Oceanography in northwestern Europe during the last interglacial from intrashell δ18O ranges in Littorina littorea gastropods

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Quaternary Research 70 (2008) 121 – 128 www.elsevier.com/locate/yqres

Oceanography in northwestern Europe during the last interglacial from intrashell δ18 O ranges in Littorina littorea gastropods Johan Burman a,⁎, Tore Påsse b a

Department of Earth Sciences, Marine Geology, University of Gothenburg, Box 460, SE-405 30 Göteborg, Sweden b Swedish Geological Survey, Guldhedsgatan 5A, SE-413 20 Göteborg, Sweden Received 22 November 2006! Available online 2 June 2008

Abstract Coastal sea-surface temperature (SST) and sea-surface salinity (SSS), including seasonality, in northwest (NW) Europe during the early phase of the Eemian interglacial ca. 125 ka ago were reconstructed from Littorina littorea (common periwinkle) gastropods. The results were based on intra-annual δ18O analyses in recent and fossil shells, mainly originating from the sea of Kattegat (Sweden) and the English Channel (United Kingdom), and confined to intertidal settings. The Eemian L. littorea shells indicated annual SSTs in the range 8–18°C for the English Channel and 8–26°C for Kattegat. All specimens from the Eemian sites experienced summer SSTs of ca. 1–3°C above recent conditions. The estimated winter SST in the English Channel during the Eemian was comparable to modern measurements of ca. 8°C. However, the Kattegat region displayed Eemian winter SST approximately 8°C warmer than today, and similar to conditions in the western English Channel. The recent-fossil isotope analogue approach indicated high SSS above 35 practical salinity units (psu) for a channel south of England in full contact with the North Atlantic Ocean during the last interglacial. In addition, the Kattegat shells indicated a SSS of ca. 29 psu, which points out a North Sea affinity for this region during the Eemian. © 2008 University of Washington. All rights reserved. Keywords: Eemian; Last interglacial; Oxygen isotopes; Sea-surface temperature; Sea-surface salinity; Seasonality; Littorina littorea; Gastropod shells

Introduction The Eemian stage (approximately equivalent to marine isotope stage (MIS) 5e; Kukla et al., 2002) has for the past century been considered a period with warm climatic conditions, and reconstructions have shown mean annual temperatures 1–3°C above present levels in the Northern Hemisphere (Sejrup and Larsen, 1991; Zagwijn, 1996). Isotopically defined to ca. 132– 116 ka (Shackleton et al., 2003), the Eemian stage further represents a setting with low global ice volume and high sea level (Fairbanks and Matthews, 1978; Chappell and Shackleton, 1986). It is also associated with peak solar radiation as a function of greater orbital eccentricity (Berger, 1978) and atmospheric carbon dioxide levels close to 300 ppm (IPCC, 1990). This makes the Eemian interglacial a potential paleo-analogue candidate for a ⁎ Corresponding author. Fax: +46 31 786 19 86. E-mail address: [email protected] (J. Burman).

future greenhouse climate in NW Europe. Previous Eemian studies have shown differences in the hydrographic configuration of the epicontinental basins in northern Europe compared to recent conditions, which resulted in salinities above today's levels (Funder et al., 2002 and references therein; Head et al., 2005). This different land-to-sea ratio compared to today mainly prevailed as a consequence of the eustatic sea level rise leading the isostatic rebound, which took place after the retreat of the Saalian ice sheet (Lambeck et al., 2006). The objective of the present study was to estimate climate seasonality in NW Europe through intra-annual coastal SST and SSS reconstructions, during the early phase of the Eemian interglacial period, approximately 125 ka. The methodology applied was stable oxygen isotope (δ18O) analyses of recent and fossil (Eemian) Littorina littorea Linné gastropod shells. The main sampling areas were located along the Swedish west coast (Kattegat) and in the United Kingdom (western English Channel). The intertidal species L. littorea occupies the surrounding

