AN ANALYSIS OF RADIOCARBON SAMPLING METHODS ON LANDSLIDES IN THE NORTH FORK STILLAGUAMISH RIVER VALLEY, WASHINGTON, USA

The 2014 Oso (or SR530) landslide that tragically killed 43 people in the North Fork Stillaguamish (NFS) River valley, Washington, has sparked an increase in landslide research in the Pacific Northwest. The approach to assessing the hazard and associated risk of landsliding in the NFS River valley has been determining the frequency of slide events since the last glaciation about 16.4 k.y. ago (Porter and Swanson, 1998). Determining the ages of landslides is integral to determining frequency of slide events. In an effort to obtain ages and thus, frequency, recent studies have designated a surface roughness value to these landslides. These surface roughness values are correlated with numerical ages of landslides based on radiocarbon dates of wood samples from landslide debris (Booth et al., 2017; LaHusen et al., 2016). Wood samples for radiocarbon dating are found in gullies that carve into landslide deposits, fluvial terraces that cross cut landslides, and landslide ponds. In collecting four new wood samples for radiocarbon dating, I identified which methods of sampling suit different ages of landslides as well as the limitations of each method. This project presents: (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.

References

Booth, A. M., S. R. LaHusen, A. R. Duvall, and D. R. Montgomery (2017), Holocene history of deep-seated landsliding in the North Fork Stillaguamish River valley from surface roughness analysis, radiocarbon dating, and numerical landscape evolution modeling, J. Geophys. Res. Earth Surf., 122, 456–472.

LaHusen, S. R., Duvall, A.R., Booth, A.M., and Montgomery, D.R., 2016, Surface roughness dating of long-runout landslides near Oso, Washington (USA), reveals persistent postglacial hillslope instability, Geology, 44, 111-114.

Porter, S. C., and Swanson, T.W., 1998, Radiocarbon age constraints on rates of advance and retreat of the Puget lobe of the Cordilleran ice sheet during the last glaciation, Quaternary Research, 50(3), 205-213.