by M. Weinelt 1, M. Sarnthein 1, E. Jansen 2, H. Erlenkeuser 3 and H. Schulz 1

Transcription

1 Younger Dryas cold spell and a Holocene-style circulation in the Nordie Seas by M. Weinelt 1, M. Sarnthein 1, E. Jansen 2, H. Erlenkeuser 3 and H. Schulz 1 I Geologisch-Paläontologisches Institut und Museum. Universität Kiel, Olshausenstr D Kiel, Germany 2 Department ofgeology. University of Bergen. Allégaten 41. N-5007,Bergen. Norway 3 C u -Labor. Institut für reine und angewandte Kernphysik, Universität Kiel. Leibnizstr. D- Kiel. Germany Reconstructions of the Younger Dryas circulation in the Nordic Seas may contribute in various ways to a better understanding of this phase of abrupt climatic setback, by the reconstruction of (1) the heat flux into high latitudes, and (2) of potential meltwater fluxes th at re duce the intensity of the Atlantic salinity conveyor belt. More than hundred stabie isotope records of Neogloboquadrina pachyderma sin., 19 ofwhich are AMS 14-C-dated, provide a detailed record ofthe various water masses in the Nordic Seas and in the northeastern North Atlantic during the last deglaciation, ifmapped as time slices. A detailed documentation ofthe stabie isotope data base and datings will be published in Sarnthein et al. (1994b). Paleosalinity and -density estimates we re derived from combined and sea surface temperature-records based on planktonic foraminifera assemblages (fig. 1), and serve to detect paleodensity gradients, driving the thermohaline circulation. Unexpectedly the and 8 13 C-distribution pattern reconstructed for the Younger Dryas cold spell does not differ significantly from the modern one. In the modern pattern low values decrease slowly from 0.7 west ofireiand to 3.00/00 at the Fram Strait, follow meridional patterns at the east side of the Nordic Seas, and mirror the inflow ofthe Norwegian Current over the Iceland Faroe-ridge. The low saline, cold water masses of the East Greenland Current on the west side of the basin are documented by relative heavy oxygen isotopes ( %0). Maximum carbon isotope values ( /00) occur in combination with maximum 8 13 C-values ( /00) north of Iceland and north of Jan Mayen reflect the dense and weil ventilated watermasses of the Arctic gyres, where deepwater formation takes place. Analogous structures with similar isotopic composition (if values are 109

2 meanglobal 10 '}f> ')11 ~ ~ ~ a os 8. E.l!l." ~I) sali'1ly 1%0] Fig. I. Younger Dryas water masses from the Nordie Seas (dots) and the northeast Atlantic (squares) in the temperature/salinity/density field (after Labeyrie et al., 1992). Isopygnals are solines. Surface water salinity has been derived from values of N. pachyderma sin., corrected for summer SSTs, following the method of Duplessy et al. (1991). Positions of NADW, NAIW, and NSDW were derived from benthic ó l8 0-values (after Labeyrie et al. 1992). ó l8 0-fractionation 1ines were based on a Younger Dryas mean global sa1inity of 35.35%0 and a mean ocean composition of 0.870/00, assuming a modern like slope. corrected for the global ice volume effect of 0.63%0, Fairbanks, 1989) are found for the Younger Dryas scenario (fig. 2 a and b). Nevertheless the Younger Dryas versus modern anomaly map (fig. 3) shows a maximum anomaly of 1-1.5% in the eastern Nordic Seas, indicating that the advection of heat into high latitudes was strongly reduced then and the arctic gyres we re extended/shifted towards SE to the central Nordic Seas and east of Iceland. Summer SSTs decrease from 13 C in the northeast Atlantic to only 3.5 C off middle Norway with a much stee per gradient than today (15-16 to 8-1O C). In the northeast Atlantic, the area close to west Ireland, was also affected by a distinct cooling, while further west temperatures hardly differed from Holocene ones (fig. 4). Heavy Ö 13 C values of /00 (corresponding to 1.23 to 1.420/00 in DIC, Labeyrie and Duplessy, 1985) occured in the central basin and northeast of Iceland which match precisely the Ö13C-composition of the deepwater outflow ( %0) into the North-Atlantic as traced by Sarnthein et al. (1994) by means of benthic foraminifera. Thus we consider this water mass as a potential source area for Younger Dryas deepwater. The Younger Dryas circulation mode contrasts markedly the and 8\3C- 110

3 Fig. 2. Distribution of ÓIHO (a) and Ó 13 C (b) patterns of N. pachyderma sin. in the NOTdie Seas and in the northeast Atlantic during the Younger Dryas. 111

4 Fig. 3. Younger Dryas vs modern anomaly map (difference values have been corrected for the global ice volume effect of 0.63%0 (Fairbanks et al., 1989». patterns ofthe LGM ( C_yrs BP). During the LGM, the central Nordic Seas we re marked by a highly saline water mass which probably was fed by centra I N-Atlantic water via the Denmark Strait (paleo-irminger Current), whereas in the northeast Atlantic an extended meitwater lense inhibited the incursion ofnorth Atlantic water over the Iceland Faeroe ridge (Weinelt, 1993; Sarnthein et al, 1994b). Deepwater formation then probably took place in the central North Atlantic, where the most saline surface water masses have been documented (Duplessy et al, 1991). Unfortunately only few SST-records of the central Norruc Seas have a time resolution sufficient to resolve the short Younger Dryas event and allow for quantitative estimates of paleosalinity and density, following the methods described by Duplessy et al. (1991) and Labeyrie et al. (1992). To avoid a 112

