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doi:10.2204/iodp.proc.347.109.2015

Biostratigraphy

Diatoms

Hole M0065A was qualitatively analyzed for siliceous microfossils. Unevenly distributed samples were analyzed in Core 347-M0065A-15H (~43 mbsf) (Fig. F3). All 69 diatom taxa found at Site M0065 were identified to species level, with the exception of Chaetoceros resting spores, which were recorded if present (Tables T2, T3). Chrysophyte cysts were divided into different morphotypes based on the morphology of their silica cell walls.

The results of the qualitative diatom analyses of Hole M0065A are summarized in a graph showing the number of taxa found divided into different salinity affinities and life forms (planktonic, periphytic, and sea ice) (Fig. F3). 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 recorded (Fig. F3).

2 to ~9 mbsf

The preservation of diatoms is considered poor in this sequence based on the finding of corroded valves and the absence of finely silicified taxa. There is also a high abundance of fractured diatoms and a low overall abundance of diatoms in general throughout this sequence, in accordance with previous studies from the area (Abelmann, 1985; Thulin et al., 1992; Andrén et al., 2000b). Poor preservation may result in overrepresented valves with thick cell walls (e.g., Paralia sulcata or the spines of Pseudosolenia calcar-avis). This sequence records a brackish-marine assemblage of medium diversity with a high proportion of periphytics. The ice-associated taxon Fragilariopsis cylindrus is recorded at one level (Fig. F3).

The silicoflagellate Dictyocha speculum and ebridian Ebria tripartita are both recorded at two levels, and chrysophyte cysts are recorded with various morphotypes throughout the sequence (Fig. F3).

9–11.4 mbsf

This sequence is dominated by taxa that indicate large lake conditions (i.e., Aulacoseira islandica, Stephanodiscus neoastraea, Cocconeis disculus, Navicula jentzschii, Aneumastus tusculus, and Cymatopleura elliptica) (cf. Hedenström and Risberg 1999). This probably indicates the conditions of the final freshwater phase of the Yoldia Sea stage of Baltic Sea history and/or 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 (Sohlenius et al., 1996). However, more detailed study with careful diatom counts may show a response to the transgression that defines the transition from the Yoldia Sea stage to Ancylus Lake (Andrén et al., 2011). Abelmann (1985) records high primary production at the beginning of the Ancylus Lake and suggests that the silica supply was high and that valves were less affected by dissolution. A similar observation is made by Andrén et al. (2000b), who found that the preservation of diatoms is good and redeposition of valves less likely. There is no obvious transition (i.e., initial Littorina Sea stage) recorded between this sequence and the succeeding brackish-marine sequence.

Chrysophytes are recorded occasionally in this sequence, exclusively as a morphotype with smooth cell walls.

Core depth interval 11.9 mbsf

A single sample records a diatom assemblage with >50% brackish-freshwater and brackish water taxa. It contains a fairly diverse assemblage with both planktonic and periphytic components (e.g., brackish-freshwater Gyrosigma acuminatum and Thalassiosira baltica and brackish Brachysira aponina and Chamaepinnularia witkowskii) (Fig. F3). This single sample might reflect the brackish phase of the Yoldia Sea stage of Baltic Sea history, but it does not contain the dominance of T. baltica that characterizes the assemblage in other parts of the Baltic Basin (cf. Lepland et al., 1999; Paabo, 1985; Andrén et al., 2000a, 2002). According to Abelmann (1985), the Arkona Basin and the southern Bornholm Basin contain no brackish Yoldia Sea stage flora. This is further discussed in Andrén et al. (2000b) in a study from the Bornholm Basin. Because we analyzed a single sample, a more thorough study should be conducted to enable conclusions about the southern extension of the brackish water influence during the Yoldia Sea stage to be drawn.

Core depth interval 12.6 to ~43 mbsf

The lowermost analyzed sequence of Hole M0065A was devoid of siliceous microfossils (Fig. F3).

Foraminifers

Results are summarized for the samples taken offshore and onshore (i.e., samples taken from core catchers and regular sections). A total of 55 samples were processed from Holes M0065A, M0065B, and M0065C for the presence of foraminifers (Table T4).

Although Sites M0064–M0066 (located in Hanö Bay and Bornholm Basin) are located close to each other, Site M0065 is the only one at which foraminifers occur. Benthic foraminifers occur from the first core (which begins at 2 mbsf) to 8.90 mbsf with the maximum abundance between 3.17 and 6.62 mbsf (common to abundant) (Fig. F4). A single foraminifer occurs in Sample 347-M0065A-4H-CC at 12.12 mbsf.

