The fallout isotope 207Bi in a Delaware salt marsh: a comparison with 210Pb and 137Cs as a geochronological tool

We report the presence of the fallout isotope 207Bi (half-life = 38 years) in salt marsh sediments collected from Wolf Glade Salt Marsh, Lewes, Delaware and evaluate its usefulness as a transient tracer for geochronology. The commonly used geochronological tracers of salt marshes. 210Pb and 137Cs, were also measured to corroborate the results. The average sedimentation rate based on a constant excess 210Pb flux model in three cores (2 plus 1 duplicate) is estimated to be 3.2 ± 0.1 mm/year. Based on 137Cs and 207Bi data and an assumption that the fallout activities of these nuclides peaked during 1963, average rates are 3.5 ± 0.7 and 3.7 ± 0.2 mm/year, respectively. The scatter on the down-core 210Pb activity is relatively large which may be ascribed to variations in the atmospheric deposition rate of 210Pb, core compaction, or sedimentation. The 137Cs is present in sediments older than 50 years and suggests that these sedimentation rates may be compromised by the post-depositional mobility of 137Cs in salt marsh environments. The presence of 207Bi is detected at similar depths in all three cores which is consistent with a pulsed input of this nuclide into the environment from nuclear detonations and a post-depositional immobility. A higher degree of reliability in sedimentation rate estimates may thus be obtained using 207Bi. Based on the measured flux of 207Bi into salt marsh and an assumed inventory ratio of View the MathML source, the total production of 207Bi from nuclear weapon testings is estimated to be on the order of 1015 Bq.