NEW AGE CONSTRAINTS ON HOLOCENE GLACIATION IN THE SIERRA NEVADA, CALIFORNIA

Multiple sediment cores from two paternoster lakes fed by outwash from the Palisade Glacier, Sierra Nevada, preserve the most detailed and complete record of Holocene glaciation yet recovered in the range. The Palisade Glacier is the largest (~1.3 km²) and presumably longest-lived glacier in the range. Distinct late-Holocene (Matthes) and late-Pleistocene (Recess Peak) moraines that lie between the glacier and the lakes mark the maximum extents of the glacier since retreat of the much larger Tioga-age (LGM) glacier. Thus, the lakes have received continuous sedimentation during the past ~15,000 yr and capture rock flour related to all post-LGM advances.

The lake cores, up to 5-m in depth, bottom in coarse, inorganic sand and silt that we interpret as outwash or slopewash related to Tioga deglaciation. Peaks in magnetic susceptibility (MS) in all sediment cores indicate that both lakes record similar multiple episodes of clastic sedimentation. These episodes indicate 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 during the Holocene. A preliminary age model suggests Neoglaciation began ~3500 cal. yr B.P., followed by glacier maxima at ~2700, ~1500, ~1000, and ~150 cal. yr. B.P., the most recent peak being the largest. The cores also record a pronounced clastic-sedimentation event between ~6000-7000 cal. yr. B.P. Clastic sedimentation is lowest between ~12,200-7000 cal. yr B.P., suggesting the glacier did not exist during that period. The maximum Holocene peak at the top of the cores confirms the regional moraine record, which indicates that the maximum Holocene advance of Sierran glaciers occurred during the late Little Ice Age (last ~200 yr). Multiple AMS radiocarbon dates, to be presented at the meeting, will refine these preliminary age assignments. Additional analyses of the cores, including visual and x-ray imaging, particle size analysis, organic content, tephrochronology, diatom assemblages, and palynology, are also in progress and will further constrain the character of the environmental fluctuations related to the rock-flour flux.