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Early Last Interglacial palaeoenvironments in the western Baltic Sea: benthic foraminiferal stable isotopes and diatom-based sea-surface salinity KAREN LUISE KNUDSEN, HUI JIANG, PETER KRISTENSEN, PHILIP L. GIBBARD AND HEIKKI HAILA

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Knudsen, K. L., Jiang, H., Kristensen, P., Gibbard, P. L. & Haila, H. 2011: Early Last Interglacial palaeoenvironments in the western Baltic Sea: benthic foraminiferal stable isotopes and diatom-based sea-surface salinity. Boreas, Vol. 40, pp. 681–696. 10.1111/j.1502-3885.2011.00206.x. ISSN 0300-9483. Stable isotopes from benthic foraminifera, combined with diatom assemblage analysis and diatom-based sea-surface salinity reconstructions, are used for the interpretation of changes in bottom- and surface-water conditions through the early Eemian at Ristinge Klint in the western Baltic Sea. Correlation of the sediments with the Eemian Stage is based on a previously published pollen analysis that indicates that they represent pollen zones E2–E5 and span 3400 years. An initial brackish-water phase, initiated c. 300 years after the beginning of the interglacial, is characterized by a rapid increase in sea-surface and sea-bottom salinity, followed by a major increase at c. 650 years, which is related to the opening of the Danish Straits to the western Baltic. The diatoms allow estimation of the maximum sea-surface salinity in the time interval of c. 650–1250 years. After that, slightly reduced salinity is estimated for the interval of c. 1250–2600 years (with minimum values at c. 1600–2200 years). This may be related to a period of high precipitation/humidity and thus increased freshwater run-off from land. Together with a continuous increase in the water depth, this may have contributed to the gradual development of a stratified water column after c. 1600 years. The stratification was, however, particularly pronounced between c. 2600 and 3400 years, a period with particularly high sea-surface temperature, as well as bottom-water salinity, and thus a maximum influence of Atlantic water masses. The freshwater run-off from land may have been reduced as a result of particularly high summer temperatures during the climatic optimum. Karen Luise Knudsen (e-mail: [email protected]) and Peter Kristensen (e-mail: peter.h.kristensen@ geo.au.dk), Department of Earth Sciences, Aarhus University, DK-8000 Aarhus C, Denmark; Hui Jiang (e-mail: [email protected]), Key Laboratory of Geographic Information Science, East China Normal University, Shanghai 200062, China; Philip L. Gibbard (e-mail: plg1@ cam.ac.uk), Cambridge Quaternary Department of Geography, University of Cambridge, Downing Street, Cambridge CB2 3EN, UK; Heikki Haila, Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, FIN-00014, Finland; received 21st December 2010, accepted 21st February 2011.

The Last Interglacial (Eemian Stage; c. 128-115 ka) corresponds approximately to Marine Isotope Stage ´ (MIS) 5e (e.g. Sanchez-Go ni ˜ et al. 1999, 2000; Kukla et al. 2002; Beets et al. 2006; Brewer et al. 2008; see discussion below). The Eemian climate was generally warmer than that during the Holocene (CLIMAP 1984; Sejrup & Larsen 1991; Funder et al. 2002), and global sea level was probably a few metres higher than at present (e.g. Shackleton et al. 2003; Kopp et al. 2009), after reaching modern levels by around 129 ka (Overpeck et al. 2006). The Northern Hemisphere summer insolation peaked between 131 and 127 ka (Cape Last Interglacial Project Members 2006). During the Eemian, relatively deep marine environments (150–250 m) prevailed in northern Denmark, as initially outlined by Jessen et al. (1910) on the basis of sediments and mollusc assemblages from the classical Skærumhede boring (Fig. 1) and later described in detail by several authors (e.g. Knudsen 1991; LykkeAndersen & Knudsen 1991; Knudsen et al. 2009 and references therein). However, the southern and western areas were characterized by relatively shallow marine environments, the deposits of which are exposed in coastal cliffs in the western Baltic area and have been

DOI 10.1111/j.1502-3885.2011.00206.x

studied in cores along the west coast of Denmark and northern Germany (e.g. Konradi 1976; Knudsen 1985, 1994; Kosack & Lange 1985; Funder et al. 2002). Previous studies of interglacial sediments in the classical Ristinge Klint section (54.511N, 10.381E; Fig. 1) in the western Baltic Sea area show the presence of a continuous early Eemian sediment succession, gradually changing from a freshwater environment in the basal shell-free Shiny Clay and the overlying thin layer of Freshwater Sand to a marine interglacial greenish-grey clay, the Cyprina Clay with a sparse mollusc fauna (e.g. Madsen et al. 1908; Sjørring et al. 1982; Kristensen et al. 2000; Head 2007; Nielsen et al. 2007). The Cyprina Clay is about 300 cm in thickness and is, in turn, overlain by Weichselian glaciofluvial deposits and tills (Figs 2, 3; for further details, see references above). The entire Ristinge Klint succession was pushed into repeated thrust slice blocks by Late Weichselian ice advances (Madsen et al. 1908; Rosenkrantz 1944; Sjørring et al. 1982). Pollen chronology, combined with the study of foraminifera and ostracods from Ristinge Klint (Kristensen et al. 2000), as well as of dinoflagellate cysts (Head 2007), reveals a transition from freshwater lacustrine environments to marine con-

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ditions about 300 years after the beginning of the interglacial, and marine conditions persisted until about 3400 years into the interglacial (cf. Kristensen et al. 2000). The Baltic Sea is a large brackish-water inland sea, which is heavily influenced by the inflow of fresh water from rivers in the catchment area. This freshwater inflow generally causes a higher water level in the Baltic Sea than in the Kattegat and Skagerrak (Fig. 1), which forces the brackish surface waters to flow out of the Baltic Sea through the Danish Belts and drives an inflow of highsalinity bottom waters (Kullenberg 1981; Sjoberg 1992). ¨ The present-day water exchange with the Skagerrak and the Atlantic Ocean is restricted by the limited capacity of the Danish Belts, Lillebælt, Storebælt and Øresund (at a ratio of 1:7:3). Critical areas for entrance of saline water into the basin are at Drogden in Øresund, where the water depth is 8–10 m, and at the Darss Sill, which stretches in a southeasterly direction from the island of Falster (Fig. 1), with an average water depth of 17 m. At present,

