Sediment cores retrieved from landslide-dammed Loon Lake recorded events back to the 5th century AD in a forested, mountainous catchment, thereby providing an opportunity to compare the impacts of known recent perturbations, including floods and timber harvesting with those of an early period in the cores, floods, fires, and earthquakes. High-resolution multi-parameter (grain size, %TC, %TN, and magnetic susceptibility) data allowed the core stratigraphy to be classified as background sedimentation and events. 137Cs and radiocarbon dating, as well as a varved record in the last 75 years provided age control. 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, was 0.79 (0.74–0.92, 95% C.L.) g cm-2 y-1, significantly higher than mean rates of both the more recent contemporary period (coincident with the passing of the legislation that regulated harvesting practices in the region), 1979–2012 AD, at 0.58 (0.48-0.70) and the entire early period, 0.44 (0.41–0.46). Several event deposits are coeval with independently estimated ages of eight Cascadia subduction zone earthquakes in the early period, including the 1700 AD Mw 9.0 event. These deposits are predominantly formed by hyperpycnal flows, as are the known event deposits in the contemporary period. The high mass accumulation rate and greater frequency of thick event deposits during the early contemporary period point to the extraordinary role of timber harvesting in priming the landscape for subsequent sedimentary delivery during floods.
Bibliographical noteFunding Information:
The authors would like to thank individuals, groups, and agencies that contributed both small and large amounts of time, equipment, effort, and expertise. Those who participated in fieldwork, sample processing and other support roles: Ken Richardson, Venice Richardson, Zion Richardson, Esther Pettersen, Kim Braasch, Franco Bola?o-Guerrero, Christine Wheatcroft, Max Taylor, Laura Woodbury, Greta Krost for Coos Bay District Bureau of Land Management (BLM). OSU faculty, staff, and labs: Dr Joe Stoner, Dr Phil Mote, Dr Mary Santelmann; Dr Wheatcroft's Sediment Dynamics Lab and group; Dr Hatten's Forest Soils Lab and group and manager Yvan Alleau; Miguel Goni's geochemistry lab; Fred Prahl's lab and staff; NSF Marine Geology Repository and staff; College of Forestry statistics support Ariel Muldoon. Other agencies and labs: National Energy and Technology Lab, Albany, Oregon, Kelly Rose and Corinne Disenhof; University of Washington Chuck Nittrouer and lab group; University of Minnesota LacCore Lab and Anders Noren; Weyerhauser, Maryanne Reiter, hydrologist; NOSAMS Woods Hole Oceanographic Institution; BLM ? Coos Bay District, Mike Kelly and staff. Those who provided feedback at various phases of the research: Dan Gavin, Patrick Pringle, Jerry Phillips, Ray Simms, Ann Morey Ross, Sarah Lewis, Fred Swanson, Catalina Segura, Jim O?Connor, Bill Burns. Funding entities: US Environmental Protection Agency grant #83518602 for research assistantship and material funds; College of Earth, Ocean, and Atmospheric Sciences teaching assistantship.
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- lake sedimentation
- Oregon Coast Range
- timber harvesting