J. Bjo ¬ rck Variations of atmospheric 14C concentrations over the Allero/d-Younger

Abstract Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolu-tion AMS 14C-dated records, Lake Gos « cia9 z5, and a ßoating Late Weichselian glacial varve chronology at the Aller+d-Younger Dryas transition allowed us to assess in detail atmospheric *14C changes between late Aller+d and early Preboreal. The combined data set shows a drastic rise in *14C during the Þrst 200 years or so of Younger Dryas and the two following about 500 year- long 14C plateaux. Model experiments which included variations in the geomagnetic Þeld, atmo-spheric CO 2 variations and a drastic reduction in North Atlantic Deep Water ßux at the onset of Younger Dryas allowed to reproduce the distinct rise in *14C during the Þrst 200 years of Younger Dryas fairly well. Also the drop in *14C 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 *14C signal in the middle of Younger Dryas remains an open question