JOURNAL OF QUATERNARY SCIENCE (1995) 10 (4) 385-390 0 1995 by John Wiley & Sons, Ltd.

CCC 0267-81 79/95/04038546

Rapid Communication

Last interglacial and Holocene climatic development in the Norwegian Sea region: ocean front movements and ice-core data HANS PETTER SEJRUP, HAFLlDl HAFLIDASON and DORTHE KLITCAARD KRISTENSEN Department of Geology, University of Bergen, N-5007 Bergen, Norway SICFUS J. JOHNSEN Niels Bohr Institute, Department of Geophysics, University of Copenhagen, Haraldsgade 6, DK-2200 Copenhagen N, Denmark, and Science Institute, Department of Geophysics, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland

Sejrup, H. P., Haflidason, H., Kristensen, D. K. and Johnsen, S. 1. 1995. Last interglacial and Holocene climatic development in the Norwegian Sea region: Ocean front movements and ice-core data. lournal of Quaternary Science, Vol. 10, 385-390. ISSN 0267-81 79

Received 16 June 1995 Accepted 22 August 1995

ABSTRACT: Planktonic foraminiferal evidence suggests that the ocean front systems between Polar and Atlantic surface waters in the Norwegian Sea generally were located closer to Greenland during Oxygen isotope Substage 5e than in the Holocene. During both these periods oscillations have occurred in the position of the fronts. In the western Norwegian Sea region, the substage 5e influence of warm Atlantic waters was interrupted by a return to polar conditions. These findings support both ice-core data and evidence from Europe that the last interglacial was a period of rapid climatic shifts. Journal of Quaternary Science

KEYWORDS: interglacial; planktonic foraminifers; palaeoceanography; climatic instability.

Norwegian Sea; GRIP ice-core; deep-sea sediments; Eemian interglacial; Holocene

Investigations of Greenland ice-cores have had a large impact on our knowledge of Late Quaternary climate change and on the focus of palaeoclimate research (e.g. Johnsen et a/., 1992a; Dansgaard et a/., 1993; GRIP members, 1993; Grootes et a/., 1993; Taylor et a/., 1993). Stratigraphic data and variations in climate proxies were, in the past, frequently related/assigned to changes in solar insolation, even when observations show that both the number of events and rates of change often do not match the insolation record faithfully. It has been demonstrated recently (within the limits of resolution of the geological archives) that there i s good correspondence between high-frequency shifts in the isotope signal in the Greenland ice-cores (Dansgaard-Oeschger events) and sea-surface conditions in the North Atlantic and Norwegian Sea through the last glacial stage, as indicated by planktonic foraminiferal records and ice-rafted detritus (IRD) (Bond et a/., 1993; Fronval et a/., 1995). This i s reasonable because the source and tracks of moisture to Greenland are related very closely to the oceanic front systems in the Norwegian Sea and North Atlantic.

The GRIP and the GISP2 ice-cores from Summit (Fig. 1) show an identical pattern in the oxygen isotope record back to ca. 100 000 yr BP (Johnsen et a/., 1992a; 1995). However, the interpretation of the GRIP ice-core record beyond this has been disputed, and it has been suggested that the isotope

pattern of the older ice is heavily influenced by ice tectonics (Boulton, 1993; Alley et a/., 1995). One crucial point i s whether the last interglacial (Oxygen Isotope Substage 5e) was interrupted by several rapid changes to a colder climate, as opposed to the Holocene which shows a very uniform isotope signal in both Summit ice-cores. It has been demon- strated, utilizing the same proxies as Bond et a/ . (1993) used for the Weichselian, that there are no, or only minor, changes through the Eemian in central North Atlantic sea-surface conditions (McManus et a/., 1994). From the northeastern part of the Norwegian Sea, Cortijo et a/. (1994) showed that there was an early substage 5e warm period (strong influx of Atlantic water) followed by a cooling. In Field et a/. (1994) and Thouveny et a/. (1994), and now recently in Larsen et a/. (1995) and Seidenkratz eta/. (l995), evidence is presented that the terrestrial and coastal climate in Europe underwent the GRIP-type of rapid changes through the last interglacial. Citing the low Ca ion content (which reflects dust in the atmosphere as a result of exposed shelf areas) in the part referred to as substage 5e of the GRIP record, Larsen et a/. (1 995) argued that this part represents interglacial conditions, even if the number and duration of changes in isotope composition through the interglacial may have been obscured by tectonics.

