Younger Dryas cold spell and a Holocene-style circulation in the Nordie Seas

by M. Weinelt1 , M. Sarnthein1, E. Jansen2, H. Erlenkeuser3 and H. Schulz1

I Geologisch-Paläontologisches Institut und Museum. Universität Kiel, Olshausenstr.40-60. D-24118 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 8180-and sea surface temperature-records based on planktonic foraminifera assem­blages (fig. 1), and serve to detect paleodensity gradients, driving the thermo­haline circulation.

Unexpectedly the 8180- and 813C-distribution pattern reconstructed for the Younger Dryas cold spell does not differ significantly from the modern one. In the modern pattern low 8180-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 Iceland­Faroe-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 (3.2-3.5%0). Maximum carbon isotope values (3.5-3.70/00) occur in combination with maximum 813C-values (0.7-0.90/00) 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 8180-values are

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meanglobal

10

'}f>

')11

~

~ ~ a • os

8. ." E

~I) .l!l

0

·2

33 33.5 345 35 355 36 36.5

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 15180 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 ól80-values (after Labeyrie et al. 1992). ól80-fractionation 1ines were based on a Younger Dryas mean global sa1inity of 35.35%0 and a mean ocean 15180 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 8180 Younger Dryas versus modern anomaly map (fig. 3) shows a maximum anomaly of 1-1.5%08180 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 Ö13C values of 0.4-0.590/00 (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 (1.2-1.3%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 8180 and 8\3C-

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Fig. 2. Distribution of ÓIHO (a) and Ó13C (b) patterns of N. pachyderma sin. in the NOTdie Seas and in the northeast Atlantic during the Younger Dryas.

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Fig. 3. Younger Dryas vs modern 8180 anomaly map (difference values have been corrected for the global ice volume effect of 0.63%0 (Fairbanks et al., 1989».

patterns ofthe LGM (18 000-15 000 14C_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

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8.7 •

8.7 •

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 3.5-3.8°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 = 28.4-28.8) to form North Atlantic deepwater if cooled during winter (fig. 1).

In contrast to the major meltwater event 14200 to 13200 14C_yrs ago, wh en a

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Table I. Co re locations, sedimentation rates, Younger Dryas 8180-values (N. pachyderma sin.), SST-values and reconstructed paleosalinity-values used for the density reconstruction in fig. 2.

Core no Location Sedimentation 8180 SSTsummer Salinity 1%01 latitude rate during (N. pachy. sin.) IOC) longitude Termination I l%ovs PDBI

Icm/kyl

16396 61 °52'N 13.9 Il o 15'W 3.39 7 33.82

17730 72°03'N 3.8 07°19'E 3.65 3.4 34.09

23071 67°05'N 6.8 02°55'E 4.00 3.5 34.82

23074 66°40'N 21.2 04°55'E 3.96 3.8 34.56

NA 87-22 55°30'N 8.2 14°35'E 3.2 6.7 34.46

SU 90108 600 35'N 3.1 22°05'W 3.27 8.3 35.45

V 23-81 54°02'N 12.7 16°08'W 3.07 6.3 33.97

V 28-14 64°47'N 6.9 29°34'W 3.52 7.6 35.65

Central 4.25-4.4 3.5-3.8 35.35-35.77 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 8180-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.

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