Hanö Bay and Bornholm Basin today have bottom water salinities between 12 and 17 (Samuelsson, 1996), which is intermediate between the salinities of Landsort Deep and Little Belt. The foraminiferal assemblage indeed reflects this, as Elphidium spp. still dominate but show more species diversity within the genus. The faunal assemblage is composed of Elphidium excavatum f. clavata, Elphidium excavatum f. selseyensis, Elphidium williamsoni/Elphidium gunteri, Elphidium incertum, and Haynesina spp. A similar assemblage was previously described for the Pomeranian Bight, located just southwest of the island of Bornholm (Frenzel et al., 2005).

The highest diversity of species occurs in the same range where the abundance is also highest. This would suggest that bottom water salinity was slightly higher during this time period, which is also in accordance with the ostracod results (see “Ostracods”). The occurrence of Haynesina sp. around 3.8–4.7 mbsf (Hole M0065B) suggests elevated salinity (~20) compared to the previous and subsequent periods (i.e., more similar to typical assemblages from farther west) (Frenzel et al., 2005; Anjar et al., 2012). When diversity decreases, the remaining species are the Elphidium varieties, which are most adapted to harsher conditions.

Ostracods

A total of 52 samples (including 36 core catchers) from Holes M0065A, M0065B, and M0065C were examined for ostracods during the onshore phase of Expedition 347 at the Bremen Core Repository (Germany). Samples were studied in the >125 µm fraction. Ostracods were present in eight samples (Table T5).

Ostracod abundance per sediment volume from the three holes varies between 1 and 5 valves per 20 cm³ sample and up to a maximum abundance of 42 valves per 20 cm³ at ~8 mbsf (Hole M0065B) (Fig. F5). Maximum abundance of foraminifers occurs at ~3–7 mbsf, where common to abundant Elphidium tests were recorded (see “Foraminifers”). It is possible that salinity changes caused the difference in depths of abundance peaks for foraminifers and ostracods. A total of five taxa were identified for this site: Palmoconcha spp., Robertsonites tuberculatus, Cytheropteron latissimum, Sarsicytheridea bradii, and Paracyprideis sp.

Palmoconcha spp. is a predominant taxon found in six out of eight samples with ostracods in the interval of 5.73 to 8.88 mbsf (Holes M0065A–M0065C). Palmoconcha is a brackish to euhaline genus found at salinities of 10–14 and higher (Frenzel et al., 2010). The uppermost samples at 2.16 mbsf (Hole M0065A) and 3.16 mbsf (Hole M0065B) do not contain this taxon. C. latissimum, R. tuberculatus, S. bradii, and Paracyprideis sp. occur in these samples, indicating higher salinities and an open-sea marine environment.

It is possible that in the interval ~8–9 mbsf salinity is lower and less favorable for foraminifers, such that brackish water ostracods dominate. In the upper part of the record, salinity increases and foraminifers become more abundant, whereas brackish water ostracods decrease in abundance and are replaced with marine species.

Palynological results

Site M0065 is situated in the central part of the southern Baltic Sea. The vegetation of the borderlands in that region belongs to cool temperate forest zone with mixed coniferous and deciduous trees, but the boreal-forest vegetation zone is very close to the site. Palynological analyses for this site focused on Hole M0065A.

Four samples from Hole M0065A were analyzed in total. The two uppermost samples at 2.17 and 8.82 mbsf are characterized by high pollen concentrations (~64,000 and 89,000 pollen grains, respectively), whereas the two other samples (15.78 and 22.63 mbsf) are virtually barren of palynomorphs.

Taxa in samples at 2.17 and 8.82 mbsf are very similar. In both samples, Pinus sylvestris type pollen prevails (52% and 53.5%). Among broad-leaved trees, the highest percentages are for Quercus pollen (Fig. F6, No. 2): 13% for the sample at 8.82 mbsf and 21% for the one at 2.17 mbsf. Betula alba pollen occurs in low abundances: 10% and 6.5%, respectively. In the lower sample (8.82 mbsf), 4% of the total pollen encountered was Tilia cordata (Fig. F7).

Age estimates based only on these two pollen spectra are fairly difficult. Because of the location in the center of the southern Baltic Sea and the relatively large distance of Site M0065 to the coast, overrepresentation of Pinus sylvestris type and Picea pollen (both bisaccate) and far-distance transport from the eastern Baltic region into the site area cannot be ruled out. Fairly high Quercus and Picea percentages may, however, imply a late Atlantic/Subboreal age for these spectra (Kabailene, 2006). In addition to pollen grains, freshwater algae (e.g., Botryococcus) and a mandible of an aquatic insect larva (probably from Endochironomus) have been found in the uppermost sample (Fig. F6, No. 1). The only organic-walled dinoflagellate cysts encountered in the uppermost samples belong to the genus Operculodinium/Protoceratium. The specimens found show particularly short processes. The samples are likely to reflect very minor marine influence.

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