The approaching obsolescence of 137Cs dating of wetland soils in North America

Abstract

The peak fallout in 1963 of the radionuclide 137Cs has been used to date lake, reservoir, continental shelf, and wetland sedimentary deposits. In wetlands such dating is used to project the ability of wetlands to keep pace with sea level rise and develop strategies for mitigating carbon pollution using biological carbon sequestration. Here we demonstrate that reliable 137Cs profiles are increasingly difficult to obtain from wetland soils. Among 58 soil cores recently collected from a range of wetland types and 137Cs fallout densities across the United States, 25% contain no identifiable 137Cs peaks. Less than 40% of 137Cs ages are consistent with 210Pb dating. We provide a new measure of 137Cs peak clarity (τ) for our core dataset by comparing the 50% interquartile range of data around the 137Cs peak for “ideal” cores profiles determined using 137Cs fallout data to that of observed core profiles. Our results show that overall τ is approximately 10 times greater for observed cores than ideal cores. The deterioration in the 137Cs peak has occurred due to radionuclide decay, 137Cs migration in situ, which is ubiquitous in this study, and 137Cs amendments from surface waters. Such deterioration likely extends to both Mexican and non-permafrost, Canadian wetlands. We recommend continued use of 137Cs only if the full bound of dating uncertainty for both 137Cs and an additional method such as 210Pb is propagated into estimates of wetland vertical accretion and carbon sequestration.