Fig. 1. Location map with indication of the presumed maximum marine ingression in Denmark, northern Germany and southern Sweden during the Eemian climatic optimum (modified from inter alia Kosack & Lange 1985; Funder et al. 2002; Kristensen & Knudsen 2006). The present-day marine circulation is shown by thin arrows (black arrow = surface current; stippled arrow = deeper current). The suggested Eemian inflow directions of Atlantic and North Sea waters through the Kattegat–Belt seas and through northern Germany are indicated by thick stippled lines. B.C. = Baltic Current; N.C.C. = Norwegian Coastal Current; LB = Lillebælt; SB = Storebælt; ØS = Øresund; Darss S = Darss Sill; Mo = Mommark; St = Stensigmose.

a strong halocline, located between 15 and 20 m depth, separates the deep inflowing high-salinity waters from the outflowing brackish surface waters. This situation is particularly pronounced in the late spring and early summer, because of the seasonality of the meteorological cycle. A thermocline thus develops during spring, separating the warm upper layer from the cold intermediate water, restricting the vertical exchange within the upper layer until late autumn. The volume of the outflow (62%) far exceeds that of the inflow (Dietrich 1950), a typical feature of an inland sea in a humid climate (Seibold et al. 1971). The average depth of the Baltic Sea is 54 m. As a result of the threshold and basin bathymetry, there are periods of stagnant water in the basins, characterized by decreased oxygen concentration or even anoxia, with renewal of the bottom waters highly driven by storm events (e.g. Sohlenius et al. 2001; Matthaeus et al. 2008). ¨ Until recently, the evolution of the Baltic Sea basin during the Eemian Stage was poorly understood, particularly in relation to the higher global sea level of that

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period (Chappell & Shackleton 1986; Shackleton 1987; Gallup et al. 1994). There was a rapid rise in relative sea level in the western Baltic area during the first 3000 years of the interglacial (c. 131–128 ka; cf. Lambeck et al. 2006). In this early part of the stage, the Baltic Sea was considerably more extensive, with connections to the White Sea in the northeast and to the North Sea through the Danish Belts, as well as across northern Germany (e.g. Zans 1936; Raukas 1991; Funder et al. 2002; Lambeck et al. 2006), and indeed Zagwijn (1996) suggested that the presence of free interchange of seawater was responsible for the pronounced maritime climate in western and central Europe during the Eemian. The objective of the present study is to investigate changes in the hydrography in the western Baltic, particularly in the Storebælt region (Fig. 1), during the early phase of the Eemian (the first 3400 years) on the basis of foraminiferal stable isotopes and diatoms. Seabottom salinity changes for this time interval are estimated on the basis of oxygen isotope measurements of benthic foraminifera, whereas the sea-surface salinity record is reconstructed on the basis of a diatom-based transfer function. The environmental development is, furthermore, discussed in relation to previously published results from the western Baltic, including studies of for-

aminifera and ostracods (Kristensen et al. 2000), dinoflagellate cyst assemblages (Head 2007) and molluscs (Nielsen et al. 2007) from the Ristinge Klint section, as well as environmental proxies for the almost complete Eemian marine sequence at Mommark (Eirı´ ksson et al. 2006; Funder & Balic-Zunic 2006; Gibbard & Glaister 2006; Haila et al. 2006; Kristensen & Knudsen 2006). Because of their location close to the Danish Belts (Fig. 1), these records have the potential to reflect environmental details of the interglacial marine transgression into the Baltic Sea area.

Age model for the Ristinge Klint sequence The age model for the sediments analysed at Ristinge Klint is based on a correlation of the pollen record (cf. Kristensen et al. 2000) with other pollen sequences from the Eemian Stage in northwestern Europe, such as those described by Andersen (1961, 1975), Muller ¨ (1974), Menke & Tynni (1984) and Zagwijn (1996). An important characteristic of the Eemian in the region is the apparent uniformity of the vegetational development, resulting from the progressive acmes of tree taxa. This property was noted by Zagwijn (1996), who concluded that the consequent zonal boundaries

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probably therefore do not differ by more than 200–500 years across the whole of the northern European region. The establishment of a strong correlation of the Ristinge Klint interglacial sequence with the Eemian Stage is critical, in particular because it enables it to be equated with the sequence at Bispingen, Lower Saxony, from where Muller (1974) described an an¨ nually laminated lacustrine sequence, which was regarded as the ‘master chronology’ for northwestern Europe by Menke & Tynni (1984). The annual laminations span the first 2100 years of the sequence, and have been extrapolated to suggest that the interglacial lasted 11 000 years in total. However, one problem is that the Bispingen sequence itself may not start at the beginning of the interglacial, possibly implying that there may be a few hundred to one thousand years that should be added to the overall duration in northwestern Europe (cf. Kukla et al. 2002;

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Fig. 3. Correlation of the regional pollen zonation for Denmark, northern Germany and the Netherlands with the Eemian interglacial stratigraphy of Ristinge Klint. For lithological units, local pollen zones and foraminiferal/ ostracodal zones, see Kristensen et al. 2000, and for dinoflagellate cyst zones/subzones, see Head (2007). The zonal boundaries of Head (2007) have been adjusted to the age model used in this study. The time-span after the beginning of the Saalian–Eemian stage boundary is based on correlation with the annually laminated Bispingen sequence of Muller (1974). ¨

Turner 2002; Brewer et al. 2008). Nevertheless, remarkably similar estimates have been obtained from other sequences in northern Germany, in particular Quakenbruck (Hahne et al. 1994) and Gros Todtshorn ¨ (Caspers 1997). Turner (2002), Caspers et al. (2002) and Brewer et al. 2008) further discuss the duration of the Eemian Stage in the region. On the basis of this, it is possible to use these approximate ages to ‘date’ pollenzone boundaries and thus provide a floating chronology for the region (Zagwijn 1996; Caspers et al. 2002). Therefore, although the Bispingen site is 190 km south of Ristinge Klint, the apparent uniformity of the vegetational sequence and its zones means that it is valid to apply the chronology established there to the whole southwestern Baltic region (e.g. Menke & Tynni 1984; Miller & Mangerud 1985; Bjorck et al. ¨ 2000; Kristensen et al. 2000). In the present paper, we do not adopt the slightly modified chronological

