HR: 0800h
AN: PP21A-1370    [Abstracts]
TI: Onset and Multiple Fluctuations of Holocene Glaciation in the Sierra Nevada, California
AU: * Bowerman, N D
EM: bowermn@cc.wwu.edu
AF: Geology Dept., Western Washington Univ., Bellingham, WA 98225-9080 United States
AU: Clark, D H
EM: doug.clark@wwu.edu
AF: Geology Dept., Western Washington Univ., Bellingham, WA 98225-9080 United States
AB: Multiple sediment cores from two paternoster tarns (First and Second lakes) in North Fork Big Pine Creek, Sierra Nevada, preserve the most detailed and complete record of Holocene glaciation yet recovered in the range; they indicate that the glacier was absent during the early Holocene, reformed in the late Holocene, and experienced several expansions and contractions, culminating with the Matthes maximum during the last $\sim$200 years. The lakes are fed by outwash from the Palisade Glacier, the largest ($\sim$1.3 km$^{2}$) and presumably longest-lived glacier in the Sierra Nevada, and capture essentially all of the rock flour produced by the glacier. Distinct late-Holocene (Matthes) and late-Pleistocene (Recess Peak) moraines lie between the modern glacier and the lakes. Thus, the lakes have received continuous sedimentation since the retreat of the Tioga glacier ($\sim$15,000 yr B.P.), and therefore capture rock flour related to all subsequent advances. First and Second lakes occupy relatively deep bedrock basins at 3036 m and 3066 m asl., respectively. Third Lake, a shallow ($<$3 m deep), moraine-dammed lake that lies directly above Second Lake, is the only lake between the Palisade Glacier and the lower lakes. As such, it captures the coarsest (sand/gravel bedload) outwash, but abundant suspended sediment (silt/clay) continues to the lower lakes. We cored the lakes using both Reasoner and Livingston corers, to sediment depths of up to $\sim$5 m. The deepest cores bottomed in coarse, inorganic sand and silt that we interpret as outwash or slopewash related to Tioga deglaciation. Magnetic susceptibility (MS) analyses of the sediment cores indicate that both lakes record multiple late-Holocene peaks in MS, with the most recent peak being the largest. They also retain outwash near the base related to the more extensive Recess Peak advance. MS peaks in Sierran lakes typically indicate greater abundances of clastic (vs. organic) sediment. The peaks in our cores thus imply 4-5 periods of increased flux of rock flour (outwash) from the upstream Palisade Glacier, most likely related to formation and expansions of the glacier in the late Holocene. The maximum peak at the top of the cores confirms the moraine record, which indicates that the maximum Holocene advance of Sierran glaciers occurred during the late Little Ice Age (last $\sim$200 yr) At least one tephra layer, possibly related to the Mono/Inyo dome complexes, occurs in the middle depths of the First Lake cores. Other narrow peaks in MS may also be associated with tephra deposits. Ongoing detailed analyses of the sediments, including AMS radiocarbon dating, visual and x-ray imaging, particle size analysis, organic content, tephrochronology, diatom assemblages, and palynology will constrain the timing and character of the environmental fluctuations related to the rock-flour flux. We will present results of these analyses at the meeting.
DE: 3344 Paleoclimatology
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1827 Glaciology (1863)
DE: 1845 Limnology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting