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

Abstract

²¹⁰Pb geochronologies should be validated with independent tracers such as ¹³⁷Cs. In the cases with constant ²¹⁰Pb activity in the topmost sediments, the presence of a distinct ¹³⁷Cs peak within the ²¹⁰Pb 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 ¹³⁷Cs 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 ¹³⁷Cs peak and the flattening in the ²¹⁰Pb activity profile. This is demonstrated using selected examples from literature data. Thus, measured constant ²¹⁰Pb activities in the top 6 cm of a sediment core from Lake Zürich. Nevertheless, they found ⁷Be only in the uppermost layer, the distinct ¹³⁷Cs 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 ¹³⁷Cs peak. Nevertheless it fails to explain the whole ¹³⁷Cs profile when handling time series of ¹³⁷Cs 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 ¹³⁷Cs and ²¹⁰Pb, and the presence of ⁷Be only in the uppermost sediment layer. A further validation of these ideas is presented from other literature data.