Late warming and early cooling of the sea surface in the Nordic seas during MIS 5e (Eemian Interglacial)

Quaternary Science Reviews 22 (2003) 809–821

Late warming and early cooling of the sea surface in the Nordic seas during MIS 5e (Eemian Interglacial) Tine L. Rasmussena,*, Erik Thomsenb, Antoon Kuijpersc, Stefan Wastega( rdd a

The University Courses on Svalbard (UNIS), PO Box 156, N-9171 Longyearbyen, Norway b ( Department of Earth Sciences, University of Aarhus, DK-8000 Arhus C, Denmark c Geological Survey of Greenland and Denmark, Thoravej 8, DK-2400 Copenhagen NV, Denmark d Department of Physical Geography and Quaternary Geology, Stockholm University, S-106 91 Stockholm, Sweden Received 12 March 2002; accepted 1 December 2002

Abstract Geochemical identification of a tephra layer found in two cores from the NE Atlantic Ocean and the SE Norwegian Sea, respectively, and dated to 127 ka BP has enabled us to obtain a precise correlation across the Iceland–Scotland Ridge at the Marine Isotope Stage (MIS) 6/5 transition. The direct distance between the two cores is only about 200 km: South of the Iceland–Scotland Ridge, sea surface temperatures rose abruptly at 130 ka BP at the onset of MIS 5e and at least 2–3000 years earlier than north of the ridge. Maximum sea surface temperatures south of the ridge occurred during this initial phase of MIS 5e, when temperatures in the Nordic Seas were still low. North of the ridge, the sea surface warmed rapidly at 127 ka BP. Correlations between the North Atlantic records and the Eemian of Northwest Europe tentatively indicate that the initial phase of MIS 5e correlates with the early part of the Eemian characterised by a warm, continental type of climate. The period after the warming of the Nordic seas corresponds to the slightly cooler and more oceanic middle Eemian interval in Europe. The sea surface temperatures fell gradually north of the ridge during the later part of MIS 5e and they were low during MIS 5d–5a. South of the ridge the temperatures remained relatively high. The data shows that there was no outflow of deep water from the Norwegian Sea during the later part of MIS 6. Outflow began at the MIS 6/5 transition simultaneous with the sea surface warming south of the ridge. r 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction The Greenland–Scotland Ridge separates the Nordic seas from the North Atlantic Ocean and divides the region into two very different sedimentary and faunal provinces (see references in Rasmussen et al., 1999, 2002). Today, the ridge forms a sill over which warm Atlantic surface water flows northward into the Nordic seas, while cold deep water flows southward into the North Atlantic Ocean (Fig. 1). Numerous deep-sea records indicate that during MIS 5 (Eemian and early Weichselian) the ridge formed an important oceanographic barrier influencing surface water conditions. Thus, south of the ridge, sea surface temperatures were continuously relatively high (e.g. Ruddiman and McIntyre, 1979; McManus et al., 1994; Chapman and Shackleton, 1998; Labeyrie et al., 1999; Oppo et al., *Corresponding author. Tel.: +47-7902-3331; fax: +47-7902-3301. E-mail address: [email protected] (T.L. Rasmussen).

2001), whereas north of the ridge, they were only high during a short interval in MIS 5e (e.g. Kellogg, 1976; Cortijo et al., 1994; Bauch, 1997; Fronval and Jansen, 1997; Rasmussen et al., 1999). Time-correlations of the early Weichselian deposits in the North Atlantic realm are generally based on biostratigraphy or variations in oxygen isotope values. However, correlations across the Greenland–Scotland Ridge are often hampered by the different environmental conditions in the North Atlantic and Nordic seas. For example, salinity changes caused by meltwater input are much larger north than south of the ridge, greatly influencing the d18 O records (e.g. Sarnthein and Thiedemann, 1990; Fronval and Jansen, 1997; Lototskaya et al., 1998; Bauch et al., 1999; Cortijo et al., 2000). Tephra markers are free of such problems and they are known to provide ideal correlation points over larger geographical areas (e.g. Ruddiman and Glover, 1972; Mangerud et al., 1984; Sejrup et al., 1989; Lackschewitz and Wallrabe-Adams, 1997; Fronval et al., 1998;

0277-3791/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0277-3791(02)00254-8

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in northwestern Europe as deduced from changes in the vegetation patterns.

2. Material and methods The study is based on two cores, MD95-2009 from north of the Iceland–Scotland Ridge and ENAM33 from south of the Ridge (Fig. 1). The study comprises the later part of MIS 6 and the whole of MIS 5. Piston core ENAM33 was sampled at 3 cm intervals for foraminifera and analyses of ice rafted material (see also Kuijpers et al., 1998). The samples were dried and weighed and washed over 63-mm sieves. The residues were dried and weighed and subsequently dry sieved over 106 mm sieves. At least 300 specimens of planktonic foraminifera and 300–500 specimens of benthic foraminifera were counted and identified. Ice Rafted Detritus (IRD) was counted on the > 150 mm fractions. At least 300 mineral grains were counted per sample. The concentration of IRD was calculated as the number of grains per gram dry weight sediment. The oxygen isotope analyses for ENAM33 were published in Kuijpers et al. (1998). MD95-2009 was sampled at 5, 2 and 1 cm intervals. Some foraminiferal data, the oxygen isotope data, and the IRD > 150 mm analyses were published in Rasmussen et al. (1999) and Balbon (2000). The foraminifera in MD95-2009 were counted in the > 100 mm fraction. We consider the size difference of the studied fractions in ENAM33 and MD95-2009 as negligible.

