THE NORTH-ATLANTIC ATMOSPHERE-SEA SURFACE C-14 GRADIENT DURING THE YOUNGER DRYAS CLIMATIC EVENT

We attempt to quantify the C-14 difference between the atmosphere and the North Atlantic surface during a prominent climatic period of the l ast deglaciation, the Younger Dryas event (YD). Our working hypothesis is that the North Atlantic may have experienced a measurable change i n C-14 reservoir age due to large changes of the polar front position and variations in the mode and rate of North Atlantic Deep Water (NADW ) production. We dated contemporaneous samples of terrestrial plant re mains and sea surface carbonates in order to evaluate the past atmosph ere-sea surface C-14 gradient. We selected terrestrial vegetal macrofo ssils and planktonic foraminifera (Neogloboquadrina pachyderma left co iling) mixed with the same volcanic tephra (the Vedde Ash Bed) which o ccurred during the YD and which can be recognized in North European la ke sediments and North Atlantic deep-sea sediments. Based on AMS ages from two Norwegian sites, we obtained about 10,300 yr BP for the 'atmo spheric' C-14 age of the volcanic eruption. Foraminifera from four Nor th Atlantic deep-sea cores selected for their high sedimentation rates (> 10 cm kyr(-1)) were dated by AMS (21 samples). For each core the r aw C-14 ages assigned to the ash layer peak is significantly older tha n the C-14 age obtained on land. Part of this discrepancy is due to bi oturbation, which is shown by numerical modelling. Nevertheless, after correction of a bioturbation bias, the mean C-14 age obtained on the planktonic foraminifera is still about 11,000-11,100 yr BP. The atmosp here-sea surface C-14 difference was roughly 700-800 yr during the YD, whereas today it is 400-500 yr. A reduced advection of surface waters to the North Atlantic and the presence of sea ice are identified as p otential causes of the high C-14 reservoir age during the YD.