Difficulties in interpreting fast mixing in the radiometric dating of sediments using 210Pb and 137Cs

Pb-210 geochronologies should be validated with independent tracers such as Cs-137. In the cases with constant Pb-210 activity in the topmost sediments, the presence of a distinct Cs-137 peak within the Pb-210 plateau has been used as a definitive demonstration of acceleration ( increase in the sedimentation rate in recent years) versus fast mixing. Nevertheless, some limitations can be identified in the use of semiquantitative arguments, and a global understanding of the whole Cs-137 activity profile is then required. Particularly, the incomplete mixing within the top sediment zone ( described through the Incomplete Mixing Zone model) can explain quantitatively and simultaneously the Cs-137 peak and the flattening in the Pb-210 activity profile. This is demonstrated using selected examples from literature data. Thus, Erten et al. (1985) measured constant Pb-210 activities in the top 6 cm of a sediment core from Lake Zurich. Nevertheless, they found Be only in the uppermost layer, the distinct Cs-137 maximum at 6 cm depth, and undisturbed varves. The fast mixing seemed then opposed to common sense. The constant rate of supply model shows acceleration and it adequately matches the position of the 1963 Cs-137 peak. Nevertheless it fails to explain the whole Cs-137 profile when handling time series of Cs-137 atmospheric deposition. Finally, it is shown how the incomplete mixing of the activity ( through the pore water) over a certain mass depth at the top sediment, with a finite value of the mixing coefficient, can quantitatively explain the whole activity versus mass depth profiles of Cs-137 and Pb-210, and the presence of Be only in the uppermost sediment layer. A further validation of these ideas is presented from other literature data.