Fig. 1. (a) Map of the northern part of the Atlantic Ocean and the Nordic seas (Norwegian Sea and Greenland Sea) showing the locations of the studied cores. Main surface and deep currents are indicated. The position of the Polar Front and the Subarctic Front are modified after Hurdle (1986). (b) Detail map of the Iceland–Scotland Ridge and the Faeroe Islands showing the position of the investigated cores and other cores discussed in the text. Abbreviation: I.-S. Ridge, Iceland–Scotland Ridge.

Lacasse et al., 1998; Haflidason et al., 2000; Lowe, 2001). A basaltic tephra layer found in the lower part of MIS 5e in two cores from the northeast Atlantic and southern Norwegian Sea allows for a much more accurate correlation across the ridge than previously obtained for this critical time period (Wastega( rd and Rasmussen, 2001). The purpose of the present study is, on the basis of this new marker horizon, to study changes in the surface and deep water connection between the northeast Atlantic and the Nordic seas during late MIS 6 and MIS 5. One of our goals is to obtain a more precise estimate of changes in the sea surface temperatures during the MIS 6/5e transition. The results are compared to the climatic development of the Eemian

3. Stratigraphy and age determination 3.1. Tephra layers The MIS 5e interval of MD95-2009 contains two tephra layers, a lower basaltic layer and an upper rhyolitic layer (Rasmussen et al., 1999; Balbon, 2000). In ENAM33 only a single basaltic layer has been found. Chemical analysis of the three layers show that the basaltic layers in the two cores are virtually identical and they have been described as tephra 5e-Low/Bas-IV (Wastega( rd and Rasmussen, 2001). Geochemical analyses suggest that the source of the tephra is the !ıgar volcanic system in the Eastern . Gr!ımsvotn-Lakag Volcanic Zone on Iceland (Jacobsson, 1979; Lacasse et al., 1998). The rhyolitic tephra was identified as 5eMidt/RHY previously described from cores in the Norwegian Sea (Sj^holm et al., 1991; Fronval et al., . 1998; Lacasse and Garbe-Schonberg, 2001). It has an age of about 124 ka BP based on a correlation to the SPECMAP time scale (Fronval et al., 1998). The age of the basaltic tephra is approximately 127 ka BP according to an earlier age model published in Rasmussen

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et al. (1999) and Balbon (2000) (Table 1). Thus, the basaltic tephra is present in both the Norwegian Sea and the Atlantic Ocean, whereas the rhyolitic tephra has only been observed in the Norwegian Sea. 3.2. Correlation In ENAM33 we have placed the MIS 6/5e boundary at 619 cm halfway between the heavy isotopic peak of Table 1 Age models for MD95-2009 and ENAM33 Core

Depth (cm)

Age (cal. years ka BP)

MD95-2009

1455 1535 1627 1725 1815 1830 1860 1865 1900 2250

75 84 94 105 114 124 127 128 130 135

509 551 590 619 650

105 114 127 130 135

ENAM33

811

MIS 6 and the light peak of MIS 5e in agreement with the accepted practice (Martinson et al., 1987). The boundary coincides with a rapid drop in the relative abundance of Neogloboquadrina pachyderma sinistral (s) (Fig. 2). However, the decrease in the d18 O values is difficult to use as a boundary marker north of the Iceland–Scotland Ridge due to large inputs of meltwater and extraordinary low d18 O values in the Nordic seas during late MIS 6 (Fronval and Jansen, 1997; Rasmussen et al., 1999; Balbon, 2000; Bauch et al., 2000). To correlate the MIS 6/5e boundary we instead use a characteristic change in the distribution of benthic foraminifera. In the boundary interval of ENAM33, a distinctive group of species termed the ‘Atlantic species group’ (see below) is replaced by a group dominated by Cassidulina obtusa. In the same interval of MD95-2009, the ‘Atlantic species group’ is replaced by Melonis barleeanum and Islandiella norcrossi (Fig. 2). Cassidulina obtusa, M. barleeanum, and I. norcrossi are all associated with the cold Norwegian Sea Overflow Water and the faunistic changes reflect the onset of convection in the Nordic seas (Kuijpers et al., 1998; Rasmussen et al., 1999). Therefore, the changes have to be practically synchronous on both sides of the ridge. The important 5e-low/BAS-IV ash marker occurs 29 cm above the MIS 6/5e boundary in ENAM33 and 40 cm above the boundary in MD95-2009. The identification and correlation of the MIS 5e/5d transition and of the stadials,

Fig. 2. Correlation of the upper part of MIS 6, MIS 5 and the lower part of MIS 4 in cores ENAM33 and MD95-2009. The correlation is based on variations in oxygen isotopes, relative abundance of N. pachyderma s, concentrations of IRD, relative abundance of the benthic foraminifera C. obtusa group (C. obtusa, G. subglobosa, G. praegeri, and B. pygmaea), M. barleeanum, I. norcrossi, and the ‘Atlantic species group’. Positions of interstadials (IS), stadials (S) and Heinrich events (H) are marked.

