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doi:10.2204/iodp.proc.347.107.2015

Biostratigraphy

Diatoms

Holes M0063A, M0063C, and M0063E were qualitatively analyzed for siliceous microfossils. In Hole M0063A, every section top was analyzed through Core 347-M0063A-18H (54 mbsf), and deeper than this level, every core top was analyzed (Fig. F5; Table T2). To locate the brackish phase of the Yoldia Sea stage of Baltic Sea history (with a duration of only 150 y; Andrén et al., 2002), cores from Hole M0063C were sampled between 42 and 45 mbsf at 20 cm resolution (Table T3). In addition, 15 samples from Cores 347-M0063E-13H through 22H (~24–42 mbsf) were analyzed to cover the transition from the brackish Yoldia Sea stage to the Littorina Sea stage (Fig. F6; Table T4). All 93 diatom taxa found at Site M0063 were identified to species level (Table T5), with the exception of Chaetoceros vegetative cells and resting spores, which were recorded if present. Chrysophyte cysts were divided into different morphotypes based on the morphology of their silica cell walls.

The results of the qualitative diatom analyses in Holes M0063A and M0063E are summarized in graphs showing the number of taxa found divided into different salinity affinities and life forms (planktonic, periphytic, and sea ice) (Figs. F5, F6). Diatoms were classified with respect to salinity tolerance according to the Baltic Sea intercalibration guides of Snoeijs et al. (1993–1998), which divide taxa into five groups: marine, brackish-marine, brackish, brackish-freshwater, and freshwater. Furthermore, if present, other siliceous microfossils found (silicoflagellates, ebridians, and chrysophyte cysts) are shown in Figures F5 and F6.

0–25.5 mbsf

The preservation of diatoms is considered good to very good in this section based on the presence of delicate vegetative frustules of Chaetoceros in high abundance, which is in accordance with a previous study from the area (Thulin et al., 1992). This section records a brackish-marine assemblage typical of the recent Baltic Sea and inclusive of the most marine phase of the Littorina Sea stage of Baltic Sea history (Thulin et al., 1992; Westman and Sohlenius, 1999; Andrén et al., 2000a). The section contains intervals with taxa indicating colder and warmer periods, and a more developed paleoecological/paleoclimatological division will follow when proper counts are accomplished.

The diatom assemblage is of medium diversity, and a few planktonic taxa dominate, such as the marine Pseudosolenia calcar-avis, brackish-marine Thalassionema nitzschioides, and genus Chaetoceros (both resting spores and vegetative cells). P. calcar-avis is a common marine warm-water species that also occurs seasonally in both oceanic and near-shore temperate waters (Hasle and Syvertsen, 1996; Sundström, 1986). This species is common in sediments deposited during the Littorina and post-Littorina Sea stages (Andrén et al., 2000a) and also at Site M0063, but it is not found in the present Baltic Sea (Snoeijs and Kasperoviciene, 1996).

Several brackish and brackish-freshwater plankton taxa such as Thalassiosira hyperborea var. lacunosa, Thalassiosira levanderi, and Thalassiosira baltica are abundant throughout the section together with the ice-associated Pauliella taeniata. A small number of other ice-associated taxa also occur, such as Fragilariopsis cylindrus and a few records of Melosira arctica and Nitzschia frigida. T. hyperborea is a pelagic cool-water taxon with four varieties classified according to geographical distribution (two in the Baltic Sea and two in the Arctic Ocean) and morphology (Hasle and Lange, 1989). The Baltic species we find in our samples, T. hyperborea var. lacunosa, may be the resting spore of the delicate variety more rarely found in sediment (Hasle and Lange, 1989). T. hyperborea, T. baltica, P. taeniata, F. cylindrus, M. arctica, and N. frigida have been found in samples of melted ice from the northern Baltic Sea (Huttunen and Niemi, 1986) and may be considered as “sea ice algae” (Hasle and Syvertsen, 1990).

In the most marine phase of the Littorina Sea stage, when surface water salinity was ~12–13 in the Baltic Proper compared to the present range of 6–8 (Widerlund and Andersson, 2011), the resting spores of C. mitra and T. nitzschioides have been used as marine influence indicators (Westman and Sohlenius, 1999; Andrén et al., 2000a, 2000b). Chaetoceros mitra is a common diatom in the North Sea but does not occur in the present Baltic Sea because of low salinity (Hasle and Syvertsen, 1990). The most marine phase of this section is recorded between ~15 and 24 mbsf. It is manifested by the presence of the silicoflagellate Dictyocha speculum, the diatom taxa T. nitzschioides, and C. mitra resting spores (Fig. F5).

