IN SEARCH OF A DEFINITIVE RECORD OF HOLOCENE GLACIATION IN THE SIERRA NEVADA, CALIFORNIA

A pair of small paternoster tarns in North Fork Big Pine Creek, Sierra Nevada, likely preserve the most detailed and complete record of Holocene glaciation in the range. The lakes, imaginatively named First and Second lakes, lie downstream of the Palisade Glacier, the largest glacier in the Sierra Nevada (~1.3 km²), and capture essentially all of the rock flour produced by the glacier. Distinct Holocene (Matthes) moraines lie between the modern glacier and the lakes, as do moraines related to the late-Pleistocene Recess Peak advance (Clark and Gillespie, 1995). Thus, the lakes have received continuous sedimentation since the retreat of the Tioga glacier (~15,000 yr B.P.), and therefore should contain rock flour related to all subsequent advances.

First and Second lakes occupy 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 most of the coarsest (sand/gravel) outwash, but abundant suspended sediment continues to the lower lakes. Short sediment cores from Third Lake indicate that it desiccated during the late Holocene, perhaps in response to severe droughts (Stine, 1994). In contrast, First and Second lakes are bedrock tarns, bounded by steep granitic cliffs on most sides. Detailed bathymetric surveys of both lakes, using differential GPS and a sonar depth-meter, demonstrate that both comprise single basins with broad, smooth bottoms. Maximum depths of 14 m (First Lake) and 23 m (Second Lake) occur near the middle of each lake.

Their geometry, strong seasonality, and high rock-flour flux indicates the lakes are promising candidates for preserving varved, rock-flour sequences (e.g., Zillen et al., 2003). Coring of the lakes to test this possibility, is scheduled for late March. Detailed analyses of the sediments, including magnetic susceptibility, visual and x-ray imaging, particle size analysis, organic content, diatom assemblages, and AMS radiocarbon dating, should provide the most detailed record of Holocene glaciation in the range, regardless of the presence or absence of varves. We will present initial results of these analyses at the meeting.