Graduate Thesis Or Dissertation

Role of Extreme Events on Sedimentation in Loon Lake, Oregon Coast Range, USA

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  • Sediment cores were retrieved from a landslide-dammed lake, recording events back to the 5th century AD in a forested, mountainous catchment. These cores provide an opportunity to compare the impacts of known recent perturbations, including floods and timber harvesting with those of the early period of the core, flood, fire, and earthquake. A multi-parameter approach was used to measure characteristics of sediment with high-resolution grain size, %TC, %TN, and magnetic susceptibility analysis. ¹³⁷Cs atmospheric deposition and radiocarbon dating techniques were used for age control, as well as a varved record in the last 75 years. The estimated mean mass accumulation rate from 1939 to 1978 AD, the time of peak timber harvest and a cool wet phase of the Pacific Decadal Oscillation (PDO), was 0.79 (0.74-0.92) g cm⁻² y⁻¹, significantly higher than estimated mean rates of both the more recent contemporary period, 1979-2012 AD, at 0.58 (0.48-0.70) and the entire early period, 0.44 (0.41-0.46). Several event beds are coeval with estimated ages of eight Cascadia subduction zone earthquakes in the early period, including the 1700 AD Mw 9.0 earthquake. These beds are predominantly hyperpycnites, as are the known flood deposits in the contemporary period, with the exception of one debris flow within a 23-cm thick event bed deposited at approximately 700 AD. The high mass accumulation rate and high concentration of thick event beds and layers during the early contemporary period point to the extraordinary role of timber harvesting in priming the landscape for subsequent sedimentary delivery during floods.
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