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epicontinental seas of the North Atlantic Ocean. Especially in NW Europe, it is one of the most common gastropods found in marine deposits. Its first occurrence in the geological record is in the early Pleistocene (Leifsdóttir and Símonarson, 2002). Today, it inhabits both rocky and sandy surfaces as well as macro algae in a littoral environment (Watson and Norton, 1985; Reid, 1996). In general, L. littorea growth rates are faster and more continuous during the first 1.5–2 yr of life. Later, it typically experiences a successive slowdown in growth rates and growth stops occur for various reasons (see Discussion). Modern specimens thriving in fully marine conditions can grow up to a size of ca. 35 mm in shell height. However, in low salinity environments (b10 psu), L. littorea shells are normally restricted to a height of ca. 10 mm or smaller. From a paleo-perspective, different species of gastropods have previously been used as proxy tools to deduce intra-annual climate variability (Geary et al., 1992; Jones and Allmon, 1995). Stable isotope thermometry applied to L. littorea shells, both of recent and fossil origin, has earlier proven to be useful for interpretations of oceanographic settings in different coastal areas of Scandinavia (Andreasson et al., 1999; Burman and Schmitz, 2005). Recent hydrography The epicontinental seas bordering NW Europe have special oceanographic features depending on location and distance from the fully marine North Atlantic Ocean (Fig. 1A). Typically, the oceanic water of the northeast Atlantic, south of the British Islands, displays annual SSTs between 10 and 16°C and SSSs above 35 psu (Levitus and Boyer, 1994). In terms of SSS, there is a negative gradient from the English Channel, the North Sea, Skagerrak and Kattegat, approaching surface salinities of 5– 10 psu in the Baltic proper. Today, at the oceanographic station L4 (PML, 2007) in the western part of the English Channel the intra-annual fluctuation is stable with respect to salinity (ca. 35 psu). This inshore monitoring station is situated about 20 km southwest of Plymouth (50°15′N; 04°13′W) and displays annual SSTs between 8 and 18°C. In comparison, long-term monthly mean SST for coastal waters outside Weymouth (50°37′N; 02°27′W) in the English Channel show an intraannual range of 7–17°C (Joyce, 2006). A minor part of the oceanic water that enters the North Sea flows south of the United Kingdom, which makes the Atlantic influx north of Scotland the important determinant of the hydrographic conditions in the North Sea. These settings and the northward-flowing Baltic current along the Swedish and Norwegian west coasts contribute to the counter clockwise circulation in the North Sea. This circulation is responsible for transporting saline water of 33–34 psu through the Jutland Current into Skagerrak and to the deeper waters of Kattegat (Rodhe, 1996). Commonly, low-saline surface water of Baltic Sea origin lead to stratified waters in the Skagerrak and Kattegat region (Svansson, 1975). Generally, seasonal changes in Skagerrak show SST in the range 0–20°C and SSS between 20 and 30 psu. In Kattegat, SSTs are of the same order of magnitude, but SSSs display a more brackish character (15–25 psu).

Figure 1. (A) Location map of the epicontinental seas of NW Europe. (B) Detail map of the western English Channel, showing the position of the investigated sites. (C) Regional map of the Swedish west coast and the Danish Limfjord, with sample locations marked.

Along the Swedish west coast, salinity generally shows an intra-annual variability due to stratification, the Baltic current and wind-induced mixing. This can be exemplified by data from Kattegat included in this study, which display pronounced SSS variation between the summer (mean 17 psu) and winter (mean 22.5 psu) seasons in the coastal zone. Site locations and gastropod shells The main areas investigated in this study are situated along the western English Channel and in the sea of Kattegat, in NW Europe (Fig. 1A). From these localities, Eemian and recent intertidal L. littorea gastropods were compared through 590 intrashell analyses on 11 specimens. The sites located in the United Kingdom (Fig. 1B), consist of a recent site close to the