This paper focuses on the last interglacial and Holocene

386 JOURNAL OF QUATERNARY SCIENCE

Figure 1 Location map sites mentioned in the text, and the main ocean surface currents and oceanographic fronts.

climatic and oceanographic changes in the southern Norweg- ian Sea region, and to the extent to which they are reflected in the GRIP ice-core record. We present new data from a deep-sea sediment core located close to Greenland at approximately the same latitude as Summit, and discuss these together with existing data on sea-surface conditions along a section extending from western Norway to the coast of East Greenland. The present-day inflow of Atlantic water to the Norwegian Sea is clearly reflected in the high levels of carbonate and the presence of non-polar planktonic Foramini- fera in the surface sediments (Kellogg, 1976). In the following we use these two variables as indicators of the position of the boundary between Polar and Subpolar water masses. It is usually considered that the presence of more than 5% of non-polar planktonic Foraminifera suggest an input of Atlantic water with a summer temperature above 4°C (Be and Tolderlund, 1971 ; Johannessen et a/., 1994).

Core HM68-22 (70’38.9” 15’03.8’W) was raised from 1363 rn water depth off Jameson Land on the East Greenland coast (Fig. 1). This area is situated close to the present boundary between. Polar water masses in the East Greenland Current and the Arctic water masses (mixture of Polar and Atlantic water masses) on the Iceland Plateau (Swift and Aagaard, 1981; Johannessen 1986). In this area the weak influence of Atlantic water is reflected by the low content of carbonate in the surface sediments and less than 5% of subpolar planktonic Foraminifera. This area i s thus very sensitive to movements in the ocean front systems between cold and warm waters in the Norwegian Sea.

Core HM68-22 is 210 cm long and was obtained by a 6.4 cm diameter gravity corer. The chronology of the upper 30 cm of the core is based on five AMS dates (Table 1). Figure 2 shows the grain size distribution, CaCO,, total organic carbon (TOC) and stable isotope stratigraphy of the core. The stable isotope analyses were performed on samples of Neogloboquadrina pachyderrna (sinistral). The signatures of the TOC and carbonate curves are very similar to the pattern obtained in numerous cores spanning the period from

Table 1 Radiocarbon dates used to provide an age model for gravity core HM68-22. The AMS dated samples are corrected for a reservoir age of 440 y

Laboratory number Depth (cm) Species Age (yr BPI

N. pachyderrna

N. pachyderrna

N. pachyderrna

N. pachyderrna

N. pachyderrna

TUa-102 1 &2.5 (sinistral) 1230 f 185

TUa-101 9.0-9.8 (sinistral) 6295 f 155

TUa-100 16.5-1 7.4 (sinistral) 16725 ? 215

TUa-99 23.6-24.4 (sinistral) 18075 2 275

TUa-98 29.6-31 .O (sinistral) 20265 ? 290

RAP1 D COMMU NlCATlON 387

HM68-22

Depth (cm) 50 100 150 200

1 .o I . . . . I . . . . , .

n v) W

0 .o .6

m ’ V -0.5 k? n

* m

-1.0 - I I

- 1

-2

-3

-4

-5

30

25

h

20 €9 2 v

0 50 lo0 1 so 200 A

Depth (cm)

Figure 2 Core HM68-22. Down-core variation in grain size distribution, CaCO,, TOC and stable oxygen and carbon isotope composition in Neogloboquadrina pachyderrna (sinistral). The chronological model of the core is based on a combined record of the AMS radiocarbon absolute chronology and on the stable isotope record. The location of stage 3 4 boundary is obtained using the TOC and the CaCO, wt. percent variation. The ages of the oxygen isotope stage boundaries are after Martinson et a/ . (1987).

