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 last deglaciation, the Younger Dryas event (YD). Our working hypothesis is that the North Atlantic may have experienced a measurable change in 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 remains and sea surface carbonates in order to evaluate the past atmosphere-sea surface C-14 gradient. We selected terrestrial vegetal macrofossils and planktonic foraminifera (Neogloboquadrina pachyderma left coiling) mixed with the same volcanic tephra (the Vedde Ash Bed) which occurred during the YD and which can be recognized in North European lake sediments and North Atlantic deep-sea sediments. Based on AMS ages from two Norwegian sites, we obtained about 10,300 yr BP for the 'atmospheric' C-14 age of the volcanic eruption. Foraminifera from four North Atlantic deep-sea cores selected for their high sedimentation rates (> 10 cm kyr(-1)) were dated by AMS (21 samples). For each core the raw C-14 ages assigned to the ash layer peak is significantly older than the C-14 age obtained on land. Part of this discrepancy is due to bioturbation, 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 atmosphere-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 potential causes of the high C-14 reservoir age during the YD.