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village of Branscombe (50°40′N; 03°10′W) and an Eemian locality at the southern tip of Portland Bill (50°35′N; 02°25′W). At the former, live L. littorea gastropods of various shell sizes were collected from rocky outcrops during low tide 4 July 2005. The analysed shells were 17 mm in shell height (BR1 and BR2). In addition, four (AP1–AP4) small recent (12 mm in shell height) gastropods were selected for aperture analyses (Table 1). From Portland Bill, the sampled fossil L. littorea shells (EC1EC4) were mature specimens (ca. 23 mm in shell height), derived from a raised beach deposit 6.9 m above today's mean sea level. The locality is situated at the far end of the peninsula on the eastern side of the Bill, towards the cliff edge. This beach deposit, made up by large numbers of molluscs, have previously been Uranium–Thorium dated to an age of 121 ± 14 ka (Davies and Keen, 1985). The other areas are made up of two sites associated with the epicontinental sea of Kattegat along the Swedish west coast (Fig. 1C). The recent locality is from Amund Island (57°45′N; 11°45′E), 15 km south of Göteborg city and includes two gastropods 13 mm in shell height (AM1 and AM2). The Eemian locality is situated at Gåsakulla (57°15′N; 12°20′E), 60 km farther south, which is a beach deposit that is currently located 23 m above sea level and 4 km east of today's coastline. A

Table 1 First-year oxygen isotope data along with size and total number of analyses for all gastropods δ18O‰ (Min.) Kattegat recent AM1 −4.136 AM2 −4.634 Skagerrak recent a L1 −3.7 L2 −3.7 L3 −3.6 Limfjord recent b OD-B −1.104 OD-C −1.547 Kattegat Eemian EK1 −1.910 EK2 −1.855 EK3 −1.782 English ch.recent BR1 0.698 BR2 0.450 AP1 AP2 AP3 AP4 English ch.Eemian EC1 1.487 EC2 1.734 EC3 1.657 EC4 1.710 a b c

δ18O‰ (Max.)

δ18O‰ (Range)

1.488 0.165 c

δ18O‰ (Aperture)

Shell height (mm)

Total number of analyses

5.624 6.122

13.1 12.8

33 33

2.6 1.9 1.9

6.3 5.6 5.5

22.5 23.5 21.0

63 56 33

2.828 2.681

3.932 4.228

15.1 17.2

33 90

1.975 2.324 2.129 c

3.885 4.179 3.911

18.6 17.5 17.9

71 41 50

2.753 2.559

2.055 2.109

16.8 17.1 11.8 12.6 12.3 11.4

56 62

23.1 24.0 23.2 23.3

65 62 58 59

1.152 1.289 1.240 1.320 3.688 4.023 4.101 3.943

2.201 2.289 2.444 2.233

Data from Andreasson et al. (1999). Data from Burman and Schmitz (2005). Value not used due to growth stop.

