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doi:10.2204/iodp.proc.347.105.2015

Biostratigraphy

Diatoms

Qualitative analyses for siliceous microfossils were carried out on 21 samples from Hole M0061A and 4 samples from Hole M0061B with an interval of 1.5 to 3 m between samples. Diatoms were identified to species level, and ebridians and chrysophyte cysts were recorded if present. The results of the qualitative diatom analyses for both holes are summarized in graphs showing the number of taxa found, divided into different salinity affinities and life forms (planktonic, periphytic, and sea ice) (Figs. F3, F4). The occurrence of diatom taxa in studied samples is shown in Tables T2 and T3. A species list of all 97 recorded taxa from Site M0061 is presented in Table T4. 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. The preservation in all slides containing diatoms can be considered good with lightly siliceous taxa still present. Chrysophyte cysts of various morphotypes were present in all slides where diatoms were recorded.

0 to ~2 mbsf

The assemblage recorded in this interval reflects a dynamic brackish estuary environment with large river input. The uppermost interval recorded in Hole M0061A has a relatively diverse assemblage (Fig. F3). It is a mixture of freshwater taxa (e.g., periphytic Tabellaria flocculosa, Fragilaria exigua, Eunotia implicata, and planktonic species from the genera Aulacoseira and Cyclotella), brackish-freshwater taxa (e.g., periphytic Rhopalodia gibba, Epithemia sorex, Epithemia turgida, Cocconeis pediculus, and planktonic Thalassiosira baltica), and brackish taxa (e.g., periphytic Rhoicosphenia curvata and planktonic Pauliella taeniata and Thalassiosira levanderi). The freshwater taxa in this uppermost interval probably reflect allochthonous river input from Ångermanälven River. The influence of sea ice taxa is observed in this sequence, consisting of P. taeniata, Fragilariopsis cylindrus, and Melosira arctica. P. taeniata is an Arctic planktonic sea ice diatom abundant in the entire Baltic Sea area that dominates the spring bloom after cold winters with extensive ice cover (Hajdu et al., 1997; Hasle and Syvertsen, 1990; Snoeijs, 1993–1998; Weckström and Juggins, 2006).

~2–6 mbsf

The diatom assemblage in this interval indicates a brackish water, open-coastal environment with some influence from fluvial input. It was only recorded in one sample from Hole M0061A (4.8 mbsf), so four additional samples from Hole M0061B (2.8–6.1 mbsf) were analyzed to cover this interval (Figs. F3, F4). The assemblage is dominated by brackish (e.g., planktonic Thalassiosira hyperborea var. lacunosa, T. levanderi, Cyclotella choctawhatcheeana, P. taeniata, and periphytic R. curvata and Cocconeis scutellum) and brackish-marine taxa (e.g., Tabularia fasciculata) but also some marine taxa, especially in the lowermost part (e.g., Pseudosolenia calcar-avis and Opephora marina). Brackish-marine Chaetoceros spp. resting spores are common throughout the sequence. There is also a constant record of ~10%–25% freshwater diatom taxa most probably reflecting transported river input. The ebridian Ebria tripartita is recorded at all analyzed levels.

~6–8 mbsf

The diatom assemblage at this interval suggests a freshwater (large lake) environment with weak influence of brackish water. The transition from this assemblage (recorded at 6.3 mbsf in Hole M0061A) to a brackish environment (recorded at 6.1 mbsf in Hole M0061B) is not defined and must be investigated further. The sequence was recorded in three samples from Hole M0061A, and the assemblage is dominated by freshwater taxa (e.g., the large lake taxa Stephanodiscus neoastraea, Aulacoseira islandica, Gomphocymbella ancylii, and Cocconeis disculus [Hedenström and Risberg, 1999]), but brackish-freshwater ice algae Fragilariopsis cylindrus was also common in the uppermost sample (6.3 mbsf) (Fig. F3).

9.6 mbsf to bottom of core

This interval is more or less barren of siliceous microfossils; only single frustules and fragments of diatoms and one Chrysophyte cyst were found (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 50 samples were processed and scanned from Holes M0061A, M0061B, and M0061C for the presence of foraminifers (Table T5). Foraminifers were previously not recorded for the Bothnian Sea and its estuaries. This is possibly due to the very low salinity in this part of the Baltic Sea, as bottom water salinity in the Ångermannälven River estuary is around 5 (Samuelsson, 1996). Past reconstructions of salinity based on Sr isotopes of mollusk shells suggested that during the middle Holocene salinity might have been as high as 10–12 in the Bothnian Sea, allowing a foraminiferal community to become established (Widerlund and Andersson, 2011).

Site M0061 cores contain significant benthic foraminifers for selected samples (Fig. F5). All foraminifers belong to a single species, Elphidium albiumbilicatum. E. albiumbilicatum is able to survive under very low salinity conditions (Rottgardt, 1952; Lutze, 1965). Foraminifers are found between 1.69 and 6.23 mbsf with a maximum abundance classified as “common” in the core catcher sample from Core 347-M0061C-1H (Fig. F5). The occurrence of foraminifers at Site M0061 can potentially be linked to the slightly higher salinity conditions that are reconstructed for the middle Holocene (Widerlund and Andersson, 2011). However, foraminifers were not present in every sample in this interval. This might be related to a combination of the dynamic setting within the estuary and the low salinity such that conditions were not continuously appropriate for foraminifers to form a population. As such, the presence of foraminifers is only appointed to specific samples instead of to a certain interval.