The ebridian Ebria tripartita is recorded at most levels, and chrysophyte cysts show a diverse assemblage with various morphotypes throughout the section.

25.5–27 mbsf

In Holes M0063A and M0063E, this interval records a consistent transition from a freshwater assemblage below to the overlying brackish-marine assemblage. This transition, with slightly brackish conditions, is usually referred to as the initial Littorina Sea stage in the Baltic Proper (e.g., Andrén et al., 2000a, 2000b) or the Mastogloia Sea in the Baltic coastal zone (e.g., Sundelin, 1922; Hedenström and Risberg, 1999). The transition reflects increased influence of brackish water and is visible in the diatom assemblage as a shift from dominance of freshwater taxa to dominance of brackish-marine taxa. Taxa assigned to brackish-freshwater affinity are found in high numbers during this transition (Figs. F5, F6).

Chrysophytes are recorded at most levels but are not as diverse as in the uppermost brackish-marine section (0–25.5 mbsf). E. tripartita is occasionally recorded.

27–41 mbsf

The lower part of this interval starts with a long sequence of barren samples or samples with very low number of taxa (Figs. F5, F6). The uppermost samples in Holes M0063A and M0063E show a simultaneous increase in a number of taxa that indicate large lake conditions (i.e., Aulacoseira islandica, Aulacoseira subarctica, and Stephanodiscus neoastraea). This section most probably indicates the conditions of the final freshwater phase of the Yoldia Sea stage and the Ancylus Lake. These two stages are not easily distinguished in the diatom stratigraphy in the open Baltic Basin because there is no change in salinity. A slight increase in transported periphytic taxa, due to the Ancylus transgression (Andrén et al., 2011), marks the onset of the Ancylus Lake in the Eastern Gotland Basin (Andrén et al., 2000a). The barren sequence of this section corresponds to a sediment core recovered from a shallower site in the Landsort Deep (Lepland et al., 1999) and cores from the Eastern Gotland Basin (Sohlenius et al., 1996; Andrén et al., 2000a).

Chrysophytes are recorded occasionally in this section, most commonly as a morphotype with smooth cell walls.

41–43 mbsf

In all analyzed holes (M0063A, M0063C, and M0063E) the brackish phase of the Yoldia Sea stage of Baltic Sea history was detected using diatom stratigraphy combined with its stratigraphic position. A diatom assemblage typical of the Yoldia Sea stage was found at one level in Hole M0063A, five levels in Hole M0063C, and two levels in Hole M0063E. All samples were located within 1 m interval in the core (Figs. F5, F6). The Yoldia Sea stage assemblage in this section has a low diversity but a high abundance, especially of the brackish-freshwater taxa Thalassiosira baltica, which indicates slightly brackish conditions. T. baltica has previously been recorded in this stage and tends to more or less entirely dominate the assemblage (Lepland et al., 1999; Andrén et al., 2000a).

Chrysophytes are recorded in this section.

43–102 mbsf

The lowermost analyzed section of Hole M0063A was mostly devoid of siliceous microfossils (Fig. F5).

Foraminifers

These results summarize the data on samples taken offshore and onshore (i.e., samples taken from core catchers and regular sections). A total of 194 samples were processed and analyzed from Holes M0063A, M0063B, M0063C, M0063D, and M0063E for the presence of foraminifers (Table T6).

Previous records of foraminifers from the central Baltic Basin are rare. A recent record used the abundance of Elphidium excavatum to indicate when inflows of higher salinity water occurred into the Eastern Gotland Basin, suggesting strong variations for the last millennium (Kotilainen et al., 2014). Wastegård et al. (1995) similarly used the presence of E. excavatum to link more saline/brackish conditions to the Yoldia Sea stage of Baltic Sea history in a core retrieved just north of the Landsort Deep.

