes [quillwort]) increases in significance at lower lake levels. Total organic carbon is higher during high stands and lower during low stands. Recently published analyses of the Lake Tahoe-Truckee River–Pyramid Lake drainage system indicate that some of the larger variations in lake level observed at Medicine Lake may be regional in nature, while smaller fluctuations probably reflect changing conditions affecting only the local watershed.