A PALEOCLIMATIC RECONSTRUCTION OF FOSSIL LAKE, OREGON USING MICROPALEONTOLOGY

Fossil Lake is a late Pleistocene lacustrine basin in south central Oregon that contains diverse vertebrate and invertebrate fossil assemblages. A total of five stratigraphic cores were recently taken throughout the basin. The ages of these cores ranged from 610 kya near the base to 23.3 kya near the top and are based on previously dated tephras. A major unconformity is present in all the cores from approximately 600 to 100 kya. Two cores from the northeastern and the south-central section of the basin were chosen to complete an intensive subsampling for micropaleontology. Ostracodes were hand picked from sediments samples using a brush. Ostracodes specimens are present throughout most of the core, but adult ostracode are rare in comparison to juveniles. Ostracode preservation is good but recrystallized and broken specimens are common. Ostracode faunal assemblages and their frequency are independent of lithology, but change in frequency of a single Candona sp. through time shows an anticorrelation to the GISP2 oxygen isotope record. This anticorrelation indicated a first order control by climate. Phytoliths were extracted from sediment samples using a heavy liquid flotation/centrifugation procedure. Other biosilicates, including diatoms, sponge spicules, and algal stratospores, and ash fragments were also recovered with this procedure. Diatoms are the dominant biosilicate in the samples, ranging from 75 to 95% of all biosilicates present. Phytolith concentrations ranged from 2 to 20%. Phytolith preservation was good with only minor dissolution pitting in some phytoliths. The dominant phytoliths present include grass long cells and pooid short cells. Rare grass trichomes and panicoid cross and bilobates are also observed. The abundant pooid phytoliths suggest a cool climate during the past 50 kya. Minor warming events occurred approximately at 23.3 kya and 47 kya based on the presence of panicoid phytoliths.