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interstadials, and marine isotope substages in the upper part of the investigated section is based on a combination of oxygen isotopes, benthic foraminifera, and IRD. Most of the units are fairly easy to identify (Fig. 2). However, some of the boundaries in the upper part of MIS 5 in ENAM33 are slightly uncertain due to the low resolution of the oxygen isotope record and the unusually thin interstadials in this interval. Furthermore, the interstadial deposits here appear to be to some extent reworked by synsedimentary currents (Kuijpers et al., 1998).

3.3. Time scale The time scale for the MIS 6/5e-5d time period for ENAM33 and MD95-2009 is constructed using the SPECMAP time scale of oxygen isotope stage boundaries accepting an age of 127 ka BP for the 5e-low/BASIV tephra (Rasmussen et al., 1999; Balbon, 2000) (Table 1). In addition, we used the 5e-Midt/RHY tephra as a fixpoint for the time scale in MD95-2009 (age 124 ka BP according to Fronval et al., 1998). Ages are averaged between the fix points (Fig. 3). The MIS 5c–5a interval was not timed because of the low resolution in ENAM33 (Fig. 2).

4. Distribution and interpretation of foraminiferal faunas 4.1. MIS 6/5 transition In ENAM33 south of the Iceland–Scotland Ridge, the polar planktonic species N. pachyderma s prior to the MIS 6/5 boundary makes up more than 50–95% of the planktonic assemblage (Figs. 3 and 4). At the onset of MIS 5e the percentage rapidly decreases to less than 5% indicating an increase in sea surface temperatures from lower than 4 to 121C or higher (Be! and Tolderlund, 1971; Kellogg, 1980; Johannessen et al., 1994; Pflaumann et al., 1996). The shift occurs 29 cm below the 5elow/BAS-IV tephra (Fig. 2). In MD95-2009 north of the ridge, the decrease in the percentage of N. pachyderma s occurs just above the ash layer (Figs. 3 and 4). The difference in the decrease of N. pachyderma s corresponds to a delay in sea surface warming between the two sites of at least 2–3000 years (Figs. 3 and 4). Note that the decrease in the amount of IRD occurs simultaneously at the two sites coinciding with the temperature increase in the Atlantic (Figs. 2 and 3). In contrast to the planktonic faunas, the composition of benthic foraminifera seems to change synchronously north and south of the ridge (Fig. 3). During H11 (Termination II), the ‘Atlantic species group’ (Rasmussen et al., 1996) dominates the benthic faunas on both

Fig. 3. Correlation of the upper part of MIS 6 and MIS 5e–c in cores ENAM33 and MD95-2009 on an age time scale. Parameters as in Fig. 2 with the omission of oxygen isotopes. Abbreviations: ‘WE0 ¼ ‘Warm Event0 :

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Fig. 4. Variation in the relative abundance of planktonic foraminifera N. pachyderma, d and s, T. quinqueloba, G. inflata (with a few O. universa) in cores ENAM33 and MD95-2009 compared with climatic parameters from The Netherlands (after Zagwijn, 1996). The comparison is based on the assumption that MIS 5e is approximately time-equivalent with the Eemian of NW Europe.

