AMS RADIOCARBON DATING OF ORGANIC FRACTIONS FROM VARVED LAKE-SEDIMENTS - AN EMPIRICAL-TEST OF RELIABILITY

Replicate cores of annually laminated (varved) sediments have been use d to test the accuracy and precision of chronologies of lake sediment accumulation over the last 1000 years, based on AMS C-14 measurements. The internally consistent results show that in the case of the soft-w ater lake sediments studied (Kassjon, northern Sweden), all the organi c fractions that include aquatic components give significantly older d ates than expected. Only the single terrestrial macrofossil and the fi ne, unidentified residual particulate fraction provide dates close to the true age of the sediments. The results also show that age discrepa ncies for some fractions are not constant over time. The age discrepan cies may arise from some carbon reservoir effect within the aquatic ec osystem, from resuspension and focusing of older marginal organic mate rial into the deepest part of the lake or from some combination of the se processes. More work is needed on the C-14 'age' of organic fractio ns in varve-dated sediments from a range of lake types.