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recent coastal sample site exactly equivalent to the latitudinal position of Gåsakulla was not possible due to lack of rocky habitat, which is commonly required in order to find large numbers of L. littorea gastropods. The Gåsakulla site has been palynologically dated through pollen stratigraphic analyses, which all indicated an early Eemian age (ca. 125 ka). Key indicators used in this study were tree pollen species like Picea and Carpinus, which are referable to the regional pollen zone E4 (Andersen, 1975). The mollusc assemblages are dominated by species like Nassarius reticulatus, Ostrea edulis, and Timoclea ovata (this study). The analysed Eemian gastropod shells from Gåsakulla (EK1-EK3) measured an average shell height of 18 mm. Additionally, for comparative purposes, recent isotopic (δ18O) maximum and minimum values based on 275 intrashell analyses from two Limfjord (OD-B and OD-C, 56°35′ N; 08°25′E) and three Skagerrak (L1, L2 and L3, 58°17′N; 11°32′E) L. littorea gastropods were included (Andreasson et al., 1999; Burman and Schmitz, 2005). Sample treatment and mass spectrometry Each gastropod shell was cleaned by ultrasonication in deionized water prior to the isotopic analyses, dried in a heating cabinet at 50°C for 6 h, and polished with a rounded drill bit to obtain a fresh sample area. The analysed samples were drilled using a dental device, along the whorl from the apex towards the aperture in each specimen, under a binocular microscope with a drill bit size of 0.5 mm. Aperture samples (AP1, AP2, AP3 and AP4) were collected by drilling approximately 5 mm dents parallel to the opening in each gastropod. The powdered calcite samples, 40–160 μg, were roasted in vacuum for 30 min at 400°C to remove organic matter and other volatiles. Stable oxygen isotope (δ18 O) analyses were performed using a VG Prism Series II dual inlet mass spectrometer with an Isocarb preparation system at the Department of Earth Sciences, Göteborg University, Sweden. Phosphoric acid was prepared according to Burman et al. (2005). All isotope values, reported in per mill (‰) relative to the Vienna-Pee Dee Belemnite (V-PDB) standard, were calibrated against NBS-19, using the δ-notation. Mean value and standard deviation for 219 NBS-19 standard samples, analysed over a period of 7 months together with the gastropod material, were − 2.22 ± 0.10‰ for δ18 O. The primary NBS-19 standard value for δ18 O given by the International Atomic Energy Agency (IAEA) equals − 2.20‰. Isotopic results and calculations Micro-drilled intrashell oxygen isotope (δ18 O) samples of L. littorea were analysed, in order to reconstruct seasonal hydrographical changes in the epicontinental seas bordering NW Europe during the Eemian. In order to evaluate the Eemian L. littorea results, the methodology uses an isotopic comparison between recent and fossil gastropod shells to facilitate proper reference conditions. All investigated gastropod shells in this study showed profound site-specific seasonality shifts in the oxygen isotope results, depending on geographical province

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(Figs. 2, 3 and 4). In all gastropods, the analysed area closest to the apex of each shell, including 1 maximum and 1 minimum value (first year), has been used to calculate the oxygen isotope range (Table 1). In Figure 2, the Eemian gastropod shells from Portland Bill (EC1, EC2, EC3 and EC4) displayed an average first-year oxygen isotope range of 2.29‰. The recent English Branscombe (BR1 and BR2) shells showed a mean value of 2.08‰ (Fig. 2). In addition to BR1 and BR2, four small recent shells from Branscombe (AP1, AP2, AP3 and AP4) displayed a mean aperture δ18O value of 1.25‰. The measured surface-water temperature at the time of sampling in the English Channel was 15.6°C (Table 2). For the three Eemian shells of Swedish origin (EK1, EK2 and EK3), the average first-year oxygen isotope range was 3.99‰. In Kattegat, the two recent (AM1 and AM2) shells showed a mean range of 5.87‰ when δ18O maximum in AM2 was substituted with the winter value measured in AM1 (Table 1,

Fig. 3). The recent Limfjord and Skagerrak shells displayed an average δ18O range of 4.08‰ and 5.80‰, respectively (Table 1, Fig. 4). Intra-annual δ18O ranges for all gastropod shells included in this study are plotted in Figure 4. In order to estimate absolute values of mean annual range in temperature (MART) from the analysed gastropod shells, we used a rewritten version of the Burman and Schmitz (2005) species-specific L. littorea fractionation equation: T-C ¼ 22:3  4:39 d18 OL:

littorea

 d18 OWater




ð1Þ

This “paleo”-temperature equation facilitated the use of input data against the V-PDB scale for carbonates and water values on the Vienna-Standard Mean Ocean Water (V-SMOW) scale. For calculation of δ18OWater composition against V-SMOW in Kattegat/Skagerrak and the English Channel, we used the

Figure 2. Intra-annual oxygen isotope (δ18O) results displayed in cm distance from the apex in the English Channel Littorina littorea gastropod shells. The top four graphs (EC1-EC4) show the Eemian gastropods from Portland Bill and the bottom graphs (BR1 and BR2) represent two recent shells from Branscombe (UK).

