BOLLY PROJECT - PRELIMINARY MULTI-PROXY DATA FROM HOLOCENE CORES, FALLEN LEAF LAKE, TAHOE BASIN, CALIFORNIA, USA

Sediment cores recovered from Fallen Leaf Lake in the Lake Tahoe Basin, constrain the end of the Tioga glaciation and provide a ~11 kyr record of paleolimnologic and paleoseismic events. Core locations were targeted with the aid of high resolution CHIRP images collected throughout the lake. We present initial results that include stratigraphic correlation, a preliminary age model based on AMS radiocarbon ages, and some preliminary diatom analyses. Three strong seismic reflectors in CHIRP profiles can be tied to the cores and correlated throughout the southern sub-basin, representing the glacial outwash-post glacial boundary at ~10.4 14C kyr BP, the Tsoyowata ash (7950-7730 cal years BP) , and a turbidite representing the most recent seismic event along the West Tahoe-Dollar Point fault (>2484 14C kyr BP). These events, along with other turbidites, are also correlative based on their magnetic susceptibility (MS) peaks and visual identification. The glacial outwash interval shows high MS and is composed of ~1.5m of laminated gray-blue clay marking the most recent glacial retreat in the Glen Alpine valley, southern end of the Tahoe basin. Preliminary ages from AMS radiocarbon on plant macrofossils indicate that the Holocene-Pleistocene boundary occurs within a 1-3m laminated interval with generally low MS. The laminated interval grades into a discontinuously laminated olive to yellowish-green opaline clay, and is overlain by the 8ka Tsoyowata ash, which is a strong seismic reflector and occurs in all of the cores. The post Tsoyowata package is a 6-8m mottled to faintly laminated green opaline clay. Diatom assemblages are well preserved and dominated by phytoplankton, chiefly A. subarctica and cyclotelloids, with subordinate amounts of araphid pennates. Relative abundance of diatom species vary widely down core, with a large part of the variation attributed to % A. subarctica, and to % periphyton, and show potential for a producing a strong paleoclimate signal. These cores potentially provide a continuous Holocene record of paleolimnologic variations in the Tahoe Basin and paleoclimatic changes in the High Sierra at a decadal to multidecadal scale.