Fine-tuning of Pb-210-based methods for dating vegetated saltmarsh sediments

Abstract

Saltmarshes, wetlands, inter-tidal mudflats are highly productive natural ecosystems with significant ecological value. 210Pb and 137Cs have been used for establishing chronologies at a centennial scale in these sediments. They are relevant for assessing how saltmarshes are vulnerable to climate change, natural hazards and contamination from human inputs. This paper aims to review and adapt existing methods for the 210Pb-based radiometric dating of these sediments where the subterranean production and decay of OM are sources of achronicity. This limits the use of the basic assumption of considering the sediment as a continuous medium. From the available evidence, 210Pb fluxes onto sediments may be mediated by the activities bound to the mass flows, suggesting that models assuming a constant flux should be handled with care. The activity concentrations of excess 210Pb required by CF-CS, CIC and TERESA radiometric models must be those of the sediment mineral fraction, while CRS and PLUM can work with the composite sediment matrix. A review of these issues and application of the models is illustrated with new data from two recent saltmarsh sediment cores. The effects of non-ideal boundary conditions in radiometric dating and the system time-average of 137Cs fluxes are discussed. Although specific to vegetated saltmarshes, this topic and the present results have generic applications for other vegetated coastal sediments.