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sides of the ridge. In the southern Norwegian Sea and the north Atlantic, the ‘Atlantic species group’ is known to increase in relative abundance during stadials and Heinrich events and it often becomes the most important constituent of the faunas. The most abundant species are Sigmoilopsis schlumbergeri, Eggerella bradyi/ Tosaia hanzawaia, Bulimina costata, and Epistominella decorata/weddellensis (see Rasmussen et al., 1996). Today, these species occur at mid-depth along the west European seaboard south of the Faeroe Islands, in the Gulf of Mexico, in the Labrador Sea and in the Mediterranean (see references in Rasmussen et al., 1996, 2002). The increase in the ‘Atlantic species group’ in the southeastern Norwegian Sea during stadials and Heinrich events probably signifies the intrusion of Atlantic intermediate-depth water into the Norwegian Sea below a cold low saline layer of meltwater causing a gradual warming of the bottom water. The intrusion was possible because convection had ceased during Heinrich events and outflow from the Nordic seas stopped (Rasmussen et al., 1996, 1999). In the MIS 6/5 transitional interval the ‘Atlantic species group’ is replaced by completely different faunas (Fig. 3). North of the ridge it is replaced by a fauna dominated by Melonis barleeanum ð¼ N: zaandamaeÞ and Islandiella norcrossi. This fauna is similar to the modern fauna in the area and connected to the Norwegian Sea overflow water (Belanger and Streeter, 1980; Mackensen et al., 1985; Mackensen, 1987). South of the ridge, the replacement fauna is dominated by the Cassidulina obtusa group consisting of species representing the modern fauna in the area (C. obtusa, Globocassidulina subglobosa, Gavelinopsis praegeri, and Bolivina pygmaea) (Fig. 2) (see Rasmussen et al., 2002). Globocassidulina subglobosa is attracted to areas influenced by bottom currents (Schmiedl et al., 1997) and therefore also suggesting outflow (see above). Thus, the changes in the benthic faunas at the MIS 6/ 5 transition indicate the onset of outflow of deep water across the Iceland–Scotland Ridge and, consequently, of convection in the Nordic seas. Outflow had probably completely stopped during Termination II. Our data indicate further that the convection started simultaneously with the warming of the North Atlantic and 2– 3000 years before the sea surface temperatures rose north of the ridge. 4.2. MIS 5e MIS 5e can be subdivided into two main phases based on variations in the distribution of planktonic foraminifera: A short initial phase from ca 130–127 ka BP and a much longer main phase from ca 127–114 ka BP (Fig. 4). The initial phase corresponds to the time lag between the first warming at ENAM33 and the first warming at MD95-2009. The transition between the two

phases coincides almost exactly with the 5e-low/BAS-IV tephra (Figs. 3 and 4). The composition of the planktonic fauna at the location of ENAM33 changed only insignificantly during MIS 5e (Fig. 4). The most important species are Turborotalia quinqueloba, N. pachyderma d, Globigerina bulloides (not shown in Fig. 4), and Globorotalia inflata. The virtual absence of N. pachyderma s indicates that the sea surface temperatures generally were high. As discussed above, at the location of MD95-2009 north of the Iceland–Scotland ridge, sea surface temperatures were low during the initial phase. An abrupt drop in the relative abundance of N. pachyderma s and coeval increases in N. pachyderma d and G. inflata indicate that the summer sea surface temperatures increased rapidly from about 127 ka BP and between ca 127 and 120 ka BP the temperature difference between the two sites was probably only about 31C as it is today (Dietrich, 1969; Hopkins, 1991). From about 120 ka BP sea surface temperatures, as reflected in the relative abundance of N. pachyderma s, gradually decreased north of the ridge, whereas they remained high to the south. In ENAM33, the maximum relative abundance of N. pachyderma d and G. inflata occurs close to 128 ka BP suggesting that the highest sea surface temperature in the northeast Atlantic was obtained during the initial phase of MIS 5e, when temperatures north of the ridge were still low. Hence, a very steep temperature gradient must have existed between the northeastern Atlantic and the southeastern Norwegian seas. The benthic faunas during MIS 5e were dominated by M. barleeanum and I. norcrossi at the location of MD952009 and by the C. obtusa group at ENAM33 (Fig. 3). These species are all indicative of outflow into the north Atlantic and convection in the Nordic seas. Maximum outflow was apparently reached about 127 ka BP slightly before the sea surface temperature increase north of the ridge. The high relative abundance of M. barleeanum and I. norcrossi in MD95-2009, and of the C. obtusa group in ENAM 33 indicate that the outflow rate remained high throughout MIS 5e. 4.3. MIS 5d–5a In MD95-2009 north of the Iceland–Scotland Ridge, N. pachyderma s dominates throughout MIS 5 indicating continuously low sea surface temperatures (Fig. 2). In ENAM33, N. pachyderma s is generally rare, though increasing slightly through the successive isotope stages. Note that N. pachyderma s is always more abundant in the cold stadials than in the warm interstadials (Figs. 2 and 4). The trend indicates variable but generally decreasing sea surface temperatures through MIS 5. Temperatures at ENAM33 were always significantly higher than at MD95-2009. At the location of ENAM33, MIS 5d was almost as warm as MIS 5e. At

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MD95-2009, ice rafting began early during MIS 5d (stadial S25 in Fig. 3) and increased during each stadial or Heinrich event (Fig. 2). IRD is low in ENAM33 and almost negligible in S25. The variations in the IRD content are otherwise similar at the two sites (Fig. 2). The changes in the benthic fauna during 5d–5a follows the well known pattern from the MIS 6/5 transition and from the glacial intervals in several cores from the NE Atlantic and the SE Norwegian Sea (Rasmussen et al., 1996, 1999, 2002). Melonis barleeanum and I. norcrossi or C. obtusa and G. subglobosa dominate the interstadials, whereas the ‘Atlantic species group’ dominates the stadials and Heinrich events (Figs. 2 and 3).

