Atmospheric radiocarbon at the end of the last glacial; an estimate based on AMS radiocarbon dates on terrestrial macrofossils from lake sediments.

The main purpose of this work is to reconstruct the atmospheric DELTA-C-14 in the glacial-postglacial transition, 14,000 - 10,000 BP, a range not covered by the tree-ring calibration curve. We measured C-14/C-12 ratios on series of terrestrial macrofossils from sediments of two Swiss lakes. We selected exclusively plant remains of recognizable terrestrial origin that are not affected by hard water and thus reflect atmospheric C-14 concentration. Due to the scarcity of such material, we used accelerator mass spectroscopy. Cores of two lakes were measured to eliminate local effects and to check the reproducibility of results. This requires a reliable, C-14-independent correlation of the cores, obtained through local pollen zone boundaries. C-14 ages were obtained as a function of the depth in the cores. If sedimentation rates are known, ages can be converted into DELTA-C-14 values. We also attempted estimating sedimentation rates; calculations are based on the Swedish varve chronology. Results were combined to form an entire data set. The DELTA-C-14 curve shows an increase with time during the Allerod and decreases during Preboreal and Bolling periods. Probabilities for these C-14 variations are discussed.