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Figure 3. Intra-annual oxygen isotope (δ18O) results displayed in cm distance from the apex in the Kattegat Littorina littorea gastropod shells. The gastropods above represent the Eemian Gåsakulla site (EK1 and EK3) and the shells below the recent (AM1 and AM2) Amund Island locality (Sweden).

Fröhlich et al. (1988) and LeGrande et al. (2004) linear relationships with salinity (SSS) for southern Scandinavia (2) and the North Atlantic Ocean (3), respectively: d18 OWater ¼ 0:272ðSSSÞ  8:91

ð2Þ

d18 OWater ¼ 0:604ðSSSÞ  20:84

ð3Þ

As continental runoff has an impact on the SSS in coastal waters, the mean salinity at the Branscombe site in the English

Channel was estimated to 34 psu (Jones et al., 2004a,b). Using the North Atlantic relationship and a mean salinity of 34 psu, the seasonally induced oxygen isotope range for the recent Branscombe gastropods corresponded to a 9°C (9–18°C) difference between winter minimum and summer maximum Table 2 Summary of measured and gastropod-derived surface-water properties for the present and the Eemian localities

Kattegat recent Skagerrak recentc Limfjord recentd English Ch. recent 4 July 2005 Kattegat Eemian English Ch. Eemian a

T°C (meas.)

T°C (calc.)

17/22.5a

− 4.3/− 2.8b

0–24

3.5–22.7

20/25a

− 3.5/− 2.1b

0–22

3.7–23.2

30.2

− 1.6e

0–21

3.2–21.2

34

− 0.3

8–18f 7–17g 15.6

9.3–18.4

Salinity psu (estimated)

29

− 1.0

15.5 8.5–25.9

35.5

0.6

7.6–17.7

Measured mean summer/winter salinity values. Calculated mean summer/winter δ18O water values. c Data from Andreasson et al. (1999). d Data from Burman and Schmitz (2005). e Measured mean annual value. f Data from Plymouth Marine Laboratory (2007) station L4. g Data from Joyce (2006). b

Figure 4. Total oxygen isotope (δ18O) range is shown for all Littorina littorea gastropods included in this study across NW Europe. The columns are defined by the measured maximum and minimum values in each individual shell, with crossbars indicating the mid-point value.

δ18OWater‰ (calculated)

Salinity psu (measured)

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temperatures. The mean aperture value of 1.25‰ (AP1-AP4) and an ambient temperature of 15.6°C resulted in a δ18OWater of − 0.28‰, according to Eq. (1). This test showed that Eq. (3) and a mean salinity of 34 psu is a valid approximation for the western English Channel, as the calculated survey temperature equalled 15.5°C (Table 2). For the recent Kattegat δ18OWater calculations, we used Eq. (2) and mean summer (17 psu) and winter (22.5 psu) salinities measured at Amund Island (Fig. 5), and the gastropod winter value of AM1 (due to indication of a growth stop in AM2; see Discussion). This resulted in an estimated temperature range of 4–23°C and can be compared with the Amund Island SST record, which displays a seasonal range of 0–24°C (Table 2, Fig. 5). Applying an intra-annual shift in salinity (summer 20 psu and winter 25 psu) from Andreasson et al. (1999), the same temperature range (4–23°C) was obtained from the three (L1-L3) Skagerrak shells (Table 2). As there is no direct paleo-proxy for measuring salinity, we indirectly estimated the salinity for the Eemian sites through the negative covariance in carbonate δ18O of the shells with geographic decrease in SSS towards the Baltic Sea. In Figure 4,

