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. 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 818 0and sea surface temperature-records based on planktonic foraminifera assemblages (fig. 1), and serve to detect paleodensity gradients, driving the thermohaline circulation.
Unexpectedly the 818 0- and 813 C-distribution pattern reconstructed for the
Younger Dryas cold spell does not differ significantly from the modern one. In
the modern pattern low 818 0-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 813 C-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 818 0-values are
109
meanglobal
10
'}f>
')11
~
~
~
aos
•
."
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 15 18 0 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 ól8 0-values (after Labeyrie et al. 1992). ól8 0-fractionation 1ines
were based on a Younger Dryas mean global sa1inity of 35.35%0 and a mean ocean 1518 0 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 818 0 Younger
Dryas versus modern anomaly map (fig. 3) shows a maximum anomaly of
1-1.5%0818 0 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 Ö13 C 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 818 0 and 8\3C-
110
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.
111
Fig. 3. Younger Dryas vs modern 818 0 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
112
YOUNGER DRYAS SST (summer)
8.7
•
8.7
•
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
113
Table I. Co re locations, sedimentation rates, Younger Dryas 818 0-values (N. pachyderma sin.),
SST-values and reconstructed paleosalinity-values used for the density reconstruction in fig. 2.
Core no
Location
latitude
longitude
Sedimentation
rate during
Termination I
Icm/kyl
16396
61 °52'N
Il o 15'W
72°03'N
07°19'E
67°05'N
02°55'E
66°40'N
04°55'E
55°30'N
14°35'E
60 35'N
22°05'W
54°02'N
16°08'W
64°47'N
29°34'W
13.9
17730
23071
23074
NA 87-22
SU 90108
V 23-81
V 28-14
Central
Nordic Seas
0
8180
(N. pachy. sin.)
l%ovs PDBI
SSTsummer
IOC)
Salinity 1%01
3.39
7
33.82
3.65
3.4
34.09
4.00
3.5
34.82
3.96
3.8
34.56
3.2
6.7
34.46
3.27
8.3
35.45
3.07
6.3
33.97
3.52
4.25-4.4
7.6
3.5-3.8
(assumed)
35.65
35.35-35.77
3.8
6.8
21.2
8.2
3.1
12.7
6.9
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 818 0-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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