EXTREME HOLOCENE PALEO-RUNOFF EVENTS RECORDED IN SEDIMENT CORES FROM LAKE TAHOE, CALIFORNIA AND NEVADA

Two piston cores were collected from the southern and northern basins of Lake Tahoe with the intent of obtaining the Holocene history of sediment accumulation. Though no tephra was found in either core, the Tsoyowata ash bed (7015±45 years B.P.) was found in a companion core 3 cm below the base of the southern basin core, providing an initial age marker for these sediments. A variety of analyses were done on these sediments at high-resolution sampling intervals, including 1) particle-size analysis by laser diffraction, 2) paleomagnetic and environmental magnetic measurements, 3) total organic carbon, and 4) d¹³C of the organic carbon component. Six radiocarbon dates determined from pine needles, charcoal, and small twigs indicate a long-term sedimentation rate of ~25 cm/ky.

The grain-size data, determined at 1 cm intervals, exhibit a background sedimentation dominated by coarse silt that is interspersed with centimeter-scale pulses of coarser material ranging up to fine and medium sands. The pulses of coarser grains exhibit relatively sharp bases, graded beds, low-angle crossbedding and abrupt shifts back to finer grain sizes, suggesting rapid deposition by turbidity currents. These turbidite intervals are matched by comparable pulsed increases in d¹³C_org and magnetic susceptibility, and by abrupt decreases in TOC. Our model for these turbidites suggests that the majority of them represent runoff from severe storms. For example, even though the bases are sharp relative to the background record of accumulation, almost all of the turbidites show a gradual coarsening upward trend over the basal few centimeters. We interpret this as reflecting a gradational increase in storm intensity and runoff with time. The corresponding increases in magnetic susceptibility likely reflect increased concentrations of ferrimagnetic material derived from terrigenous sources. The coincident enrichment in d¹³C_org and concomitant reduction in TOC could well be due to the isotopic signature of terrigenous plant debris arriving with storm runoff and the dilution of background organic concentrations, respectively. These preliminary results provide new information about the Holocene paleoclimatic history (and perhaps the paleoseismic history) of the Lake Tahoe watershed.