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The samples are processed to prepare Pb activities.
This technique is more sensitive than gamma spectrometry and is suitable when a small quantity of sample is available for analysis.
In all three lakes, rates calculated from Pb 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 distributions occurred at the same depth in each pair of 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 and trace metals (both up to 2 ×) 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.
Pb profiles in certain lakes (most in New England) and pollen dates suggest that bioturbation, physical mixing, changes in sedimentation rate, or acidification may be occurring, but this is insufficient to account for the deviant Cs at prefallout sediment depths is due mostly to net downward molecular diffusion and adsorption, and that surface concentration is due to net upward diffusion and adsorption (and possibly also biological uptake), recycling of sedimentary Cs in this sediment may be a function of release by organic decomposition and scarcity of clay minerals for adsorption.
30-50g of dried sample is required for analysis using a standard gamma spectrometer or 5g using a Well detector.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 Pb in the recent cores.Pb 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.