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.