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interpretation of Head (2007), because this does not take account of the interpretations of Andersen (1961, 1975) for the Danish area (see also the discussion in Kristensen et al. 2000). Applying this approach, the base of the Cyprina Clay (232 cm; cf. Kristensen et al. 2000) at Ristinge Klint corresponds to the IIa/IIb zonal boundary at Bispingen (Muller 1974), which is dated at 300 years into the Ee¨ mian (Fig. 3). The IIb/IIIa zonal boundary at Bispingen (750 years into the Eemian) equates to 167 cm in the Ristinge Klint record (cf. Kristensen et al. 2000; placed at 190 cm by Head 2007), and the IIIc/IVa zonal boundary at Bispingen, which is dated at about 3000 years into the Eemian (Muller 1974), corresponds to ¨ 39.5 cm at Ristinge Klint (cf. Kristensen et al. 2000; placed at the top of the sequence by Head 2007). The top of the sequence is here estimated to correspond to c. 3400 years (cf. Kristensen et al. 2000). For error estimates for the pollen zone boundaries, see the pollen record and correlation in Kristensen et al. (2000). How the Bispingen chronology relates precisely to that derived from the deep-sea record remains problematic in the absence of numerical ages for the terrestrial sequences. Comparison with the ages in the deep-sea record indicates that the first appearance of trees in both Ristinge Klint and Bispingen could have occurred as long as 3–4000 years after the Termination II event recognized in the marine isotope sequence, that is, the ´ MIS 6 to 5 transition (Sanchez-Go ni ˜ et al. 1999, 2000, 2002; Kukla et al. 2002; Shackleton et al. 2003), which is dated to c. 130 ka. This implies that the first appearance of trees in northern Europe, the point taken as the base of the Eemian interglacial, occurred c. 127–126 ka, according to Kukla et al. (2002).

Material and methods The sample material for this study comprises part of that used for the previously published local pollen zonation and foraminiferal/ostracodal zonation (Kristensen et al. 2000), as well as for the dinoflagellate cyst zonation (Head 2007), deriving from thrust slices 12 and 23 in Ristinge Klint (Fig. 2); see also Kristensen et al. (2000).

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each sample when possible. At some levels, the limited preservation of diatoms in the sediments prevented that number from being reached (see also below). Diatom percentages were calculated based on the sum excluding Chaetoceros spores, and diatom assemblage zones were determined using cluster analysis. Diatom data from surface sediments, together with modern environmental variable data in the Skagerrak–Kattegat, were used as the modern training set (Jiang et al. 1998). This data set has been used: (i) because the diatom species found in most of the samples from Eemian sediments in the southern Baltic Sea are comparable with those from the surface sediments in the Skagerrak–Kattegat today (cf. Jiang 1996), and (ii) because the general Eemian current pattern in the Skagerrak–Kattegat–Baltic Sea is assumed to have been similar to that prevailing at present (cf. Birks 1995), with high-salinity bottom waters flowing into the Baltic Sea and lower-salinity surface waters flowing out of the Baltic Sea through the Danish Belts (Fig. 1). Thus, comparable environments are expected during these two time periods. However, because a few samples from the Eemian sediments are dominated by brackish and freshwater species, the modern data set was enlarged by adding some surface sediment samples from the Baltic Sea and from a lake in northern Germany (cf. ˚ Hakansson et al. 1998). In total, 36 samples and 95 taxa, with relative abundances of 41% and occurring in at least two samples, were included in the modern data set. Diatom taxa from Ristinge Klint with a relative abundance of 41% were included in the fossil data set for numerical analysis. The taxa mentioned in the text are listed in Table 1. The C2 software (Juggins 2007) was used to reconstruct quantitatively the summer sea-surface salinity. Six numerical reconstruction methods were tested for the modern data set. Weighted averaging partial least squares (WA-PLS) using five components was employed to quantitatively reconstruct summer sea-surface salinity, because it has a low root mean squared error (2.28) of prediction based on the leave-one-out jackknifing (RMSEP(Jack)) and maximum bias (0.82), and a high coefficient of determination (r2 = 0.995) between observed and predicted values (Birks & Koç 2002).

Diatoms

Stable oxygen and carbon isotopes

Samples for diatom analysis were treated with 10% HCl to remove the calcareous matter, washed with distilled water and treated with 30% H2O2 to destroy the organic material. Samples with high clay contents were washed repeatedly by suspending and dispersing the material in distilled water, the supernatant being decanted off only after at least 3 h. Diatom slides were made using Naphrax (refraction index = 1.73) ˚ (Hakansson 1984). Over 300 valves were counted in

Calcareous foraminifera are known to reflect the oxygen isotopic composition of the water mass, in which they grow their shells. The isotopic composition is dependent on temperature and salinity and is therefore a valuable measure in connection with the interpretation of palaeoenvironmental conditions (cf. Shackleton 1974). Foraminifera for stable isotope analyses were processed in accordance with the methods for foraminiferal studies by Feyling-Hanssen et al. (1971) and

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Table 1. List of the Ristinge Klint diatom taxa mentioned in the text, with taxonomic and ecological references. Species

References

Actinocyclus octonarius Ehrenberg 1838 Actinoptychus senarius Ehrenberg 1838 Campylodiscus clypeus Ehrenberg 1840 Campylodiscus echensis Ehrenberg 1840 Coscinodiscus asteromphalus Ehrenberg 1844 Dimeregramma minor (Gregory) Pritchard, 1861 Diploneis didyma (Ehrenberg) Cleve 1894 Ellerbeckia arenaria Crawford 1988 Grammetophora oceanica Ehrenberg 1841 Odontella rhombus (Ehrenberg) Kutzing 1849 ¨ Paralia sulcata (Ehrenberg) Cleve 1873 Stephanopyxis turris (Greville) Pritchard 1861 Thalassionema nitzschioides (Grunow) Mereschkowsky 1902

Hasle & Syvertsen (1997: pp. 120–121); Hendey (1964: pp. 83–84) Hasle & Syvertsen (1997: p. 141) Snoeijs (1993: p. 31) Hendey (1964: p. 291) Hendey (1964: p. 78) Hendey (1964: p. 156) Hendey (1964: p. 226) Krammer & Lange-Bertalot (1991: pp.17–19) Hustedt (1959: pp. 44–46); Hendey (1964: p. 170) Sims (1996: p. 406) Hasle & Syvertsen (1997: p. 91); Hendey (1964: p. 73) Hendey (1964: p. 92) Hasle & Syvertsen (1997: pp. 257–262); Hendey (1964: p. 165)

