Salt marsh soil lead-210 and dating for Clayoquot Sound, British Columbia, Canada

The goal of this project was to provide the first estimates of blue carbon stocks and carbon accumulation rates in the high and low marsh zones of salt marshes from the Pacific Coast of Canada, within the Clayoquot Sound UNESCO Biosphere Reserve and Pacific Rim National Park Reserve on the Pacific Coast of Canada. Sediment cores were collected from seven salt marshes during summer (June-September) 2016. The cores were collected using a simple percussion coring technique in which a length of two-inch (57 mm) diameter, PVC vacuum tubing fitted with a plastic core catcher (AMS Inc.) was hammered into the ground until the depth of refusal. At the GBK location, a steel sledge corer (AMS Inc.) was used to extract four cores before mechanical problems required switching to the simple percussion method. Radioisotope (210Pb and 226Ra) measurements were conducted on eight cores (four from the high marsh zone and four from the low marsh zone) from marshes CBE, CRF, GBK, and TMF, to create age models for estimating carbon accumulation rates. Activities of 210Pb were determined by alpha spectrometry through its granddaughter 210Po, assumed in secular equilibrium. The atmospheric or excess 210Pb (210Pbxs) fraction used to derive the age-depth model was determined as the difference between the total 210Pb activity and its parent nuclide 226Ra activity. 226Ra activities were determined by α-spectrometry at Flett Research using calibrated geometries in a glass vessel, Spectech UCS 30 Alpha Scintillation Spectrometer purged with helium, and sealed for at least 11 days. Samples were sealed and stored for two hours before counting to ensure secular equilibrium of 226Ra daughters. 226Ra was determined through counting 222Rn activity for 60,000 seconds (Minimum Detectable Activity (MDA) = 0.0167 Bq kg-1). Sediment chronologies were estimated using the Constant Flux: Constant Sedimentation (CF:CS) model, which assumes a constant atmospheric deposition of 210Pb to the marsh surface and a constant mass accumulation rate (MAR, [g cm-2 yr-1]). Under these assumptions, the MAR was obtained from the slope [cm2 g-1] of the linear best-fit line of the relationship between the natural log of the excess 210Pb activity against the cumulative mass [g cm-2] using MAR (g cm-2 y-1) = 0.0311 (yr-1)/slope (cm2 g-1), where 0.0311 yr-1 is the radioactive decay constant of 210Pb. We emphasize that we have estimated 210Pb-derived accumulation rates in terms of cumulative mass (g cm-2) instead of depth, to avoid issues of compaction associated with the coring process.