Oxygen Isotope Stage 6 to 7 in the Norwegian Sea (Kellogg, 1980; CLIMAP Project Members, 1984; Sejrup et a/., 1989). Characteristic features are the high amount of CaC03 in substage 5e, also reflected in the grain size distribution, and high TOC values close to the stage 4-5 boundary. Core HM68-22 is different from more easterly and southerly positioned cores in its low content of carbonate in the Holocene and a conspicuous double peak in substage 5e. The stable oxygen isotope record on planktonic Foraminifera is generally noisy for part of this time span in this region, reflecting possible changes in salinity superimposed on ice- volume and temperature signals. However, by comparing

both the carbon and the oxygen stable isotope curves from core HM68-22 with other records from the North Atlantic and the Norwegian Sea (Kellogg, 1980; Sejrup et a/., 1989; Baumann et a/., 1995; Nam et a/., 1995), we have tentatively located the stage &5e, 5e-5d and 5 a 4 boundaries in the core (Fig. 2).

By using the chronology of Martinson et al. (1987) for these isotope stage boundaries, and assuming uniform sedimentation rates throughout isotope stage 5, we have established a chronology for the lower part of the core. It should be emphasized that, owing to low and possibly variable sedimentation rates, the events recorded within stage

388 JOURNAL OF QUATERNARY SCIENCE

5e are not precisely dated. However, no signs of hiatus or disturbance were found in the core. In Fig. 3A the carbonate data for the lower part of the core are plotted on this time- scale together with the isotope record from the GRIP ice- core, and in Fig. 39 the number of planktonic Foraminifera per 1OOg sediment and the content of N. pachyderma (sinistral) are plotted on the same time-scale. Because of the low percentages of subpolar Foraminifera (non-N. pachyderm (sinistral)) more than 600 individuals per sample were counted through this part of the core.

The fact that carbonate content in substage 5e (Eemian) i s higher than in the Holocene suggests that the peak Eemian was more strongly influenced by Atlantic water in this area.

This is also reflected in data from coastal sections on jameson Land (Fig. I ) , which suggest summer temperatures 3 4 ° C higher during parts of the Eemian than during the Holocene (Funder et a/., 1995). The two very high Eemian carbonate peaks in core HM68-22 make this core particularly interesting relative to the GRIP ice-core record. From Fig. 3B it is evident that the carbonate content is partly reflected in the number of planktonic Foraminifera. The lowest percentages of subpolar planktonic Foraminifera are found prior to the first and between the two stage 5e foraminiferal maxima (Fig. 39). It should be emphasised that the number of subpolar Foramini- fera i s very low, and the only firm conclusion we can draw is that the sea-surface summer temperatures were probably

-30 30 A

98

N

00 9

96 A n

n 5 m W

d 94

i

92

- 25

- 2 0 q

- l5 .!3

. o "

00

8 - 1 0 % " - 5

- 0

100 110 120 130 140

Age ka BP

Figure 3 Comparison of the oxygen stable isotope (d '"0) record obtained from the GRIP Summit ice-core, Greenland, and the planktonic foraminferal record from deep-sea core HM68-22 from Norwegian Sea region. (A) Substage 5e oxygen isotopes in the GRIP ice-core according to the time-scale developed for that core (Dansgaard et a/., 1993). The calcium carbonate content (shaded) for substage 5e in the deep-sea core HM68-22 is plotted on the same time scale as the GRIP isotope record. (6) Relative frequency of the planktonic foraminiferal species Neogloboquadrina pachyderma (sinistral) plotted on the same time-scale for substage 5e as well as the concentration of planktonic Foram in ifera (shaded).

