Atmospheric Radiocarbon at the End of the Last Glacial: An Estimate Based on AMS Radiocarbon Dates on Terrestrial Macrofossils from Lake Sediments

Abstract

The main purpose of this work is to reconstruct the atmospheric Delta-14C in the glacial-postglacial transition, 14,000 -10,000 BP, a range not covered by the tree-ring calibration curve. We measured 14C/12C 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 14C 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 required a reliable, 14C-independent correlation of the cores, obtained through local pollen zone boundaries.14C ages were obtained as a function of the depth in the cores. If sedimentation rates are known, ages can be converted into Delta-14C 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-14C curve shows an increase with time during the Allerød and decreases during Preboreal and Bølling periods. Probabilities for these 14C variations are discussed.