USGS Pacific Tidal Marsh Soil Core Surveys

To parameterize accretion for SLR models, we measured historic rates of mineral and organic matter accumulation at each site by collecting deep soil cores with a Russian peat borer. At each site, we obtained cores in each of three vegetation zones: low, medium, and high marsh. Two replicate cores were sampled from each station for a total of 6 cores per site (except Coos Bay where 7 cores were taken). Coring locations were determined by RTK GPS elevation and tidal inundation data. Transects for core sampling were determined in ArcGIS, using a digitial elevation model and site-specific tidal datums to choose station locations below MHW (low), between MHW and MHHW (mid), and above MHHW (high). Sediment cores were 50 cm deep and 5 cm in diameter. In the lab, we cut cores into 1 cm sections to process for bulk density, porosity, and organic matter composition using loss on ignition in a muffle furnace at 550ºC for 8 hr. Only half of the cores collected were processed for bulk density, organic matter and Cesuim dating (one replicate). We used Cesium-137 (137Cs) isotope dating techniques to determine accumulation rates in deep soil cores. Atmospheric nuclear testing prior to 1964 resulted in the spread of 137Cs across the globe creating a reliable marker horizon in soils. We used a gamma spectrometer at the Oregon State University Radiation Center to detect 137Cs activity, measured in picocuries (pCi), in 1 cm core samples for 24 hr. We standardized the 137Cs activity of each sample to its mass. The depth of the 137Cs peak activity indicated the 1964 marker horizon, which we used to determine average soil accretion rates over the last half century. Not every processed core had a distinct peak of Cesium 137.