Foraminifers at Site M0063 occur continuously from the sediment surface to ~30 mbsf (Section 347-M0063B-14H-CC), with maximum abundances in the upper 20 mbsf (few to abundant). Between 20 and 30 mbsf, abundance decreases from few to very rare (Fig. F7). The taxonomic diversity is low, mainly based on analysis of the upper samples (0–7 mbsf), as preservation is poor deeper than this interval, and identification was often only possible to the genus level. In addition to signs of dissolution, many specimens are covered by what may either be organic material or inorganic precipitate coating. The fauna mainly consists of Elphidium spp., with Elphidium excavatum f. clavata and Elphidium excavatum f. selseyensis dominating. Elphidium albiumbilicatum and possibly Haynesina spp. are the minor components of the fauna. The close similarity of the different Elphidium spp., the occurrence of ecophenotypes, and the increasingly poor preservation downcore make it impossible at this stage to determine any clear variations in their relative distribution (Schweizer et al., 2008; Pillet et al., 2013).

It has been suggested that during the late–middle Holocene, salinity in the Baltic Proper was higher than modern bottom water values (~10), potentially as high as 15–20 (e.g., Gustafsson and Westman, 2002). It is unlikely that values would have been higher, as this would likely have resulted in a more diverse foraminiferal assemblage, for example in the Eemian Peski section along the Gulf of Finland where Bucella frigida occurred when salinity became >20 (Miettinen et al., 2014). Between 20 and 30 mbsf, the contribution of E. albiumbilicatum increases, suggesting that conditions might have become slightly fresher.

The very rare occurrence of several E. excavatum and E. albiumbilicatum specimens at 45 mbsf (Section 347-M0063C-23H-2, 15 cm) might be attributed to the Yoldia Sea stage (based on diatom results). The additional very rare occurrence in Section 347-M0063A-22H-CC (45.48 mbsf) of Elphidium spp. cannot directly be linked to a specific stage and may not be significant.

Ostracods

A total of 225 samples (including 129 core catchers) from Holes M0063A, M0063B, M0063C, M0063D, and M0063E were examined for ostracods during the onshore phase of Expedition 347. Samples were studied in the >125 µm fraction. Ostracods were present in 25 samples (Table T7).

Ostracods were mainly found in Holes M0063B, M0063C, and M0063D and only in two samples in Hole M0063A. Abundance per sediment volume from the three holes (M0063B, M0063C, and M0063D) was very low and did not exceed 4 valves/20 cm³ sediment sample (Fig. F8). Seven taxa were identified: Sarsicytheridea bradii, Heterocyprideis sorbyana, Cytheropteron pseudomontrosiense, Paracyprideis sp., Palmoconcha spp., Leptocythere spp., and Candona sp.

Ostracods occur in low abundance in the upper 18 m (Holes M0063B–M0063D). Deeper than this interval, they were only recorded at ~26 and ~39 mbsf (Hole M0063D).

Preservation of ostracods is very poor, and valves often bear signs of dissolution. Similarly, preservation of foraminifers is generally moderate and worsens downcore (see “Foraminifers”). Low abundance and poor preservation prevent paleoenvironmental interpretation of the ostracod data.

Previous work suggests that dissolution of calcareous microfossils is one of the major taphonomic processes in the Baltic Sea. Benthic foraminiferal work shows major differences in the abundance and assemblages of living versus dead foraminifers (Murray and Alve, 1999). Ostracods are even more susceptible to dissolution, and their low abundance together with dissolution signs on both ostracods and foraminifers indicate postmortem changes (Swanson and van der Lingen, 1997).

Palynological results

Site M0063 is situated in the boreal-forest vegetation zone but close to the transition to cool temperate forest with mixed coniferous and deciduous trees that characterizes the terrestrial realm of the southeastern Baltic region. For this site, palynological analyses focused on Hole M0063A. Generally, one sample per core was examined for palynomorphs. Bisaccate pollen grains are included in the pollen reference sum, with average percentages of ~50% of the total pollen assemblage.

From Hole M0063A, a total of 29 sediment samples were analyzed. Generally, sediments from the upper part contained palynomorphs in excellent preservation (Fig. F9; see PalyM0063.xls in PALYNOLOGY in “Supplementary material”) and sufficient numbers to yield statistically relevant palynological results (Figs. F10, F11), whereas only a few samples deeper than 32.86 mbsf could be analyzed in detail.