5 YOUNGER DRYAS SST (summer) Fig. 4. Distribution of summer sea surface temperatures in the Nordie Seas and in the northeast Atlantic during the Younger Dryas cold spell, reconstructed on planktonic foraminifera assemblages using the SIMMAX 26 modern analogue approach (Pflaumann et al., 1994). smoothing of the signals by bioturbational mixing, we evaluate in our paleodensity reconstruction only records with sedimentation rates exceeding 3 cm per 1000 years (tabie 1). We assume that if the Younger Dryas event lasted for 1200 calendar years (according to the chronology of the Greenland ice cores (Alley et al., 1993; Taylor et al., 1993), it should leave areliabie signature in this records. Based on the assumption that summer sea surface temperatures amounted to C also in the central Nordic Seas where the heaviest stabie isotope values occur (fig. 2 and 3), here the watermass could reach a density high enough (ao = ) to form North Atlantic deepwater if cooled during winter (fig. 1). In contrast to the major meltwater event to C_yrs ago, wh en a 113

6 Table I. Co re locations, sedimentation rates, Younger Dryas values (N. pachyderma sin.), SST-values and reconstructed paleosalinity-values used for the density reconstruction in fig. 2. Core no Location Sedimentation SSTsummer Salinity 1%01 latitude rate during (N. pachy. sin.) IOC) longitude Termination I l%ovs PDBI Icm/kyl 'N 13.9 Il o 15'W 'N 'E 'N 'E 'N 'E NA 'N 'E SU 'N 'W V 'N 'W V 'N 'W Central Nordic Seas (assumed) huge meltwater lense dominated the Eastern Nordie Seas and caused an inversion of the surface circulation (Sarnthein et al., I994b). Little evidence is found for meltwater flux during the Younger Dryas. Small amounts of meltwater are registered in the northern Norwegian Sea, where low values occured at low SSTs and moreover in the region immediately off Ireland (fig. 1, 2,4). We thank the Sonderforschungsbereich 313 for supporting this study. REFERENCES AlIey, R.B., D.A. Meese, e.a. Shuman, A.J Gow, K.e. Taylor, P.M. Grootes, J.w.e. White, M. Ram, E.D. Waddington, P.A. Mayewski and G.A. Zylinski - Abrupt increase in Greenland snow accumulation at the end ofthe Younger Dryas. Nature 362, (1993). Duplessy, Je., L. Labeyrie, A. Juillet-Lec\erc, F. Maitre, J Duprat and M. Sarnthein - Surface salinity reconstruction of the North Atlantic Ocean during the last glacial maximum. Oceanologica Acta 14, (1991). Fairbanks, R.G. - A year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature 143, (1989). Labeyrie, L.D. and J.e. Duplessy - Changes in the oceanic 13C/12C ratio during the last years: high latitude surface water records. Paleogeogr., Paleoclimat, Paleoecol. 50, (1985). Labeyrie, L.D., Je. Duplessy, J Duprat, A. Juillet-Lec\erc, 1. Moyes, E. Michel, N. Kallel and N. Shackleton - Changes in the vertical structure of the North Atlantic Ocean between glacial and modern times. Quat. Sci. Revi. 11, (1982). 114

7 Pfiaumann, u.. 1. Duprat, C. Pujol and L.D. Labeyrie - SIMMAX, a transfer technique to de duce Atlantic Sea Surface Temperatures from planktonic foraminifera - the EPOCH approach, (manuscript submitted to Paleoceanography). Sarnthein, M., K. Winn, S.1.A. Jung, l.e. Duplessy, L.D. Labeyrie, H. Erlenkeuser and G. Ganssen - Changes in East Atlantic Deep Water Circulation over the Last Years: An Eight Time-Slice Record. Paleoceanography 9, (1994). Sarnthein, M., E. lansen, M. Weinelt, M. Arnold, 1.-C. Duplessy, H. Erlenkeuser, M. Maslin, T. Johannessen, N. Koe, A. Flatey, G. Johannessen, S. Jung, U. Pfiaumann and H. Schulz - Variations in Atlantic surface ocean paleoceanography, N: A time-slice record of the last years. 53 pp. (manuscript submitted to Paleoceanography). Taylor, K.e., G.w. Lamorey, G.A. Doyle, R.B. Alley, P.M. Grootes, P.A. Mayewski, 1.W.e. White and L.K. Barlow - The 'flickering switch' of late Pleistocene c1imate change. Nature 361, (1993). WeineIt, M. - Veränderungen der Oberflächenzirkulation im Europäischen Nordmeer während der letzten Jahre - Hinweise aus stabilen Isotopen.- Berichte aus dem Sonderforschungsbereich , 106 pp. (1993). 115

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