THE NATURE OF CLIMATICALLY DRIVEN FLOOD DEPOSITION IN LAKE QUINAULT, WA

Lake Quinault is a 15 km², ~70 m deep moraine-dammed lake located on the western front of the Olympic Mountains in Washington. Analysis of multiple piston cores retrieved from the lake indicates that the sedimentary record has been dominated by flood-induced deposition over the past ~3500 years. Presumably linked to Quinault River discharge, the thicknesses of flood layers follow a power-law distribution and, based on radiocarbon dating of terrestrial plant debris, occur at a frequency of every 3.5 yrs in the proximal end of the lake and every 17 yrs in the distal end.

Following visual core analysis, grain size samples were collected at various cm-scale intervals, and color and μXRF data were collected from split cores at 2 and 5 mm resolution, respectively. L* (total reflectance) has a moderate correlation with mean grain size, whereas b* (blue-to-yellow index) roughly parallels the contribution of clay-sized sediment to the overall grain size distribution. With respect to elemental data, Ti, Si, Ca, Fe, K, Rb, and Sr are all well correlated, which suggests their abundances are related to the flux of detrital material into Lake Quinault. If color and XRF data are controlled by the delivery of clastic sediment, which is related to both the discharge of the Quinault River and the availability of sediment for transport, then time-series analysis should provide insight into any climate variables potentially influencing the sedimentary record of the lake.

To test this assumption, the periodicity of color, μXRF, and Quinault River monthly and annual average discharge data were evaluated using the SLOMBS program, which combines the Lomb-Scargle periodogram with a permutation test to determine confidence levels. Results show that color parameters have high power and confidence at periodicities of 5-7, 15-20, and 50-70 years; XRF elemental abundances at periodicities of 10-20, 35, and 50-70 years, and Quinault River discharge at periodicities of 7 and 15 years. The overlap among these results provides further evidence that the Lake Quinault sedimentary record is dominated by the deposition of terrigenous material delivered via the Upper Quinault River. In addition, the power at these intervals suggests that the flux of sediment may also be controlled by such climate variables as ENSO and the Pacific Decadal Oscillation.