Ostracods

Ostracods were examined from 49 samples (including 23 core catchers) from Holes M0061A, M0061B, and M0061C during the onshore phase of Expedition 347 at the Bremen Core Repository. Samples were studied in the >125 µm fraction. Ostracods were present in eight samples (Table T6).

Ostracod abundance per sediment volume from the three holes was low and is shown in Figure F6. Four species were identified: Hirschmania viridis, Paracyprideis fennica, Sarsicytheridea punctillata, and Heterocyprideis sorbyana.

P. fennica is the most common species at this site, occurring in the interval 0.17–4.45 mbsf (Holes M0061A–M0061C). Deeper, at 5.11 mbsf (Core 347-M0061A-3H-1, 30–32 cm), only one species, H. viridis, was recorded. The interval immediately deeper, to 28.68 mbsf (Holes M0061A–M0061C), was barren. Similarly, foraminifers were only found from 1.69 to 6.23 mbsf (see “Foraminifers”). The highest number of ostracods was recorded at 2.82 mbsf (Hole M0061B) (Table T6) in the only sample where S. punctillata and H. sorbyana were recorded.

Such low abundance and taxonomic diversity implies harsh environments for ostracods. P. fennica, S. punctillata, and H. sorbyana are all known to tolerate a wide range of salinities from 4 to fully marine conditions; P. fennica can also be found in low-oxygen environments (Frenzel et al., 2010). Low salinity is not a limiting factor for ostracod abundance, as many taxa tolerate fresh and brackish water environments, but similarity between ostracod and foraminifer records implies that salinity decreases toward the uppermost samples. Another important factor is the very high sedimentation rates that decrease in the upper part of the record (see “Lithostratigraphy”), allowing microfossils to be preserved.

Palynological results

For Site M0061, palynological analyses focused on Hole M0061B. Generally, one sample per core was examined for palynomorphs. Today, the regional vegetation in the terrestrial area around Site M0061 is dominated by pine (Pinus), birch (Betula), and spruce (Picea) trees and belongs to the boreal-forest vegetation zone with taiga-like conditions.

Nine sediment samples were analyzed from Hole M0061B. The uppermost four samples contained enough palynomorphs to get statistically relevant results (Figs. F7, F8). Bisaccate pollen was included in the reference sum, but at least 50 nonsaccate pollen grains were counted per sample in order to cope with the high percentages of bisaccate pollen.

0.13–6.62 mbsf

Pollen concentrations are between ~40,000/cm³ and ~180,000/cm³ in the four samples analyzed from this interval (see PalyM0061.xls in PALYNOLOGY in “Supplementary material”). Pinus is the dominant pollen taxon in all samples. The second most frequent pollen type in most samples is Betula pollen. The two uppermost samples analyzed show some difference compared to the two deeper samples. Picea (spruce) pollen shows percentages up to 17% in the two uppermost samples (0.13 and 1.53 mbsf; Fig. F7). It is accepted that spruce arrived at the Swedish-Bothnian coast around 3500 cal y BP (Huntley and Birks, 1983; Segerström and von Stedingk, 2003). This would imply that the age of the sample at 1.53 mbsf dates to at least 3500 y BP. Worthy of mention here is the presence of Betula nana pollen, a dwarf birch growing under arctic and cool temperate climatic conditions (for instance, central and northern Sweden). The two uppermost samples are also characterized by the high-frequency presence of Radiosperma corbiferum (Fig. F8), a dubious taxon which has so far been described as an acritarch, mollusk egg, or algal cyst (also called “Sternhaarstatoplast,” e.g., Nehring, 1994). Picea is virtually absent in the deeper samples from this interval (4.42 and 6.26 mbsf). In some samples, pteridophyte spores are also common, with recurrent presence of Dryopteris (wood fern), particularly in the sample from 6.26 mbsf. Dinocysts and other marine palynomorphs were extremely rare in these samples, except for one sample at 4.42 mbsf that contained several specimens of Operculodinium centrocarpum/Protoceratium reticulatum. All of them have particularly short processes, which may indicate low salinity (Mertens et al., 2009). This finding is congruent with foraminifer-based results, which indicate a slightly stronger marine influence between 3.40 and 4.90 mbsf indicated by slightly increased abundances of foraminifers (genus Elphidium) and lower frequency/absence of foraminifers shallower than these depths. Generally, the rarity of marine taxa in this interval points to a strong terrestrial influence, with particularly low influence in the upper samples.

13.0–26.15 mbsf

Five samples from this interval were analyzed, but none of them contained enough pollen to generate statistically relevant results. Most samples were completely barren of palynomorphs. This is in accordance with sedimentological findings, which indicate that the samples originate from silty/sandy sediments. Thus, palynomorphs either occur in very low concentration in this material or were completely degraded because of oxidation.

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