Variations of atmospheric 14C concentrations over the Allerod-Younger Dryas transition

Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolution AMS C-14-dated records, Lake Gosciaz, and a floating Late Weichselian glacial varve chronology at the Allerod-Younger Dryas transition allowed us to assess in detail atmospheric Delta(14)C changes between late Allerod and early Preboreal. The combined data set shows a drastic rise in Delta(14)C during the first 200 years or so of Younger Dryas and the two following about 500 year- long C-14 plateaux. Model experiments which included variations in the geomagnetic field, atmospheric CO2 variations and a drastic reduction in North Atlantic Deep Water flux at the onset of Younger Dryas allowed to reproduce the distinct rise in Delta(14)C during the first 200 years of Younger Dryas fairly well. Also the drop in Delta(14)C at the Younger Dryas/Holocene boundary seems reasonably explained by changes in North Atlantic Deep Water circulation. However, the reason behind the anomalous behaviour of the Delta(14)C signal in the middle of Younger Dryas remains an open question.