ELSEVIER Earth and Planetary Science Letters 126 (1994) 275-287 EPSL The North Atlantic atmosphere-sea surface 14C gradient during the Younger Dryas climatic event Edouard Bard a Maurice Arnold b Jan Mangerud c, Martine Paterne b Laurent Labeyrie b, Josette Duprat a, Marie-Antoinette M61i~res e, Eivind Scnstegaard f, Jean-Claude Duplessy b a CEREGE, CNRS-Universitd d'Aix-Marseille III, JE 192, sce 431, Facultd de St J~r6me, 13397 Marseille Cedex 20, France b Centre des Faibles Radioactivitds, CNRS-CEA, F-91198 Gif-sur-Yvette Cedex, France c Department of Geology, University of Bergen, All~gt. 41, N-5007 Bergen, Norway d Departement de Gdologie et Ocdanographie, Universitd de Bordeaux 1, Bordeaux I, France e Laboratoire de Glaciologie et Gdophysique de l'Environnement, BP96, 38402 St Martin d'H~res, France f Sogn og Fjordane College, N-5801 Sogndal, Norway Received 29 March 1994; revision accepted 14 July 1994 Abstract We attempt to quantify the 14C 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 14C 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 I4C 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' a4C 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 14C ages assigned to the ash layer peak is significantly older than the 14C 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 14C age obtained on the planktonic foraminifera is still about 11,000-11,100 yr BP. The atmosphere-sea surface aac 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 14C reservoir age during the YD. I. Introduction Numerous studies have demonstrated that the mode, rate and location of North Atlantic Deep Water (NADW) production were quite variable during the Quaternary and, in particular, during two recent cold periods, the last glacial maximum (LGM, about 18 kyr BP [1-5]) and the Younger 0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00157-X