Knudsen (1998) using mesh sizes of 0.1 and 1.0 mm (see also Kristensen et al. 2000). The foraminifera in the 0.1–1.0 mm fraction were concentrated using the heavy liquid CCl4 (specific weight 1.59 g cm 3). About 20 specimens (size 0.3–0.5 mm) of each of the three shallow infaunal benthic taxa Ammonia beccarii (Linne´), Buccella frigida (Cushman) and Elphidium excavatum (Terquem) forma selseyensis (Heron-Allen & Earland) were subsequently extracted from the light fraction for isotopic analysis. In order to obtain a complete benthic isotopic record, a series of overlapping samples of A. beccarii and E. excavatum was measured, which allowed normalization of the A. beccarii record to E. excavatum. It has been demonstrated that E. excavatum incorporates the oxygen isotopes in equilibrium with the ambient seawater, and that isotopic fluctuations are accurately recorded by this species (e.g. Klingberg 1998; Scheurle & Hebbeln 2003). Changes in the isotopic record of E. excavatum therefore directly reflect long-term changes in environmental conditions, or maybe seasonal changes in reproduction or growth (cf. Schonfeld & Numberger 2007a). ¨ Measurements (totalling 110 samples) were carried out at the Stable Isotope Laboratory, School of Ocean Sciences, University of Wales, Bangor, UK. The samples were treated with anhydrous phosphoric acid at 801C in individual reaction vessels of a PDZ-Europa automated carbonate preparation system (CAPS) coupled to a Europa 20-20 isotope-ratio mass spectrometer. Precision for an internal standard run with the samples (n = 24) was d13C11.95 (1s 0.05), d18O 1.47 (1s 0.06).

Diatom zonation and palaeoenvironments In total, 20 samples of which three were barren were analysed from Ristinge Klint (Fig. 4). The sample at 220 cm contained 32 specimens, most of which were fragments of Campylodiscus clypeus and C. echensis. The sample is included in the zonation in spite of the low content of diatoms, because the diatom assemblage

composition is similar to that of the adjacent sample (at 214 cm). A correlation with the local and regional pollen zones, as well as with previously published foraminiferal/ostracodal and dinoflagellate cyst stratigraphy, is shown in Fig. 3.

Zone RKD I (220–211 cm) Campylodiscus species dominate the zone, mainly C. clypeus and C. echensis. Another important taxon is the freshwater species Ellerbeckia arenaria. These three species account for up to about 90% of the total assemblage. The total number of diatom valves is low. Campylodiscus clypeus and C. echensis can be found in the Baltic Sea today and are very common along the German coast at salinities of around 5–9 psu (Snoeijs 1993). A similar diatom assemblage, the so-called Clypeus flora (Gronlund 1994), can also be found in shallow ¨ sediments of the southern Bothnian Sea (salinity around 5 psu), indicating brackish, shallow, lagoonal conditions (Snoeijs 1999). The assemblage suggests a restricted, slightly brackish environment. In the Kiel Bight in the western Baltic, which is close to Ristinge Klint (Fig. 1), the present August sea-surface salinity is 12–18 psu (8 m water depth), much higher than reflected by the diatom components of zone RKD I. Zone RKD I represents part of regional pollen zone E3 (Fig. 3).

Zone RKD II (211–181 cm) There is a major change in the components of the diatom assemblage in zone RKD II compared with zone RKD I. Marine diatoms dominate the assemblage, with the main species being Actinoptychus senarius and Coscinodiscus cf. asteromphalus, although there are still some freshwater diatoms, for example Epithemia spp., in the assemblage. Most specimens of C. cf. asterophalus occur as fragments. Actinoptychus senarius is a cosmopolitan species (Hendey 1964; Hasle & Syvertsen 1997). It has frequently

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Fig. 4. Distribution of diatoms in the Ristinge Klint record (depth scale). The local diatom assemblages and correlation with the regional pollen zonation are shown on the right-hand side.

been reported in the neritic plankton all round the British coastline, and is hardly ever absent from littoral sampling at almost any time during the year (Hendey 1964). Sancetta (1982) found that A. senarius is most consistently present in the samples from water depths of 30–50 m in Norton Sound and off Bristol Bay, but it never accounts for more than 2% of the flora in the Bering and Okhotsk seas. It is a common species in assemblages related to the high-salinity Jutland Current entering the Skagerrak (cf. Jiang 1996). Coscinodiscus asteromphalus is a marine pelagic planktonic species with a world-wide distribution (Hendey 1964). Thus, the diatom assemblage in zone RKD II reflects a considerably higher salinity than that of zone RKD I, indicating a strong marine ingression, possibly coinciding with a rapid relative sea-level rise. The relatively small total number of diatom valves, as well as the occurrence of diatom species with a heavily silicified structure, may imply strong currents. Zone RKD II represents part of regional pollen zone E3 (Fig. 3).

Zone RKD III (181–121 cm) The diatom assemblage in this zone is still dominated by A. senarius, but it is characterized in particular by the occurrence of various marine and brackish species such as Actinocyclus octonarius, Odontella rhombus, Thalassionema nitzschioides, Dimeregramma minor, Diploneis didyma and Grammetophora oceanica.

Freshwater diatoms such as Epithemia spp. and E. arenaria disappear at the transition to zone III. Thalassionema nitzschioides and O. rhombus are both neritic species (Hendey 1964). The former is also one of the most common species in an assemblage representing the high-salinity Jutland Current flowing into the Skagerrak (Jiang 1996), and is the main component of the Norwegian–Atlantic Current assemblage in the Greenland, Iceland and Norwegian seas (Koç Karpuz & Schrader 1990). The diatom assemblage indicates a coastal palaeoenvironment, which was presumably more openly connected to the sea than during the previous period. Zone RKD III represents the upper part of regional pollen zone E3 and the lower part of zone E4 (Fig. 3).

Zone RKD IV (121–63 cm) The diatom assemblage in zone RKD IV is similar to that of zone RKD III, with the exception of a decrease in O. rhombus, A. octonarius, T. nitzschioides and C. cf. asteromphalus and an increase in A. senarius and P. sulcata. This change probably indicates a slightly decreased salinity compared with zone RKD III. Zone RKD IV represents part of regional pollen zone E4 (Fig. 3).

Zone RKD V (63–48 cm) Paralia sulcata and Stephanopyxis turris replace A. senarius as the main components of the assemblage in

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zone RKD V. Stephanopyxis turris is reported as a widespread member of neritic plankton in temperate waters (Hendey 1964) and in temperate to warm waters (Hasle & Syvertsen 1997). The change in diatom components of the assemblage suggests an increase in the salinity, presumably as a result of a stronger influence of Atlantic waters, and possibly also higher water temperatures. Zone RKD V represents the upper part of regional pollen zone E4 and the lower part of zone E5 (Fig. 3).

