Challenges and limitations of the 210Pb sediment dating method: Results from an IAEA modelling interlaboratory comparison exercise

The Pb-210 sediment dating is the most widely used method to determine recent (similar to 100-150 years) chronologies and sediment accumulation rates in aquatic environments and has been used effectively for reconstruction of diverse environmental processes associated with global change. Owing to the relative accessibility of the Pb-210 methodology, many environmental chronologies have been produced, but not always critically assessed. Sometimes, sedimentary processes such as compaction, local mixing, erosion, or episodic sedimentation are not taken into account, nor the validity of the fundamental premises and proper estimation of uncertainties assessed. A Pb-210 dating interlaboratory comparison modelling exercise was designed within the framework of the IAEA (International Atomic Energy Agency) Coordinated Research Project "Study of temporal trends of pollution in selected coastal areas by the application of isotopic and nuclear tools" (CRP K41016), to identify potential problems associated with the use of Pb-210 dating models and to suggest best practices to obtain reliable reconstructions. The exercise involved 14 laboratories worldwide with different levels of expertise in the application of the Pb-210 dating methods. The dating exercise was performed using Pb-210, Ra-226 and Cs-137 activity data from two sediment cores (coastal and lacustrine sediments), and the participants were requested to provide their Pb-210 chronologies based on dating models. This modelling exercise evidenced the limitations and constraints of Pb-210 method when supplementary and validation information is not available. The exercise highlighted the relevance of solid understanding of the fundamentals, assumptions and limitations of the Pb-210 dating method and its validation, and allowed identifying key aspects to improve the reliability of Pb-210 dating process, including: a critical examination and interpretation of the Pb-210 activity depth profile; an appropriate selection of the Pb-210 dating model according to the characteristics of the Pb-210 activity profile and the environmental setting taking into account sediment compaction in the calculations; a sound identification of the Pb-210 equilibrium depth and the estimation of the Pb-210 inventory ensuring the best possible estimation of interpolated Pb-210 values when needed; and the use of independent markers to corroborate the age models.