An Analysis of Radiocarbon Sampling Methods on Landslides in the North Fork Stillaguamish River Valley, Washington, USA
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Tragedy in the North Fork Stillaguamish (NFS) River valley, Washington, USA has sparked an increase in landslide research in the Pacific Northwest. In 2014, the town of Oso, WA lost 43 lives to the deadliest landslide in North American history. The Oso Landslide (formally referred to as the State Route (SR) 530 Landslide) began as a rotational slide and became a debris flow when it reached the NFS River (Keaton et al., 2014). Since this devastating event, researchers have been trying to understand and, ultimately communicate, landslide hazard risk in this river valley. The approach to assessing the hazard and associated risk of landsliding in this valley has been determining the frequency of slide events since the last glaciation about 16.4 k.y. ago (Porter and Swanson, 1998). The Fraser glaciation deposited permeable advance outwash sand and gravel over impermeable glaciolacustrine silt and clay layers. Heavy precipitation events in the winter months, a signature of western Washington climate, saturate the permeable sand units and weaken the cohesion of the steep hillslopes in the NFS River valley. In addition to lithology, climate, and topography, the NFS River meanders throughout the valley and interacts directly with the hillslopes. This occurs most commonly in the narrowest portion of the valley where the Oso Landslide is located (Keaton et al., 2014). Recent studies have designated a surface roughness value, using standard deviation of slope, to these landslides. These surface roughness values are correlated with numerical ages of landslides based on radiocarbon dates of wood samples (LaHusen et al., 2016). These wood samples represent the time of the slide event, which would have presumably killed and entombed these trees. The more radiocarbon dates of landslides in the NFS River valley, the more accurate the age-roughness model is for estimating landslide ages. In an effort to further calibrate the numerical age-roughness model, this project collected four new wood samples for radiocarbon dating and developed a protocol for determining best methods for sampling organic material from landslides. Wood samples for radiocarbon dating are found in gullies that carve into landslide deposits, fluvial terraces that cross cut landslides, and landslide ponds. In this project, I identify which methods of sampling suit different ages of landslides as well as the limitations of each method. I present here: (1) a protocol for determining which method to use to sample landslides for radiocarbon dating, (2) an explanation of how to remotely identify features, such as ponds, to support field work, and (3) an analysis of the accuracy of each method according to the four new radiocarbon dates that I obtained.