Sea-bottom salinity estimates, foraminifera and stable isotopes The relative abundances of the most important foraminiferal taxa in the early Eemian succession at Ristinge Klint (data from Kristensen et al. 2000) are shown in Fig. 5, together with the oxygen and carbon isotope results. Subsequently, the oxygen isotopes for Ammonia beccarii in zone RKFO 3 and the lower part of RKFO 4 are normalized to the E. excavatum record and shown on a floating time scale (Fig. 6). For comparison, additional stable isotope analyses were performed on B. frigida when possible (the interval between 142 and 6 cm (Fig. 5), that is, most of zone RKFO 4 and zone RKFO 4). Ammonia is a widely distributed foraminiferal genus, which is often found to be a dominant taxon. It has been recorded in salt marshes in both fine-grained and coarser sediments, and as a part of the infauna, as well as of the epifauna. Ammonia is extremely tolerant of both salinity (1–90 psu) and temperature (0–351C) conditions (Walton & Sloan 1990), but it has quite specific requirements concerning these environmental parameters for reproduction and for growth of the shell. At the highest latitudes, it reproduces in protected bays, where the temperature reaches 17–201C during at least one month of the year, and the lower salinity boundary for reproduction is 15 psu (Walton & Sloan 1990). Lutze (1965) reported the species Ammonia beccarii living in the deeper parts of the Kiel Bight and in the Fehmern Belt, where it seems to prefer the incoming ‘high’-salinity (15–30 psu) water from the Kattegat. Moreover, it has recently colonized areas above the discontinuity layer but never below it in Gelting Bay, Flensborg Fjord (Fig. 1), in the southwestern Baltic Sea (cf. Exon 1972; Polovodova et al. 2009). Oxygen isotope measurements of living A. beccarii from Havstens Fjord in the eastern Skagerrak show only minor seasonal variation and indicate that A. beccarii is very likely to experience its major shell growth and calcification in the late spring/early summer. This level is then retained and carried over into the following year (Winn et al. 1998). The vital effect of A. beccarii has been calculated from the d18O time series in

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Havstens Fjord to be between 0.5 and 1% (Winn et al. 1998). In our Ristinge Klint record, however, the isotopic difference between the A. beccarii and E. excavatum records is about 2%, which would suggest either a larger vital effect for A. beccarii than 1% and/ or a negative vital effect for E. excavatum.

Foraminiferal environmental indication and stable isotopes The foraminiferal/ostracod assemblage zone (called ‘foraminiferal zones’ in the following) RKFO 3 (232–187 cm) is totally dominated by Elphidium albiumbilicatum Weiss, with A. beccarii as the only accessory species in the upper part of the zone (cf. Kristensen et al. 2000; Fig. 5). These are typical assemblages for shallow, brackish-water environments. At present, E. albiumbilicatum is found at salinities as low as 3.5 psu (Rottgardt 1952), indicating extremely low salinity at the base of zone RKFO 3 at Ristinge Klint. The zone is also characterized by extremely low, but continuously increasing, oxygen isotope values of A. beccarii from around 5 to 4% (Fig. 6), coinciding with an increase in the carbon isotope values from around 4 to 3%. Assemblage zone RKFO 4 (187–66 cm) is characterized by the appearance of several new foraminiferal species, with Haynesina orbiculare (Brady) and E. excavatum as dominant species and with B. frigida as an important accessory species, increasing in abundance in the upper part of the zone (Kristensen et al. 2000). In addition, Kristensen and co-workers found that Elphidium williamsoni Haynes occurred frequently in the lower part (187–120 cm), whereas Elphidium incertum (Williamson) has peak abundance at the base of the zone. This assemblage composition indicates a considerable increase in the influence of highsalinity Atlantic water masses at the base of zone RKFO 4. For instance, E. incertum requires a salinity exceeding 22 psu (cf. Lutze 1965; Polovodova et al. 2009). In addition, the occurrence of the lusitanian species Elphidium lidoense Cushman throughout zone RKFO 4 indicates sea-bottom temperatures of at least 31C higher than today (cf. Le´vy et al. 1969; Daniels 1970). The increasing trend in oxygen isotope values (E. excavatum) continues through the lower part of zone RKFO 4 (Fig. 5), followed by an interval of almost constant values between 150 and 110 cm, and an abrupt change to a relatively heavier level between 110 cm and the top of the zone. The oxygen isotope values for B. frigida are relatively constant between 140 and 80 cm depth, followed by slightly lighter values. It is interesting to note that there are some short-term intervals of inverse patterns for the oxygen isotope values of these two species (see discussion below).

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δ C (benthic)

δ O (benthic)

Zones

Buccella frigida

Haynesina orbiculare

Ammonia beccarii

Elphidium albiumbilicatum

Depth (cm)

Elphidium excavatum


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RKFO 5

B. frigida E. excavatum A. beccarii

0

50

RKFO 4

100

RKFO 3

150

200

0

40

(%)

80 0 10 20 30 0 20 40 60 0

(%)

(%)

20 40 60 0

(%)

10

(%)

20

1

0

–1 –2 –3 –4 –5 –6

–5

–4

(‰)

–3

–2

–1

0

(‰)

Fig. 5. Percentage distribution of five selected benthic foraminiferal taxa from Ristinge Klint (cf. Kristensen et al. 2000), including those used for the oxygen and carbon isotope measurements, as shown on the right-hand side (depth scale). The local foraminiferal zonation is also shown.

Haynesina orbiculare and B. frigida both reach their maximum frequency in assemblage zone RKFO 5, and Haynesina nivea (Lafrenz) becomes an important element (Kristensen et al. 2000). This assemblage indicates high-salinity conditions and relatively deep waters. A persistent sea-level rise, implying a deepening at the site, may have resulted in the stratification of the water column and the establishment of relatively cool, highsalinity bottom waters below a pycnocline. Throughout most of zone RKFO 5, the oxygen isotope values of both E. excavatum and B. frigida continue at almost the same level as seen in the upper part of zone RKFO 4, but a gradual change to heavier values is seen for B. frigida above around 20 cm, coinciding with a change to lighter values for E. excavatum. There is a continuous slight increase in carbon isotope values for both E. excavatum and B. frigida throughout zones RKFO 4 and RKFO 5, although with some fluctuation (Fig. 5).

