241AM DATING OF LAKE-SEDIMENTS

Am-241 derived from decay of fallout Pu-241 is now frequently detected in analyses of lake sediments by low-background gamma assay, and offers an alternative to weapons test Cs-137 in dating recent sediments at those sites where the Cs-137 record has been degraded by post-depositional mobility or obliterated by Chernobyl fallout. Calculations of the in-growth of Am-241 from Pu-241 indicate a nominal distribution broadly similar to that of Cs-137, with the maximum Am-241 activity occurring in fallout dating from 1963. Results from a number of sites suggest that Am-241 is significantly less mobile in lake sediments than Cs-137, and that its distribution in cores reflects more closely the fallout record. Since further decay of existing weapons debris will increase Am241 concentrations by about 24% over the next 40 years, Am-241 is likely to play an increasingly important role in assessing the validity of Pb-210 dates at sites with varying sediment accumulation rates.