The Northwestern Atlantic Surface and Deep Water Variability at 400 Yrs Resolution Over the Last 150 Kyrs and the Evolution of the Laurentide Ice Sheet

L.Labeyrie Internet: laurent.labeyrie@lsce.cnrs-gif.fr; Leclaire H.; Cortijo E. (L.S.C.E., CNRS-CEA, Av. de la Terrasse, 91198 Gif sur Yvette Cedex, France); Auffret G. (IFREMER, BP70, 29260 PlouzanT, France)

Pastouret and coauthors (Can. J. Earth Sci. 12, 1019-1035, 1975) have shown that the sediment core CH 69-K09 (41045'N 47021'W 4100m water depth), contains a succession of ice rafted events (IRD) which mark the oscillations of the Laurentide ice sheet over the last 160 kyrs. The core is located at the Northern boundary of the Gulf Stream, at the South East of the New Foundland margin. It is therefore ideally suited to study the relationship between the evolution of the Laurentide ice sheet and the deep thermohaline circulation. We have resampled the core about each 3 cm, measured d18O and d13C foraminiferal isotopes in the benthic species C. wuellerstorfi and U. peregrina and the planktic species G. bulloides to follow the evolution of the transitional waters and N. pachyderma s. for polar waters, Changes in IRD and in the distribution of planktic foraminifera species (for estimation of the SST) have been analysed simultaneoulsy. The time scale is constrained by 21 foraminiferal 14C AMS dates and isotopic correlation with SPECMAP. The records are of exceptional quality, with a mean sedimentation rate around 10 cm/kyr. All the large IRD peaks, known as Heinrich events, are present. These peaks present the same structure as described in Bond and Lotti (1995) much further east, with a well defined enrichment in Islandic volcanic ash at the bottom of each layer. Input of isotopically light melt water is observed for all the Heinrich events in both the planktic and benthic foraminiferal records. Benthic foraminifera Cibicides wuellerstorfi d13C shows simultaneously well defined decreases of ventilation of the deep water.

The records present also a well defined variability at the thousand years scale corresponding probably to the D/O variability of the Greenland ice records. The Laurentide ice sheet margin (as evidenced by theIRD record) does not seem to react significantly at these rapid changes, in opposition to what is observed in the Irminger Basin (Elliot et al. submitted) and south of Island (Bond and Lotti 1995). The SST, planktic foraminifera d18O and benthic d13C show however well defined oscillations, proving that changes in heat transfer by the Gulf Stream and deep thermohaline circulation were involved in these climatic oscillations.