5. Palaeoceanographic conditions in the NE Atlantic and Nordic seas during MIS 5 with a comparison to the Eemian of NW Europe 5.1. MIS 6/5 transition and MIS 5e The correlation of the ENAM33 and MD95-2009 records reveals that the sea surface warming at the MIS 6/5e transition was delayed with about 3000 years from the southern to the northern side of the Iceland– Scotland Ridge. Simultaneous with the surface warming south of the ridge, convection started in the Nordic seas. The planktonic foraminifera suggest that the highest temperatures in the NE Atlantic were obtained during the initial phase of MIS 5e, when surface temperatures north of the ridge were still low (Figs. 2 and 4). This early episode of maximum surface water temperatures south of the ridge is also evident from diatom studies of this interval in core ENAM33 (Witak and Kuijpers, 2001). During the middle part of MIS 5e, sea surface temperatures were high on both sides of the ridge. At the location of MD95-2009 north of the ridge, the sea surface started to cool in the later part of MIS 5e and from the MIS 5e/5d transition conditions were cold in the Nordic seas. South of the ridge, temperatures remained high. The changes in the planktonic faunas at the transition from MIS 6 to 5 in ENAM33 resemble the changes described from other Northeast Atlantic cores (e.g. McManus et al., 1994; Oppo et al., 1997, 2001; Chapman and Shackleton, 1998), and the ENAM33 record is apparently typical for the area. The record from MD95-2009 resembles the records from cores LINK06, LINK15, and LINK16 in the southeastern Norwegian Sea and the Faeroe Shetland Channel (Nielsen et al., 2000) (Fig. 1b), and there is no doubt that it is representative for the area north and northwest of the Faeroe Islands. Detailed correlations of the crucial initial phase of MIS 5e between our cores and cores further north in the Nordic seas are somewhat

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uncertain, as the important tephra marker 5e-low/BASIV has not been recorded in the more northern parts of the SE Norwegian Sea, so far. However, large oscillations in the abundance of N. pachyderma s before 127 ka BP in core ODP 644 from the V^ring Plateau have been interpreted to indicate unstable sea surface conditions during the initial part of MIS 5e and a late warming (Fronval and Jansen, 1997; Fronval et al., 1998). The difference in summer sea surface temperature during the initial phase of MIS 5e from ENAM33 to MD95-2009 was probably at least 81C: The distance between the two cores is only about 200 km (Fig. 1) and such a steep temperature gradient has no parallel in the region today. However, the gradient seems comparable to the present situation at the Polar Front west and northwest of Iceland, where the North Atlantic Water and the Irminger Current meet the Polar water of the Greenland Current (Fig. 1a). During summer time, the temperature difference here between the cold and the warm water masses is in the order of 7–81C; increasing southward and decreasing northward (Dietrich, 1969; Hopkins, 1991). The transitional zone is only a few hundred kilometres wide. At present, the Polar Front follows the eastern boundary shelf margin along the east coast of Greenland with a bulge along the north coast of Iceland (Hurdle, 1986) (Fig. 1a). Our results indicate an enhanced influence of Polar water in the Nordic seas during the initial phase of MIS 5e. We suggest that the steep north–south temperature gradient across the Iceland–Scotland Ridge marks the position of the Polar Front at that time. Thus, the front was probably running northwest–southeast rather closely following the crest of the Iceland–Scotland Ridge (Fig. 5a). Highest primary productivity as indicated by an early MIS 5e diatom peak in core ENAM33 (Witak and Kuijpers, 2001) and MD95-2009 (Rasmussen et al., 1999) may demonstrate the vicinity of such a frontal zone where surface water mixing is intensive. Fronval and Jansen (1997) and Fronval et al. (1998) suggested that the general cold conditions in the Nordic seas during the early part of MIS 5e was associated with a strong east–west temperature gradient caused by the spreading of cold meltwater in the western part and inflow of warm Atlantic surface water in a narrow corridor along the Norwegian coast. Our results support this hypothesis. The benthic faunas of both MD95-2009 and ENAM33 indicate that there was convection in the Nordic seas and outflow of deep water during the initial phase of MIS 5e (Fig. 3). This suggests that there must have been some compensating inflow of surface water from the Atlantic. However, the evidence from the MD95-2009 and the LINK cores show that the inflow could not follow the main paths of the modern North Atlantic Current (Fig. 1), which is close to the Faeroe Islands. The inflow must have been restricted to a

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narrow pathway on the eastern side of the Faeroe– Shetland Channel and close to the west coast of Norway (Fig. 5a). Alternatively, part of the inflow of warm, saline Atlantic water may have been at sub-surface depth. The validity of the reconstruction discussed above is supported by a considerable resemblance to the paleoceanographic development at the end of the last glacial period. North of the Iceland–Scotland Ridge, including the MD95-2009 site, the sea surface temperatures remained low until after the Younger Dryas cold event at ca 10 ka BP (e.g. Jansen et al., 1983; Sejrup et al., 1984; Koc- and Jansen, 1992; Koc- et al., 1996; Hald and Aspeli, 1997). In contrast, south of the ridge, the temperatures increased rapidly at the MIS 2/1 transition, ca 13 ka 14 C BP, coinciding with the beginning of a relatively strong deep water outflow from the Nordic seas (e.g. Jansen et al., 1983; Sejrup et al., 1984; Rasmussen et al., 1996; Kroon et al., 1997; Kuijpers et al., 1998; Rasmussen et al., 2002). On the Scottish shelf, the highest sea surface temperatures during the entire deglaciation and early Holocene occurred during the B^lling interstadial (Kroon et al., 1997). Strong surface warming at the beginning of the B^lling interstadial (ca 13 ka 14 C BP) has also been recorded from a narrow zone close to the coast of Norway (Lehman and Keigwin, 1992; Haflidason et al., 1995; Klitgaard-Kristensen et al., 1998) and it is probable that a restricted inflow of warm Atlantic surface water into the Norwegian Sea occurred along the Norwegian shelf (Koc- et al., 1993). In the vicinity of the Faeroe Islands, the inflow was presumably restricted to the Scottish

