MODERN SEDIMENTATION RATES IN WESTERN WASHINGTON LAKES – TIES TO SEISMIC ACTIVITY AND ANTHROPOGENIC INFLUENCES

Lead-210 and Cesium-137 dating of recent lake sediment in western Washington is in progress to determine linkages to changes in erosion rates and mass transport deposits due to seismic activity and anthropogenic influences. Eight lakes (Ozette, Whatcom, Washington, Chelan, Kachess, Keechelus, Cle Elum, and Sammamish) have been cored and sampled; they represent different elevations, climatic zones, and depositional environments spanning the outer coast of the Olympic peninsula to the Cascades. The lakes also represent a transect over the Cascadia megathrust and vary in proximity to active crustal faults.

Constraining recent sedimentation rates is critical to interpreting the timing of earthquakes in the region and to understanding potential anthropogenic effects. For example, Ozette Lake cores show very good concordant sedimentation rates derived from ²¹⁰Pb an ¹³⁷Cs. The deepwater depocenters have sedimentation rates of 2.4 mm/yr while an inter-basin ridge is about half of that, with sedimentation rates of ~1 mm/yr. A common 2-3 cm thick sequence of clay lamina throughout the lake, just below the base of excess lead-210, may represent a marker of early European homesteading around the watershed from ~1890 to 1920. The cores also record evidence of mass transport deposition triggered by Cascadia megathrust earthquakes, the most recent occurring at 1700 CE (beyond the range of this technique). However, understanding sedimentation rates during the last 150 years augments radiocarbon dating of event deposits and helps constrain Bayesian age models that have shown a recurrence interval of ~420 years of megathrust earthquakes.