the stepwise displacement towards more negative δ18O values with decreasing salinity was displayed by the change from the recent English Channel (34 psu), through the North Sea-affected Limfjord locality (30.2 psu), and the less saline Skagerrak and Kattegat sites (23 and 20 psu, respectively). When extrapolated, this isotopic relationship resulted in an estimated Eemian salinity of 35.5 psu at Portland Bill and 29 psu at Gåsakulla. By applying a salinity of 35.5 psu into Eq. (3) for the Portland Bill locality, δ18OWater was calculated to 0.6‰ for the Eemian English Channel, which equals measurements at latitude 45°N in the Bay of Biscay (Fairbanks et al., 1992). The Portland Bill shells displayed an average first-year intra-annual δ18O range of 2.29‰, and calculated MART for the Eemian English Channel equalled 8–18°C (Table 2, Fig. 5). Using the same technique and Eq.(2) for the Gåsakulla shells (average first-year intraannual δ18O range equal to 3.99‰) and a fixed estimated salinity of 29 psu, the resulting Kattegat sea-surface seasonality showed temperature variation between 8–26°C for southern Sweden in the early Eemian (Table 2, Fig. 5). Discussion and conclusions

Figure 5. Gastropod (Littorina littorea)-derived coastal SST and SSS in Kattegat (Gåsakulla) and the western English Channel (Portland Bill) during the Early Eemian interglacial, compared with regional monitoring data.

The species-specific L. littorea fractionation relationship of − 0.22‰ δ18O v. V-PDB) per °C can be used to extract an overview of the temperature span to which these gastropods have been exposed (Burman and Schmitz, 2005). This is a valid approach, as carbonate secretion by shell-building organisms normally show a fractionation between − 0.20‰ and − 0.25‰ per °C (Epstein et al., 1953; Shackleton, 1974). However, to estimate absolute values of MART from the analysed gastropod shells, we used a rewritten version of the species-specific L. littorea fractionation Eq. (1). Firstly, it should be mentioned that this and similar kinds of expressions will always be sensitive to absolute salinity and δ18OWater input values, when SST is calculated. In regions where precipitation exceeds evaporation, the δ18OWater–salinity relationship is linear (Epstein and Mayeda, 1953). This can be expressed as a mixing line (see Eqs. (2) and (3)), which is based on a two-component model, with mixing between seawater and precipitation/continental runoff. For low salinity environments in NW European coastal waters, mixingline equations (e.g., the δ18OWater–salinity gradient) are sensitive to seasonal changes (Austin et al., 2006). Accordingly, as there are measured seasonal SSS changes taking place in southern Scandinavia, we used different winter and summer input values in salinity to compensate our recent SST calculations for Kattegat and Skagerrak. In the western English Channel, intra-annual shifts in SSS are less than 1 psu and thus are not included when SST are calculated. Secondly, temperature estimates from Littorinidae gastropods are best executed over the first 1.5–2 yr of the aggregated shell material (the intra-annual variation closest to the apex), due to ontogenetic decrease in growth rate (Ekaratne and Crisp, 1984). In turn, reproductive status is perhaps the most important factor controlling cessation of shell growth, and L. littorea gastropods are believed to be mature after ca. 2 yr of age (Chow, 1987). Moreover, light, which affects food availability, consumer pressure (Harley, 2002), and temperatures below ca.

