Lead-210 and radium-226 measurements by direct gamma assay can now provide a record of changing concentrations in lake sediments sufficiently reliable and precise to form a suitable basis for age/depth and drysedimentation-rate calculations . There are additional benefits in terms of non destructive sample preparation and simultaneous assay for other environmentally significant gamma-emitting radioisotopes (e.g . 137 Cs and 241 Am) . Results from L . Fleet, S . W. Scotland illustrate the value of this approach especially in lakes with disturbed catchments where variable input of supported 210Pb has occurred .
241Am derived from decay of fallout 241Pu is now frequently detected in analyses of lake sediments by low-background gamma assay, and offers an alternative to weapons test 137Cs in dating recent sediments at those sites where the 137Cs record has been degraded by post-depositional mobility or obliterated by Chernobyl fallout. Calculations of the in-growth of 241Am from 241Pu indicate a nominal distribution broadly similar to that of 137Cs, with the maximum 241 Am activity occuring in fallout dating from 1963. Results from a number of sites suggest that 241Am is significantly less mobile in lake sediments than 137Cs, and that its distribution in cores reflects more closely the fallout record. Since further decay of existing weapons debris will increase 241Am concentrations by about 24% over the next 40 years, 241Am is likely to play an increasingly important role in assessing the validity of 210 Pb dates at sites with varying sediment accumulation rates.
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