BURIED AND SUBMERGED FORESTS OF WASHINGTON AND OREGON—TIME CAPSULES THAT RECORD PALEO- EARTHQUAKES, VOLCANIC ERUPTIONS, AND LANDSLIDES

Dendrochronology (tree-ring dating) in concert with radiocarbon analysis has yielded more precise ages for subfossil trees killed by catastrophic Holocene geologic events in the Pacific Northwest. Buried forests are found in most river valleys that drain Cascade Range stratovolcanoes. Their study has improved the histories of many volcanoes and precisely dated eruptions and geomorphic events at Mounts Rainier, St. Helens, and Hood. Submerged sites along coastal Oregon and Washington, in Puget Sound, in the Columbia River, and in lakes (most landslide-dammed) contain subfossil snags that record the timing of geologic events including paleo-earthquakes. The ages of more than 25 landslides have been estimated by radiocarbon dating of associated subfossil wood. However, only 25 percent of these allow constraint of the calendric age of tree mortality, and more sampling will be required to obtain outer wood. Additional tree-bearing lakes have been discovered but not sampled, and more than two dozen candidate sites lack reconnaissance. Many likely record paleo-earthquakes.

Both ring-width and tree-ring density parameters of annual growth rings are useful for cross dating. Low density, “light” rings record cool summers and are an effective cross-dating tool, because they can record conditions of regional-to-hemispheric extent, and thus allow analysis beyond microclimate boundaries defined by soil moisture. Light rings useful as “pointers” in the Pacific Northwest include 1302, 1347, 1371, 1395, 1438, 1448, 1509, 1546, 1560, 1579, 1601, 1641, 1643, 1696, 1801, 1806, and 1809. Light rings are useful at elevations >750 m in Washington and Oregon, or where trees from higher elevations have been carried downslope, for example, by lahars.

Systematic sampling and dating of subfossil trees can improve the quality and accuracy of radiocarbon ages, allow correlations among sites, and precisely date geologic events. The oldest published ring-width chronologies in Washington and Oregon extend only to the late 1200s, however, tree-ring dating of subfossil trees can extend that record. Dendrochronology is the key to precisely dating subfossil forests and to refining and extending the record of prehistoric earthquakes, volcanic eruptions, and landslides.