Abrupt Climatic Changes During the Last Deglaciation in the North-West Pacific

E Bard; F Rostek (both at: CEREGE, CNRS, University Aix-Marseille III, France; ph. +33 442971561; fax +33 442971549; Internet: bard@cerege.fr)

High resolution isotopic profiles were previously measured in cores located along the Japanese continental margin (Kallel et al. 1988 Oceanol. Acta 11, 369; Labeyrie et al. 1990, Oceanol. Acta 10, 329; Duplessy et al. 1989 Radiocarbon 31, 493). Two different interpretations were proposed to explain an abrupt delta-O18 decrease by 1.3 per mil at about 15,000 cal-yr-BP in core CH84-14, followed by a delta-O18 maximum between 13,000 and 11,500 cal-yr-BP, in apparent synchrony with the Younger Dryas cold event. The first explanation (Kallel et al. 1988) involves major SST changes in phase with the North-Atlantic deglaciation as studied in core SU81-18 (Bard et al. 1987 Nature 328, 791). The second interpretation is based on the spread of low salinity waters (Labeyrie et al. 1990; Keigwin & Gorbarenko 1992, Quat. Res. 37, 346). We have measured alkenone unsaturation ratios in all three cores, CH84-14, CH84-04 and SU81-18, in order to quantify SST variations and better interpret delta-O18 variations. The alkenone SST records clearly show that the prominent delta-O18 minimum which occurred off Japan between 15,000 and 13,000 cal-yr-BP is not due to a temperature increase. Consequently, the subsequent delta-O18 increase between 13,000 and 11,500 cal-yr-BP should not be used as an example for a far reaching Younger Dryas cooling. Further support for the low salinity layer hypothesis comes from the consideration of vertical gradients based on delta-O18 and delta-C13 of both benthic and planktonic foraminifera and of ventilation ages reconstructed by C14.