TREE-RING ANALYSIS, A TOOL TO ASSIST IN DATING OF SUBFOSSIL FORESTS KILLED BY GEOLOGIC EVENTS—VICTIM TREE EXAMPLES FROM MOUNT RAINIER LAHARS AND THE BONNEVILLE LANDSLIDE, WASHINGTON AND OREGON, USA

Bark-bearing old-growth tree victims of past earthquakes, landslides, and volcanic eruptions and lahars allow wiggle-matching of radiocarbon dates and higher precision ages, which helps narrow the focus of tree-ring cross correlations. Examples from Mount Rainier from the past 1600 years include trees buried near Kent, Washington (~518–550 CE), at Fife and Auburn (~900 CE), and by the Electron Mudflow in the Puyallup River valley (~1490–1510 CE)(Vallance and Pringle, 2008). Wood samples obtained with borers and saws were mounted, polished, and scanned at 0.01mm resolution. ImageJ software was used for tree-ring measuring, and programs Cofecha and Arstan for analysis along with visual examination and crossdating methods, such as noting narrow rings and low-density “light rings”. At Kent we cross correlated five Douglas-fir (Pseudotsuga menziesii); three displayed minor earlywood beneath bark, suggesting tree death in spring (Moran et al., 2017). The Fife subfossil trees include sub-horizontal logs at ~3-4 m depth and one standing Western redcedar (Thuja plicata) rooted at a depth of ~5 m. Trees of that age at Auburn had been buried by an eruption-triggered lahar that appears correlative with lahar-derived gravelly-sand deposits at the Port of Seattle and those in Fife. Radiocarbon ages of the trees in Fife and Auburn are similar to dates on trees killed by the Seattle Fault (Atwater, 1999) and Tacoma Fault (Sherrod et. al., 2004) and a number of large landslides in the Olympic Mountains, thus, future efforts will seek to locate additional subfossil tree victims and evaluate them with radiocarbon wiggle-matching and tree-ring analysis in order to obtain higher-precision ages for each of these events.

Trees killed by the Electron Mudflow include 28 Douglas-firs recovered near Orting and from Lake Kapowsin, which was dammed by the Electron Mudflow, and whose submerged trees can be crossdated with the subfossil trees in Orting. A long (341 years) tree-ring chronology derived from twelve Electron trees can be used to crossdate tree victims of the mid-1400s Bonneville landslide in the Columbia River gorge, 133 km to the south, which predated the Electron my 61 years. Oral stories of the indigenous peoples appear to describe lahar events at ~900 CE, the Electron Mudflow (Kearns and Pringle, 2015), and the Bonneville landslide.