Pb-210 and Cs-137 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. Evid ence from some lakes suggests that this radionuclide may be adequately mobi le to compromise dating reliability. This study provides one test of that p ossibility by comparing recent measurements of Pb-210 and trace metals to o nes carried out more than 20 yrs in the past. Cs-137 dating is used to conf irm sediment accumulation rates in the recent cores. In the three Connectic ut, USA, lakes studied, sediment accumulation rates changed abruptly to hig her 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 abo ve and below this horizon agreed, within measurement uncertainty, in recent and older cores. Furthermore, when the older data were corrected for 20 yr s of burial, the changes in slope in Pb-210 distributions occurred at the s ame 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 i mportant differences were also observed. For two of the three lakes, there was a significant difference in average sediment accumulation rate during t he past 33 yrs as calculated from Cs-137 and Pb-210 in the recent cores. Mo st potential causes for this difference can be ruled out, and it appears th at 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 t race 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 i n the past.