Core depth interval ~0.06–19.70 mbsf

The uppermost interval contains pollen assemblages that reflect Holocene conditions. Several samples were palynologically analyzed for this interval. Counting sums vary around 100 pollen grains per pollen spectrum (Fig. F11) because of relatively high pollen concentrations between 20,000 and 130,000 grains/cm³. The pollen spectra of this depth interval are dominated by Pinus sylvestris type (maximum = 62%), Betula alba type (maximum = 28.5%), Alnus glutinosa type (maximum = 17%), and Quercus (18.5%). Pollen of other deciduous trees like Fraxinus, Ulmus, Tilia cordata type, and Carpinus were noted. Percentage shares of Corylus avellana pollen vary between 1% and 10%. In the uppermost sample, pollen grains of Secale type and Triticum type (Fig. F9, No. 1) were found, as well as a single occurrence of Fagus pollen (Fig. F9, No. 3).

The most striking feature of this interval is the presence of Picea pollen, with percentages increasing from <1% at the bottom of the interval to 5% at the top. The increasing upward Juniperus pollen curve (from 5.5% to 9.97%) allows for correlating this pollen interval with radiocarbon dated sediments from Landsort Deep for ~5630 ± 75 C¹⁴ y BP at the bottom and for ~1605 ± 100 C¹⁴ y BP at the top (Thulin et al., 1992). Comparison of our pollen diagram with dated sediments from Lake Judesjön in northern Sweden suggests an age of the bottom interval of ~3600 y BP (Wallin, 1996).

Radiosperma (Fig. F9, No. 8) and Thecamoeba remains (Fig. F9, No. 9) are frequent in most samples from this interval, and freshwater algae occur in several samples. Dinoflagellate cysts also occur in several samples. A particularly high dinocyst/pollen ratio was encountered in two samples at 16.95 and 19.70 mbsf, but the dinocyst assemblages are almost monospecific, with Operculodinium/Protoceratium reticulatum showing percentages of ~75%. Most specimens are characterized by short processes. The two samples with a relatively high dinocyst/pollen ratio also contain Copepod eggs.

22.95–39.45 mbsf

Several samples examined from this interval are barren of palynomorphs or show low pollen concentrations (~10,000 or less pollen grains/cm³). Three pollen spectra were achieved in this interval. Pinus sylvestris type pollen prevail in all of them and fall from 55% at the bottom of the interval to 40% at the upper sample. The percentages of Betula alba type pollen are significantly lower compared to the overlying interval and increase from 9.5% at the bottom to 17% at the top. Quercus and Corylus avellana achieve the highest shares in the entire pollen succession: 19.5% and 11.5%, respectively. Together with Ulmus pollen (4.5%), these percentages may point to a late–middle Holocene age for this interval (Antonsson et al., 2006).

56.80 mbsf

Only one pollen spectrum was achieved in this part of the core. It is dominated by Pinus sylvestris type (47.50%) and Betula alba type (26.50%) pollen. Picea pollen is 3.5%. Pollen of deciduous trees like Quercus, Alnus glutinosa type, Fraxinus, Carpinus, and Corylus avellana were present in amounts exceeding 1%. It is very difficult to attribute even roughly the possible age of this spectrum. The presence of thermophilous tree taxa and Picea pollen suggests contamination from the upper part of the sediments. On the other hand, in a pollen succession from Lake Gilltjärnen, northern central Sweden (Antonsson et al. 2006), similar pollen spectra were recorded around 10,500–11,000 cal y BP. The lack of Picea pollen in the spectra from this lake may be explained with eastern migration of spruce into Sweden.

66.18 mbsf

The only pollen spectrum in this part of Hole M0063A is overwhelmingly dominated by Pinus sylvestris type pollen (77%), whereas Betula alba pollen are 7%. Relatively high values (the highest in the entire pollen succession from Hole M0063A) are achieved by Chenopodiaceae (6%) and Artemisia (3.5%) pollen. The presence of these pollen types points to open steppe-like communities. It may imply a pre-Holocene age for this spectrum. Late glacial interstadials Bølling/Allerød cannot be ruled out. Again, there is also the possibility of contamination (see “Lithostratigraphy”). In this sample, Thecamoeba remains could also be found.

69.40–101.94 mbsf

Pollen concentration in the 10 samples analyzed for this interval varies between 0 and 1000 grains/cm³; therefore, it was not possible to gain statistically relevant data for these samples. As a consequence, this interval is not depicted in the detailed pollen diagram (Fig. F10).

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