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4°C (Burman and Schmitz, 2005) have the potential to inhibit Littorinidae shell growth. The majority of the shells in this study are relatively small (Table 1), and when larger specimens have been used (e.g., EC1–EC4) only the areas close to the apex have been investigated. However, first-year calculations were tested against total mean for all δ18O maximum and minimum values (except for a single winter value in AM2, due to scar formation indicating growth stop for this specimen). In this test, the temperature difference between these two approaches were found to be less that ± 0.4°C. This can be considered as ca. 50% of the total uncertainty of the calculated coastal SSTs, and it does not conclusively exclude the use of up to 3 yr of intraannual variability as a foundation when calculating seasonality from L. littorea gastropod shells. The primary component for the variability of the calculated recent English Channel SST range (9–18°C) is accounted by the seasonal temperature variations. In these coastal waters, oceanographic data from the Plymouth station L4 and the Weymouth monitoring site show an intra-annual average temperature range between ca. 7–8 and 17–18°C (Fig. 5). From these SST records, it should be noticed that there are a marked inter-annual variability. But, the yearly temperature anomaly in recent time (i.e., the last 4–5 yr) has been around + 1°C above normal, as compared to long-term records (PML, 2007; Joyce, 2006). Recent coastal SST calculations display a temperature span between 4 and 23°C for both Kattegat and Skagerrak, using a difference of approximately 5 psu between summer and winter salinity. This is in agreement with summer coastal SSTs from monitoring data along the Swedish west coast (Andreasson et al., 1999 and references therein) and the Amund Island record, which display summer temperatures of ca. 22–24°C (Fig. 5). All of the recent results are thus consistent, and calculated field temperatures average around ± 0.6°C compared with proxy data, except for temperatures below 4°C. The latter indicates growth stoppage for L. littorea shells below this temperature, as previously stated (Burman and Schmitz, 2005). The reconstructed gastropod Eemian summer SSTs are 1– 3°C above recent conditions, in agreement with a warmer climate during the last interglacial (Zagwijn, 1996; Montoya and von Storch, 2000). However, the interpreted SSS of 29 psu and a minimum winter SST of 8°C for southern Scandinavia are far from the conditions existing today. Past studies have shown that the epicontinental seas in NW Europe during the early Eemian were a part of the Lusitanian biogeographical zone (Funder et al., 2002). This warm Atlantic zone (characterized by Mediterranean species) is indicative of winter SST above 9°C and summer SST higher than 16°C. Today, this zone occupies the western part of the English Channel and has its boundary geographically to the west of the British Islands. These results, and similar findings based on dinoflagellate cysts in the Baltic and Danish Belt Seas, indicate SSS around 20–25 psu for the Baltic proper and 25–30 psu for Kattegat in the early Eemian (Funder et al., 2002; Head et al., 2005). In addition, marine shelf records from northern Denmark show foraminiferal changes from a high Arctic to a Boreal–Lusitanian fauna at approximately the same time (Knudsen et al., 2002). These conditions are generally explained by a strong North Atlantic Current, a

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possible high-index state of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO), a high sea-level stand, and increased oceanic connectivity between the epicontinetal seas of NW Europe during the early phase of the Eemian interglacial (Weaver and Hughes, 1994; Funder et al., 2002; Fells et al., 2004). In this study, slightly higher summer SSTs are indicated for NW Europe during the Eemian. But, of more vital importance is the finding of equivalent winter SSTs around 8°C for both the Eemian English Channel and Kattegat sites. In addition to the latter, high SSS (29 psu) at the Gåsakulla site in Sweden support the general Eemian oceanographic settings. Conclusively, these results indicate that the English Channel was in full contact with the North Atlantic Ocean, and that the surface water in Kattegat had a strong North Sea affinity during the early Eemian. If a near-future rise in sea level combined with a strengthening of the North Atlantic Current will lead to similar conditions as during the last interglacial (Overpeck et al., 2006), southern Scandinavia could expect inundation of Atlantic water and a possible warming of winter SSTs by 6–8°C. However, this scenario is not likely, as long as the effect of isostatic emergence in the Skagerrak area prevails over the anthropogenic sea level rise (Funder and Balic-Zunic, 2006). Thus, a continuation of these conditions will gradually restrict the Skagerrak entranceway and lead to diminish oceanic inflow and an increased isolation of the Baltic Sea. Acknowledgments We gratefully acknowledge Dr. Anna Godhe, Professor Björn Malmgren and two anonymous reviewers for the valuable and constructive comments on the manuscript, Owe Gustafsson for the technical assistance with the Prism II, James Fishwick at PML together with NERC Oceans 2025 Theme 10 at the Western Channel Observatory for the L4 data, and Professor Birger Schmitz and the Adlerbertska foundation for supporting this project financially.

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