RAPID COMMUNICATtON 389

below 4°C for most of the time (Be and Tolderlund, 1971; Johannessen et a/., 1994). However, the fluctuation in the number of specimens suggests that the distance to the warm Atlantic water, which has been recorded in several cores situated further east on the Iceland Plateau (CLIMAP Project Members, 1984), varies throughout stage 5e. This suggests that in this part of the Norwegian Sea the ocean front system oscillated along an east-west axis throughout the last interglacial, and this may be reflected in the changes in isotopic composition of the stage 5e record in the GRIP ice- core (Larsen et a/., 1995).

Koc Karpuz and Jansen (1 992) and Haflidason et a/. (1 995) have shown (utilizing diatoms and planktonic Foraminifera respectively) that the Holocene oceanographic front system also varied considerably in the southern Norwegian Sea. Between ca. 10 ka and 5 ka these systems were situated much farther to the west than in the late Holocene. This is illustrated by variations in the abundance of subpolar plank- tonic Foraminifera along an east-west transect (Fig. 4). The changes in inflow of Atlantic water to the Norwegian Sea during the Holocene correspond to changes in several other climate proxies in northwest Europe, but none are reflected in the Summit ice-cores (Larsen et a/., 1995). This could be explained by the topographic effect that the build-up of Holocene ice and subsequent lowering at Summit had on the isotopic composition of the ice (johnsen, unpublished), for in the Renland section on the east coast of Greenland a

Planktonic forams Subpolar (%)

0 20 40 60

Figure 4 The amount of subpolar planktonic Foraminifera along an east-west transect between Norway and Greenland for the last 20 ka BP. The time-scale is based on a number of AMS dates and regional tephra layers (Haflidason et a/., in preparation).

Holocene change is reflected in the isotopic composition of glacial ice (Johnsen et a!., 1992b).

During the Late-glacial the ocean front oscillations occurred in a more southerly and easterly part of the Norwegian Sea relative to our core. Lehman and Keigwin (1992) and Haflidason et a/. (1995) have shown a very good correspon- dence between oscillations in the influx of Atlantic water (reflected in planktonic foraminifera1 data) from high-resolution records (10-1 5 ka) from the Troll area in the North Sea (Fig. 1 ) and the shifts in isotopic composition on Greenland.

These findings suggest that in the North Atlantic area the high-frequency shifts in isotope composition in Greenland ice-cores reflect movements in the ocean front system between Polar and Atlantic water masses. During both the Holocene and the Eemian interglacial periods the oceanic fronts migrated close to Greenland and therefore the stratigraphic record from this area corresponds most closely to the ice-core stratigraphy for these periods. During glacial episodes the oscillations are found further to the east in the Norwegian Sea (Fronval et a/., 1995, Haflidason et a/., 1995) and in the North Atlantic (Bond et a/., 1993; McManus et a/., 1994). The data from the Norwegian Sea region indicate a close correspondence between movements of the ocean front system and the atmospheric conditions over Greenland in the Holocene and Eemian interglacials, as well as during the last glacial stage. They also support evidence from the GRIP ice-core which shows that in the Norwegian Sea area neither the Eemian nor the Holocene have been periods of very stable climatic conditions (Johnsen, in preparation).

Acknowledgements This paper was made possible by funding to H. Haflidason and H. P. Sejrup from the Norwegian Research Council (NFR) through the KLIMBRE programme, by funding to H. P. Sejrup through the ENAM (European Continental Margin; sediment pathways, processes and fluxes) programme (ENAM is funded by the EU Commission through the MAST II programme), and to S. Johnsen through the international Greenland Ice-core Project (GRIP) organised by the European Science Foundation.

Sincere thanks to K. Flood and K. J. Karlsen for providing us with some of the petrographic analyses, to V. C. Hope for the planktonic Foraminifera analyses, to S. Gulliksen of the Trondheim Radiocarbon Laboratory for providing atomic mass spectroscopy dates at the Uppsala Accelerator Facility, to R. Saras and 0. Hansen for mass spectrometer operation, to E. L. King for correcting the English language and finally to Dr William Austin and Dr Karen Luise Knudsen for critical and valuable reviews.

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