High-precision accelerator-mass-spectrometer radiocarbon dating of buried tidal-marsh soils - An approach to estimating the frequency and coastal extent of subduction zone earthquakes in Oregon and Washington

Accurate and precise radiocarbon dating of buried tidal-marsh soils in estuarine stratigraphic sequences in the Pacific Northwest can help distinguish soils submerged during regional plate-boundary earthquakes from soils submerged during local upper-plate earthquakes or by non-tectonic processes. More precise ages than those resulting from conventional methods of radiocarbon analysis can be obtained by averaging multiple accelerator-mass-spectrometer (AMS) 14C ages of rigorously selected and pretreated plant macrofossils at the abrupt upper contacts of tidal-marsh soils. An initial test of this method in Coos Bay, Oregon, shows that standard deviations on AMS ages can be reduced to ± 25-40 radiocarbon years. But consideration of the total analytical errors in AMS analysis and age differences due to variations in the rate of 14C production in the atmosphere over time indicate that 95% confidence limits on calendar-corrected ages for submergence events range from 50 to 450 years