Stable isotope environmental indication The oxygen isotope values (normalized to E. excavatum) of the Ristinge Klint record (Fig. 6) have been used to estimate the relative changes in the sea-bottom salinity through the early Eemian. In addition, our knowledge about lower limits of temperature and salinity for the reproduction and growth of A. beccarii (see above) has been applied. If the temperature at the sea floor was constant during the studied time interval, the isotope curve would directly reflect the changes in sea-bottom salinity, although, for the lower part of the record, there may also be an unknown global ice-volume correction. Such a correction would result in generally lower oxygen isotope values, but with a gradually decreasing difference upwards through the time interval studied. For the corresponding early Holocene interval, the correction would amount to between 0.45 and 0.20%,


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Regional pollen zones

δ O, normalized to E. excavatum

Diatom - based Sea Surface Salinity (SSS)

Age (years)

690

E5 3000

RKFO 5 RKD V

2000

RKD IV

E4 RKFO 4

RKD III

1000

RKD II

E3

RKFO 3 RKD I

Low

High Salinity

0 10

20 (psu)

–4

–3

–2 –1 (‰)

0

Fig. 6. Diatom-based sea-surface salinity reconstruction for Ristinge Klint based on a transfer function calculation shown together with the benthic foraminiferal oxygen isotope record (floating age scale). The oxygen isotope values for A. beccarii have been normalized to E. excavatum, with the scale showing an increase in salinity (and/or decrease in temperature) to the right. The local diatom and foraminiferal zonations are marked in each of the diagrams, and the regional pollen zonation is indicated on the right-hand side.

corresponding to around 2–1 psu using the d18O–salinity equation of Winn et al. (1988, 1998). Compared with the actual oxygen isotope changes in the present record (Fig. 6), this correction would not change our interpretation significantly. A possible regional correction for meltwater concentration from the Scandinavian Ice Sheet has not been taken into account, although this might have been important for the early part of the record. At the first appearance of E. albiumbilicatum in the lower part of RKFO 3 at Ristinge Klint (Fig. 5), the salinity may have been as low as 3.5 psu (cf. Rottgardt 1952), whereas a salinity of at least 15 psu must have prevailed at the first appearance of A. beccarii (cf. Walton & Sloan 1990). The isotopic record indicates a continuous fast increase in the salinity through the upper part of RKFO 3 as well as in the lowermost part of RKFO 4. Assuming a general rise in temperature at around 750 years after the beginning of the Eemian, as suggested by both the dinoflagellate cyst assemblages for surface waters (Head 2007) and the foraminiferal

assemblages for bottom waters (Kristensen et al. 2000), the salinity would have been even higher than indicated by the isotopic record. The pronounced increase in the carbon isotope values in the upper part of RKFO 3 (Fig. 5) may be an indication of a change in source water at the sea floor. Foraminiferal assemblages indicate relatively constant temperature conditions throughout zone RKFO 4. This would imply that, after an increase in salinity through the initial part of the zone, stable salinity conditions prevailed during the interval from about 150 to 110 cm (about 1200 to 1800 years), followed by a change to a different level with slightly higher salinity (an increase of 2–3 psu) through the remaining part of the zone (1800–2500 years). This is also in accordance with a faunal indication of an increase in salinity in the upper part of RKFO 4, and the development of stratification of the water column. The foraminiferal fauna of zone RKFO 5 indicates an increase in the salinity, presumably caused by a more pronounced stratification of the water column than for the previous zone, and maybe also a cooling of the bottom waters as a result of the stratification. This appears to be in contradiction to the relatively constant oxygen isotope values during most of the interval. Thus, an increase in the salinity, coinciding with relatively stable oxygen isotope values, would imply an increase in bottom-water temperature rather than a decrease. It is interesting to note the inverse patterns of the isotope records for B. frigida and E. excavatum in two specific intervals of the record, namely 125–110 cm (1500–1800 years) and 20–0 cm (3200–3400 years). This shows that the calcification of the two species did not occur in the same water masses within these time intervals, a difference that is difficult to explain. One possibility is that B. frigida, which is usually found today in cold-water assemblages and not in boreal and boreallusitanian assemblages as in the present interglacial record, may periodically have reproduced and calcified during a different time of the year, when bottom waters were relatively cold, for instance in the autumn or winter, instead of in the summer season. In addition, the opportunistic species E. excavatum might have changed its season of reproduction during certain intervals, a factor that would complicate the interpretation of the oxygen isotope results, for instance for zone RKFO 5 (see also the discussion in Kristensen & Knudsen 2006). It was shown by Wefer (1976) that, at relatively constant temperature and salinity, E. excavatum reproduced at different times of the years, with the main reproduction periods at the time of the deepest extension of the euphotic zone, but independently of the oxygen content. Schonfeld & Numberger (2007a, b) ¨ noted that the availability of fresh organic material at the sea floor is the decisive trigger for fast reproduction events of E. excavatum.

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Another interesting feature is that the offset between the two species B. frigida and E. excavatum is generally smaller in the time interval 80–20 cm (2200–3200 years) than during the remaining part of the record, corresponding in time to a major part of the interval of intensified stratification. This may also indicate that the reproduction and calcification of tests occurred at different times of the year. A gradually more pronounced stratification of the water column through zones RKFO 4 and RKFO 5 would imply that the depositional basin might periodically have been exposed to oxygen deficiency. However, the relatively high faunal diversity in this time interval shows that the sea floor was generally well ventilated. The slightly increasing trend in the carbon isotopes (Fig. 5) is therefore suggested to be an indication of a gradual change in water masses in the area throughout zones RKFO 4 and RKFO 5, in agreement with the faunal indication of an increasingly greater influence of Atlantic water masses.

Diatom-based summer sea-surface salinity estimates A total of 15 samples from Ristinge Klint were included in the numerical analysis for summer sea-surface salinity reconstruction (Fig. 6). The samples at 220 and 211 cm (zone RKD I; around 350–450 years after the beginning of the interglacial) were excluded from the analysis because only a few specimens of mainly three species were found in the diatom assemblage, which has no modern analogue in the data set. This particular assemblage is, however, similar to the so-called Clypeus flora assemblage, which can be found today in the shallow waters of the southern Bothnian Sea, with salinity around 5 psu. Therefore, it is assumed that the summer sea-surface salinity was around 5 psu, or lower, during this time interval. The present summer sea-surface salinity in shallow areas of the southwestern Baltic, for example in the Kiel Bight, is 12–18 psu (at 8 m water depth), which is considerably higher than that reconstructed, indicating that relatively brackish conditions prevailed during the initial part of the marine ingression in the Ristinge Klint area. An abrupt increase in the reconstructed sea-surface salinity, reaching more than 15 psu in zone RKD II (211–181 cm; about 450–750 years), indicates a strong marine transgression, possibly caused by a rapid relative sea-level rise, during this time interval. Maximum values of around 28 psu, reconstructed for the lower part of zone RKD III (750–1250 years), are followed by a gradual decrease to around 23 psu in the upper part of that zone (1250–1600 years). A salinity of around 20 psu prevailed in the initial part of zone IV (1600–2200 years), increasing to around 22 psu towards the top (2200–2600 years) and to values as high as 24–26 psu above 63 cm (zone RKD V; after 2600 years).