shelf (Austin and Kroon, 1996; Kroon et al., 1997) and the easternmost part of the Faeroe–Shetland Channel (Klitgaard-Kristensen et al., 1998; Rasmussen et al., 2002). These results agree well with records from the north Icelandic shelf, which indicate a failure of the Irminger current around Iceland during the B^lling– Aller^d interstadials (Eir!ıksson et al., 2000). Hence, it is evident that the paleoceanographic changes of the northeast Atlantic and Nordic seas during the penultimate and last deglaciations were fairly similar. The warming of the Nordic seas was in both cases delayed by about 3000 years as compared to the northeast Atlantic Ocean. Inflow was restricted to a narrow zone along the Scottish and Norwegian coast. However, there were also significant differences. For example, we have not recorded any Younger Dryas-like event with IRD deposition during MIS 5e (Fig. 3). Our model for inflow of Atlantic water during the initial phase of MIS 5e suggest that the Polar Front shifted direction east of the Faeroe Islands roughly following the boundary between the cold and the inflowing warm water masses (Fig. 5a). Following the abrupt initial warming of the Nordic seas at ca 127 ka BP, sea surface temperatures, as recorded by the relative abundance of N. pachyderma s in MD95-2009, increased gradually. From ca 125 to 119 ka BP they were probably higher than during the Holocene temperature optimum (see Rasmussen et al., 1996). Surface temperatures higher than Holocene for the Nordic seas for parts of MIS 5e were also estimated by Fronval et al. (1998) and Cortijo et al. (1994). In agreement with Fronval and Jansen (1997), we suggest

Fig. 5. Reconstructions of most likely surface current pattern and position of Polar Front in the North Atlantic and Nordic seas during the early and middle part of MIS 5e. (a) The early initial phase corresponding to the thermal optimum in NE Europe (ca 130–127 ka BP). (b) Middle part of MIS 5e after the warming of the Nordic seas (ca 127–120 ka BP).

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that the inflow of Atlantic surface water was stronger and occurred over a wider front than at present (Fig. 5b). 5.2. Comparison of MIS 5e in the NE Atlantic and Nordic seas with Eemian land records The climate of northwest Europe is strongly influenced by the oceanic and atmospheric circulation of the Atlantic Ocean and Nordic seas. There is therefore good reason to believe that large-scale oceanographic changes such as those described above would be reflected in the neighbouring land records. The Eemian sections closest to the location of our cores and including a detailed paleoclimatic interpretation are from The Netherlands (Zagwijn, 1961, 1996) (Fig. 4). It is a characteristic feature for the climate history here, as in fact for most Eemian sections in western Europe, that the temperature rise after the Saalian glaciation was extremely rapid and that the thermal optimum occurred early, generally within the first quarter or third of the Eemian time period (pollen zones E1-E4, see Fig. 4) (Zagwijn, 1961, 1996; Litt et al., . 1996; Bjorck et al., 2000; Rioual et al., 2001). The thermal optimum in The Netherlands is further characterised by a continental type climate with relatively cold winters and a low precipitation (Fig. 4). From pollenzone E5 summer temperature dropped slightly, whereas winter temperatures and precipitation increased. The climate became more oceanic. Roughly similar trends have been observed over most of western Europe from Denmark in the north to southern France in the south. In northwest Europe, a significant climatic deterioration took place during pollenzone E6 and at the end of the Eemian forest vegetation were replaced by cold steppe (Zagwijn, 1961, 1996; Litt et al., 1996; . Bjorck et al., 2000; Rioual et al., 2001) (Fig. 4). Below we will compare the northwest European Eemian records with our interpretation of the development in the North Atlantic and Nordic seas during MIS 5. In the discussion we assume that MIS 5e corresponds roughly to the Eemian, realising, however, that this is somewhat uncertain. Several authors have argued that the Eemian may include parts of MIS 5d (e.g. Kukla . et al., 1997, Bjorck et al., 2000; Lehman et al., 2002), and records off the coast of Iberia combining marine d18 O records and terrestrial pollen assemblages have indicated that the start of the Eemian, as defined by the pollen records, is slightly delayed as compared to the * et al., 1999). A more oceanic changes (Sa! nchez-Goni firm correlation between our records and the Eemian records could be established if the tephras could be found in the terrestrial records. From Fig 4 it appears that the early thermal optimum in northwest Europe corresponds to our initial phase with maximum sea surface temperatures in the northeast

