Records of radionuclide deposition in two salt marshes in the United Kingdom with contrasting redox and accumulation conditions

Abstract

Single cores from two salt marshes in the United Kingdom located near different nuclear facilities were investigated to compare chronostratigraphic estimates derived from the natural radionuclide 210Pb excess with estimates from the known times of introduction of artificial radionuclides to the environment. Both cores selected had clear visual indications of redox zonation, and evidence for diagenetic redox perturbation of the radionuclide records was also sought. In the core from Beaulieu Marsh on the south coast of England, the redox zonation was revealed by the profiles of the redox-sensitive elements Mn + I, Fe + P + As, and S, and the records of nuclear discharges were entirely contained within oxidized conditions in the upper 40 cm. The constant flux/constant sedimentation 210Pb excess accumulation estimate was 76% of that derived from the 1963 fallout 137Cs level (0.35 vs. 0.46 g cm −2 yr −1 dry mass), but the constant flux 210Pb excess method indicated that accumulation rates were lower at Beaulieu before ∼1950. On any timescale, 137Cs appears earlier in the sediment record than its introduction to the environment, but although downward diffusion of 137Cs relative to 241Am has clearly occurred, the 137Cs peak still appeared in place and there was negligible penetration of 137Cs into underlying reduced conditions. This core also contained a peak of the 60Co discharges from either or both the Winfrith and La Hague nuclear plants that peaked in 1980 and 1984, respectively. The sediments in the core from Wyre Marsh on the eastern coast of the Irish Sea had accumulated more rapidly than those at Beaulieu, and in this case the redox zonation could be established only from Mn and S profiles. Here, the constant initial activity 210Pb excess accumulation rate estimate was 125% of that derived from the 137Cs peak correlated with the 1975 Sellafield discharge maximum (0.79 vs. 0.64 g cm −2 yr −1). Sellafield discharge 137Cs swamps fallout or Chernobyl 137Cs signals in this core, but the 137Cs and 241Am sediment records are well separated and remain consistent with the slightly different discharge patterns over time. This is so despite the fact that the maximum activity levels of both isotopes are now located well into reducing conditions out of which Mn must have migrated. The 210Pb profile appeared similarly unaffected by the oxidized/reduced boundary in this case. This core was too short to define the limits of any downward 137Cs migration. 210Pb excess accumulation rate estimates for salt marshes should be viewed with some caution because of the steady-state assumptions inherent in all 210Pb excess methods and the potential for fluctuating accumulation conditions and open system behavior in salt marshes.