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The mean early Eemian salinity at Ristinge Klint (after the initial transgression) was thus considerably higher than that at present (around 17 psu) in the area.

Palaeoceanography in the western Baltic In the following, the environmental indications of foraminifera and diatoms at Ristinge Klint are discussed and combined with the indications of dinoflagellates (Head 2007; adjusted to the age model in this study) and molluscs (Nielsen et al. 2007) from the same site (Table 2). They are also compared with lake-level indications of changes in precipitation/humidity in the region (Bjorck et al. 2000). ¨ Environmental development at Ristinge Klint The interval c. 300–650 years (232–180 cm, Fig. 3). – After the marine transgression at c. 300 years, this interval is characterized by shallow, brackish-water conditions, with an initial diatom assemblage comparable to the living assemblage in shallow areas of the eastern Baltic Sea (o5 psu). This corresponds to the dinoflagellate indication of o3 psu for sea-surface salinity at that level (Head 2007), as well as with the foraminiferal indication as low as 3–4 psu, or maybe slightly higher, for the sea floor. There was a subsequent gradual increase in water depth and in ventilation of the sea floor at Ristinge Klint, and both sea-surface and sea-bottom salinity increased to 15 psu (Head 2007 and this study). Molluscs from the same record (Nielsen et al. 2007) indicate high-energy conditions in a shoreface or foreshore environment. The occurrence of laminated sediments indicates that bioturbation was not important (Nielsen et al. 2007). The interval c. 650–1600 years (180–120 cm, Fig. 3). – This interval is characterized by a rapid increase in water depth and an abrupt change to relatively high salinity. The sea-surface salinity reached its maximum of c. 28 psu for the entire record in this period (diatoms), but with a subsequent decrease, which may be related to a period of high precipitation/humidity, as indicated by an increased lake level at Hollerup (Bjorck et al. 2000). The dino¨ flagellate assemblages (Head 2007) suggest a marked increase in sea-surface temperature after 750 years. The sea-bottom salinity also increased significantly in this time interval, with a mollusc indication of 25–30 psu (Funder et al. 2002) and a foraminiferal indication of 422 psu (Kristensen et al. 2000). This major change in salinity at 650 years after the beginning of the interglacial has been attributed to an opening of the Danish Straits (Kristensen et al. 2000; Nielsen et al. 2007). Nielsen et al. (2007) found that tidal features are absent from this interval at Ristinge Klint, and they interpreted four distinct shell horizons as an indication of

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Table 2. Early Eemian palaeoenvironmental development in the western Baltic, based on data from Ristinge Klint. Sea surface

Sea surface

Sea floor

Diatoms

Dinoflagellates

Foraminifera and stable isotopes

RKD V (2600–2850 years): Increase in salinity and possibly also in sea-surface temperature. Strong influence of Atlantic waters.

RKDf 3 (2600–3400 years): Increasingly more open-marine waters, although not fully marine. Increasing distance or decreasing influence from the shore. Temperature persistently at least 26–281C. Stratification.

RKFO 5 (2600–3400 years): Increase in water depth. Salinity increase. Well oxygenated bottom-water conditions. Lowenergy environment below wave base.

2600–3400 years: Pronounced thermal stratification of the water column. Increase in Atlantic water influence. Sea-bottom and sea-surface temperature and salinity higher than today.

RKD IV (1600–2600 years):

RKDf 2 (upper part: 1600–2600 years): Strongly stratified waters develop in upper part of the zone. Temperature increase to at least 26–281C.

RKFO 4 (upper part, 1600–2600 years): Increase in water depth. Salinity increase. Well-oxygenated bottom-water conditions. Lowenergy environment below wave base.

1600–2600 years:

RKDf 2 (lower part, 650–1600 years): Significant rise in inflow of warm, saline Atlantic waters. Temperature increase to considerably higher than at present after 750 years.

RKFO 4 (lower part, 650–1600 years): Fast increase in water depth. Pronounced salinity increase, stabilizing in upper part. Welloxygenated bottom-water. Subtidal environment, periodically storm-induced transport from shallow areas.

650–1600 years:

RKDf 1 (300–650 years): Marine ingression, low salinity, progressively increasing.

RKFO 3 (300–650 years): Shallow, brackish environment, salinity increasing upwards. High-energy environment, no bioturbation.

300–650 years: Marine inundation. Warm interglacial conditions. Low, but increasing salinity, mixing of the water column.

RKFO 211 (0–300 years): Lacustrine environment (ostracods).

0–300 years: Freshwater deposits.

Slightly reduced sea-surface salinity compared with zone III.

RKD III (650–1600 years): Fast increase in water depth. Increased connection to the Atlantic. High sea-surface salinity in lower part (28 psu), slightly decreasing towards the top. RKD II1I (c. 350–650 years): Shallow water. Initially very low salinity, increasing upwards.

periodically turbulent, probably storm-induced, transport from shallow waters into an offshore environment. This is supported by a relatively high abundance of the shallow-water foraminiferal species E. williamsoni in this time interval (Kristensen et al. 2000). Elphidium williamsoni is an indicator species for inter-tidal environments, and its maximum abundance (10%) in this time interval is presumably a result of this offshore transport. A possible similar re-deposition of other shallow, inter-tidal species such as A. beccarii and E. albiumbilicatum in this time interval would not have changed our interpretation for the interval significantly. The interval c. 1600–2600 years (120–63 cm, Fig. 3). – There is an indication of a continued increase in water depth and generally well-ventilated bottom-water conditions through this time interval. The water column was strongly stratified, with persistently higher sea-surface (26–281C) temperatures than today (Kristensen et al. 2000; Head 2007). A continued decrease in sea-surface salinity (to a minimum of c. 17 psu), coinciding with the highest lake levels at Hollerup (cf. Bjorck et al. 2000), ¨

Summary

Increase in stratification towards the top. Gradual increase in Atlantic water influence. Seabottom and sea-surface temperature and salinity higher than today.