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Atlantic, a steep temperature gradient across the Iceland–Scotland Ridge, and general cold conditions in the Nordic seas (Figs. 4 and 5a). Inflow of Atlantic water into the Nordic seas was confined to a narrow zone along the Scottish and Norwegian coasts and its relative contribution to the warming of Scandinavia and the North Sea was probably relatively small. The high temperature was probably more directly related to summer insolation, which reached a maximum between 128 and 127 ka BP (Figs. 4 and 5a). Such a scheme would explain the continental type climate with warm summers and relatively cold winters (Zagwijn, 1996). Moreover, cold surface water conditions in the Nordic seas can be assumed to have had a negative effect on atmospheric cyclone activity in this region, which implies a reduction of the westerly wind intensity over NW Europe. The Dutch data strongly indicate that the global sea level was rapidly rising during pollenzones E1–E4 and first reached a maximum during pollenzone E5 (Zagwijn, 1996) (Fig. 4). This suggest that large amounts of meltwater was pouring into the northern seas supporting the idea of Fronval and Jansen (1997) and Fronval et al. (1998) that meltwater was the main cause for the unexpected cold sea surface conditions in the Nordic seas during the early Eemian. From about 127 ka BP warm Atlantic surface water expanded north of the Iceland–Scotland Ridge and warmed the Nordic seas. However, while sea surface temperatures increased in the Nordic seas they dropped slightly in the northeast Atlantic. These changes conform with the climatic development in northwest Europe during pollenzone E5, which, as noted above, became more oceanic with slightly lower summer temperatures, higher winter temperatures, and a higher precipitation (Fig. 4). Higher surface water temperatures in the Nordic seas are concluded to have resulted in an atmospheric circulation pattern with intensified cyclone formation in this region favouring west wind circulation * et al. (1999) propose over NW Europe. Sa! nchez-Goni that the lower summer temperatures during the middle part of the Eemian were due to the decreasing summer insolation. We suggest further that the more oceanic climate is related to the penetration of warm Atlantic water into the Nordic seas and further into the North Sea. During the later part of MIS 5e sea surface temperatures decreased significantly in the Nordic seas in concordance with the climatic deterioration in north western Europe. 5.3. MIS 5d–5a At the location of MD95-2009, sea surface cooling began around 120 ka BP in late MIS 5e. From ca 114 ka BP and throughout MIS 5, N. pachyderma s constitutes more than 90–95% of the fauna indicating low summer

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sea surface temperatures (Figs. 3 and 4). This agrees well with studies from the Nordic seas of e.g. Kellogg (1976), Cortijo et al. (1994, 1999), Fronval and Jansen (1997), Fronval et al. (1998), Rasmussen et al. (1999). South of the Iceland–Scotland ridge at the location of ENAM33, sea surface temperatures during MIS 5d (interstadial IS24) were almost as high as during MIS 5e (Fig. 4). This is typical for eastern North Atlantic Ocean records (e.g. McManus et al., 1994; Chapman and Shackleton, 1998; Oppo et al., 2001; Lehman et al., 2002). The ice rafting event (S25) marking the MIS 5e/ 5d transition is small in MD95-2009 and almost negligible in ENAM33 (Fig. 3) A small ice-rafting event at the 5e/5d transition has previously been observed in the North Atlantic by Chapman and Shackleton (1999), Oppo et al. (2001) and in the Nordic seas by Fronval and Jansen (1997). Thus, from about 120 ka BP, the temperature differences increased between the Atlantic and Nordic seas. The transitional zone between the cold and the warm water was also displaced in a southward direction. From the beginning of MIS 5d the zone, once again, was placed over the Iceland–Scotland Ridge, and the temperature gradient here was almost as steep as during the initial phase of MIS 5e (Fig. 3 and 4). The surface circulation probably resembled that of the early MIS 5e with inflow restricted to the easternmost part of the Nordic seas. In contrast to the surface water conditions, very little happened at the bottom during MIS 5d. The benthic faunas on both side of the ridge resembled the Holocene and Eemian faunas (Figs. 2 and 3) indicating convection in the Nordic seas and deep outflow. This result agrees well with d13 C records from the central North Atlantic, which according to Oppo et al. (1997) indicate continuous outflow over the Iceland–Scotland Ridge during MIS 5e and 5d. Continuous, though slightly weakening flow of NADW during late MIS 5e and 5d was also registered at the Blake Outer Ridge in the subtropical North Atlantic (Bianchi et al., 2001). The first significant cooling event south of the ridge is S24 (H10, also referred to as C24) (Heinrich, 1988; McManus et al., 1994; Chapman and Shackleton, 1999; Oppo et al., 2001). It marks the 5d/5c transition. In ENAM33, the relative abundance of N. pachyderma s increases to almost 20% (Figs. 2 and 3). A small but steady increase in N. pachyderma s during interstadials IS24-19 indicates gradually colder conditions through each successive interstadial. The composition of the benthic assemblages coupled with the high concentration of foraminifera indicates that all interstadials had strong outflow. This is in good accordance with previous reconstructions by Henrich (1998) based on the carbonate distribution in the sediments and by Fronval and Jansen (1997) based on the concentration and distribution of foraminifera. The high abundance of Atlantic species during H8 and H7a, b and the increase in IRD

