210Pb and 137Cs dating methods in lakes:: a retrospective study

Pb-210 has been used for more than two decades to provide the geochronology of annually deposited sediments and to construct pollution histories. Evidence from some lakes suggests that this radionuclide may be adequately mobile to compromise dating reliability. This study provides one test of that possibility by comparing recent measurements of Pb-210 and trace metals to ones carried out more than 20 yrs in the past. Cs-137 dating is used to confirm sediment accumulation rates in the recent cores. In the three Connecticut, USA, lakes studied, sediment accumulation rates changed abruptly to higher values between 40-50 yrs ago (increasing by factors of 2.2, 2.9, and 3.0). In all three lakes, rates calculated from Pb-210 distributions both above and below this horizon agreed, within measurement uncertainty, in recent and older cores. Furthermore, when the older data were corrected for 20 yrs of burial, the changes in slope in Pb-210 distributions occurred at the same depth in each pair of cores. The depth of sharp peaks in concentrations of trace metals also matched. In general, this evidence supports the idea that sediments in these lakes have simply been buried, without significant diagenetic remobilization of Pb-210 and trace metals . Nevertheless, some important differences were also observed. For two of the three lakes, there was a significant difference in average sediment accumulation rate during the past 33 yrs as calculated from Cs-137 and Pb-210 in the recent cores. Most potential causes for this difference can be ruled out, and it appears that one of the two nuclides is remobilized compared to the other. There were also significant differences in the total inventories of both Pb-210 and trace metals (both up to 2 x) between recent and older cores in some cases. This may be due to dissimilar sediment focusing, since it is not known for certain whether the new cores were collected at exactly the same sites as in the past.