Maximum sea-surface salinity. Increase in Atlantic water influence. Sea-bottom and seasurface temperature and salinity higher than today.

may be related to freshwater run-off from land as a result of high precipitation. There is also an increase in sea-bottom salinity compared with the previous interval. In general, the mollusc assemblage and preservation state suggest low-energy conditions at the sea floor, below the wave base (Nielsen et al. 2007), but an increase in organic matter and probably low oxygen contents at the sea floor were suggested at 105–120 cm (the lowermost part of this time interval) by the abundance of Corbula gibba. Decreased oxygen concentration at the sea floor is, however, not supported by any depletion in the carbon isotope values, but it is interesting to note that the interval rich in C. gibba corresponds in time to the interval of reverse pattern for the oxygen isotope curves for the two species B. frigida and E. excavatum, which is undoubtedly an indication of some kind of a change in the environment. The interval c. 2600–3400 years (63–0 cm, Fig. 3). – A further increase in water depth characterizes this time interval. As seen for the previous interval, there is a mollusc indication of low-energy conditions at the sea

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floor, below the wave base (Nielsen et al. 2007). The water column was strongly stratified, with persistently high sea-surface temperatures (26–281C; Head 2007) and a pronounced increase both in sea-surface and in bottom-water salinity, indicating intensified influence of Atlantic waters at the site. It is interesting to note that the maximum bottom-water salinity for the entire Eemian record at Mommark occurred between 3000 and 4000 years after the beginning of the interglacial, encompassing the upper part of this time interval. The sea-surface salinity increase, indicated by the diatoms between 2600 and 2850 years, coincides with a marked drop in lake level at Hollerup during the insolation maximum interval. The precipitation was presumably still high, perhaps even higher than previously, but evaporation increased as a result of increased summer temperatures (Bjorck et al. 2000). The dino¨ flagellate indication of high sea-surface salinity and increased distance from shore (Head 2007) may therefore also partly be a result of relatively less freshwater run-off from the land.

Opening of marine connections to the western Baltic There is an indication of a marine Eemian connection from the North Sea to the western Baltic through valleys and depressions in northern Germany during regional pollen zone E2 (Knudsen 1985; Kosack & Lange 1985; Menke 1985). In the eastern Baltic, however, an even earlier marine inundation has been reported from Estonia (Liivrand 1991) and Latvia (Kalnina 2001), where a late Saalian brackish-water phase was followed by ‘normal marine’ conditions in the early Eemian. This suggests that a marine connection through the Øresund and southern Sweden to the eastern Baltic (Fig. 1) had presumably already been established during late Saalian time. The earliest indication of marine Eemian environments in the western Baltic appears to be around 300 years after the beginning of the interglacial (e.g. Kristensen et al. 2000; Funder et al. 2002; Knudsen & Gibbard 2006 and references therein; Head 2007; this article), and at several sites the initial marine deposits overlie freshwater deposits from a relatively large lake (see also the discussion in Kristensen & Knudsen 2006). Owing to the continued global sea-level rise, accompanied by a decrease in the local isostatic rebound during the early Eemian, a threshold to this lake or lake system appears to have been submerged, and the area became connected to the open sea at around 300 years. After a brackish-water period with a sea-surface salinity increase from around 3 to 15 psu, as indicated by diatoms, foraminifera and oxygen isotopes at Ristinge Klint, there was a major increase in sea-surface and sea-bottom salinity at around 650 years (estimated sea-surface salinity of 28 psu). A similar major change

693

was also recorded at Mommark, although here it is less well dated because of low sedimentation rates (cf. Haila et al. 2006; Kristensen & Knudsen 2006). This abrupt environmental change has been related to an opening of the western Danish Straits (Storebælt and Lillebælt) at that time (Kristensen et al. 2000; Head 2007). It is interesting to note that the opening of the Danish Straits occurred remarkably early in the Eemian interglacial compared with the Holocene, in which an opening is suggested to have occurred at about 3000 years after the beginning of the interglacial (between 9000 and 8200 cal. a BP; Gyllencreutz et al. 2006). This difference is related to a fast relative sea-level rise in the Eemian compared with that during the Holocene (Funder et al. 2002; Lambeck et al. 2006).

Conclusions An environmental reconstruction of the early Eemian development in the western Baltic Sea is presented based on stable isotopes from benthic foraminifera, diatom assemblage analysis and diatom-based transfer function estimates of sea-surface salinity. In combination with previous studies from the area, it has been possible to give an overview of changes in sea-surface and sea-bottom salinity and temperature, in water depth and in the development of stratification in the water column. The interglacial sediments at Ristinge Klint represent pollen zones E2–E5 and span 3400 years, and the results are presented on a ‘floating time scale’ as years after the beginning of the interglacial. The initial marine transgression, c. 300 years after the beginning of the interglacial, was followed by a brackish-water phase with a gradual increase in sea-surface as well as sea-bottom salinity to 15 psu, followed by a fast salinity increase at around 650 years after the beginning of the interglacial. All the various environmental proxies point to an abrupt increase in the influence of Atlantic water at that time, presumably related to the opening of the Danish Straits to the western part of the Baltic Sea during the fast relative sea-level rise. This event was followed by the maximum reconstructed sea-surface salinity between c. 650 and 1250 years (28 psu), and both sea-surface and bottom-water salinity and temperature were persistently higher than at present in the same area throughout the remaining part of the record (top of record at c. 3400 years). The slightly reduced sea-surface salinity between c. 1250 and 2600 years, with minimum values of o20 psu between c. 1600 and 2200 years, may be related to increased freshwater run-off from land during a period with high precipitation/humidity. This interval was characterized by continuously increasing bottom-water salinity, and this is also the time interval in which stratification of the water column gradually developed.

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A particularly pronounced stratification was, however, found in the upper part of the record, c. 2600–3400 years, during a period of relatively high seasurface salinity and temperature (cf. Head 2007). This corresponds in time to a period with a drop in lake level at the Hollerup site, which was related to increased evaporation caused by high summer temperatures during the climatic optimum by Bjorck et al. (2000). The ¨ high sea-surface salinity in the western Baltic may be a result of a decrease in freshwater run-off, coinciding with the increased influence of Atlantic waters during a continued relative sea-level rise. There is also a strong indication of high sea-bottom salinity in this time interval, whereas the benthic faunal composition would suggest relatively cold bottom waters. Acknowledgements. –This article is a contribution to the European Commission’s 4th Framework Environmental and Climate Programme, the BALTEEM Project (Contract no. ENV4-CT98-0809: Palaeoenvironmental and Palaeoclimatic Evolution of the Baltic Sea Basin during the Last Interglacial (Eemian, Mikulino)). HJ acknowledges financial support from the Fund for Creative Research Groups of China (no. 41021064). We thank Paul Kennedy at the Stable Isotope Laboratory, School of Ocean Sciences, University of Wales, Bangor, UK, for providing the stable isotope measurements, and we are grateful to Kyaw Winn and an anonymous reviewer, as well as to the editor Jan A. Piotrowski, for constructive and useful comments on the manuscript.

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