suggests a deterioration in the bottom water conditions probably caused by a weakening or complete stop of the outflow (see also Rasmussen et al., 1996, 1999) (Fig. 2). A large difference in sea surface temperatures during MIS 5c–5a between the Atlantic Ocean and the Nordic seas have been demonstrated in several studies (e.g. Kellogg, 1976; Ruddiman and McIntyre, 1979; Duplessy and Labeyrie, 1992; Larsen et al., 1995). Our results narrow the area holding the temperature gradient down to the 200 km zone across the Iceland–Scotland Ridge.

6. Conclusions Correlation between core ENAM33 south of the Iceland–Scotland Ridge and core MD95-2009 north of the ridge demonstrate that during the MIS 6/5e transition, sea surface temperatures rose 3000 years earlier south of the ridge than north of the ridge. At the location of ENAM33, the temperatures increased rapidly from about 130 ka BP. At MD95-2009, the increase first began from about 127 ka BP. The distance between the two sites is only about 200 km and the temperature difference between them during summer time was at least 81C: Such a steep gradient is unknown from the area today, but recent parallels can be found between Greenland and Iceland across the Polar Front separating the cold Polar water from the warm Atlantic water. Our data show that during the early part of MIS 5e, cold Polar water covered most of the Nordic seas and the Polar Front was positioned across the Faeroe Islands more or less following the crest of the Iceland– Scotland Ridge. This is a displacement of about 1000 km toward the southeast as compared to its present position. The planktonic foraminifera demonstrate that the highest sea surface temperatures in the NE Atlantic during the entire MIS 5e period occurred during this initial phase when conditions were still cold in the Nordic seas. The benthic foraminifera indicate that simultaneous with the surface warming south of the ridge, the convection started in the Nordic seas. The deep water circulation changed from glacial mode with no outflow from the Nordic seas to interglacial mode with outflow. Compensating inflow of Atlantic water must have been restricted to a narrow zone along the Scottish and Norwegian Coasts, or may partly have occurred at sub-surface level. This initial phase of MIS 5e seems to correlate with the early thermal optimum of the European terrestrial Eemian. In northwest Europe, this period is characterised by a continental type of climate with warm summers, relatively cold winters and a low precipitation. Such climatic conditions point to reduced west wind circulation over NW Europe, which may be ascribed to reduced cyclone activity prevailing over the Nordic seas as a result of the persisting cold surface water

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conditions. From about 127 ka BP sea surface temperatures rose in the Nordic seas and between 127 and 120 ka BP the surface was warm on both sides of the ridge. This period corresponds to the middle Eemian in northwest Europe characterised by a more oceanic climate with slightly cooler summers, warmer winters and a higher precipitation. North of the Iceland–Scotland Ridge, sea surface cooling began around 120 ka BP in late MIS 5e. At the end of MIS 5e, N. pachyderma s constitutes more than 90–95% of the planktonic fauna at the location of MD95-2009, indicating low sea surface temperature. South of the ridge, the temperatures remained relatively high and the temperature gradient across the Iceland– Scotland Ridge again became steeper. However, the steepness seen during the initial phase of MIS 5e was not reached until the S25 marking the MIS 5e/5d transition. Sea surface temperatures in the northeast Atlantic during MIS 5d were almost as high as during MIS 5e. In the Nordic seas, the temperatures continued to drop. Polar surface water spread from the northwest and the surface inflow from the Atlantic water was displaced to the easternmost part of the Faeroe–Scotland Channel and the Scottish shelf as during the initial phase MIS 5e. The steep temperature gradient across the Iceland– Scotland Ridge persisted during the interstadials of MIS 5c–5a with roughly similar surface conditions as during MIS 5d. The benthic foraminifera show that there was a relatively strong outflow from the Nordic seas during all interstadials IS24-19. During the stadials, sea surface temperatures fell south of the ridge, but they were still significantly higher than north of the ridge. The Polar Front was probably positioned south of the Faeroe Islands and the overflow from the Nordic seas was strongly reduced.

Acknowledgements MD95-2009 was cored with support from the IMAGES-program (International Marine Global Change Studies) under IGBP-PAGES and ENAM33 from the EU-financed ENAM I program (European North Atlantic Margins). LINK-cores were taken by the RV Dana cruise 2000 financed by the Danish National Research Counsil (Project ‘LINK’). Isotope measurements were performed at CNRS-CEA, Gif sur Yvette under the supervision of L. Labeyrie and E. Balbon. T.L. Rasmussen was supported by the Carlsberg Foundation.

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