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doi:10.2204/iodp.proc.347.106.2015

Biostratigraphy

Diatoms

Qualitative analysis of diatom community composition was carried out on 18 samples from Hole M0062A. Samples were taken with a spacing of 1.5 m or less in the upper 8.6 mbsf of the core and at every core top for the remainder. Diatoms were identified to species level (Tables T2, T3), and chrysophyte cysts were assigned to morphotypes and present at all analyzed intervals. Diatom taxa were classified with respect to salinity tolerance into five groups (fresh, brackish-fresh, brackish, brackish-marine, and marine) as well as life forms (planktonic, periphytic, and sea ice) according to the Baltic Sea intercalibration guides of Snoeijs et al. (1993–1998). Paleoecological information is illustrated in Figure F3. Although quantitative preservational analysis is yet to be completed, diatom preservation is generally considered to be of high quality based on the presence of gracile taxa.

0–3.4 mbsf

This interval contains mainly freshwater taxa from the planktonic genera Aulacoseira and Cyclotella and the periphytic genera Gomphonema and Tabularia. Some brackish-freshwater taxa (e.g., Diatoma vulgaris) and brackish taxa (e.g., Cyclotella choctawhatcheeana) are also present at low abundances. Sea ice affiliated taxa are not present in this interval. This assemblage represents a very low salinity environment and contains freshwater taxa both from the Ångermanälven River and lake associated freshwater taxa. Notably, Chaetoceros resting spores are not present, implying very low photic zone salinity throughout this interval.

3.4–7.1 mbsf

This interval contains a relatively higher concentration of brackish and brackish-marine taxa, specifically Rhoicosphenia curvata, Thalassiosira levanderi, and Chaetoceros resting spores. Additionally, the sea ice associated taxon Pauliella taeniata is present in this interval. This species is known to occur across the Baltic Sea during the spring bloom following winters with extensive sea ice cover (Hajdu et al., 1997; Hasle and Syvertsen, 1990; Snoeijs, 1993–1998; Weckström and Juggins, 2006). Chaetoceros is associated with high-productivity water columns (e.g., Leventer, 1992; Andrén et al., 2000). This interval is interpreted as a brackish estuarine environment with input of freshwater from the Ångermanälven River. The lowermost part of this interval, from 6.8 to 7.1 mbsf, documents the transition from a freshwater environment below to an estuarine environment above.

7.1–8.6 mbsf

This interval contains mainly freshwater taxa, including many large lake-associated taxa (e.g., Stephanodiscus neoastraea and a diverse Aulacoseira assemblage). Brackish and brackish-marine taxa are very rare. This interval is interpreted as a large lake setting with minor brackish water influence.

8.6 mbsf and deeper

This interval is devoid of siliceous microfossils.

Foraminifers and ostracods

A total of 17 offshore and 5 onshore samples were prepared for foraminiferal and ostracod analysis from Site M0062. No foraminifers or ostracods were found at this site. The lack of foraminifers suggests that conditions during the entire period represented by these cores were too fresh to support foraminifers. Very few microfossils >63 µm are present. Cladoceran antennae occur occasionally and testate amoebae occur very rarely in the upper 10 mbsf, suggesting a freshwater environment. A blue precipitate presumed to be vivianite occurs in Section 347-M0062A-3H-1, suggesting degradation of organic matter in the presence of abundant dissolved iron.

Palynological results

For Site M0062, palynological analyses focused on Holes M0062A and M0062B (see PalyM0062.xls in PALYNOLOGY in “Supplementary material”). Generally, one sample per core was examined for palynomorphs. The present-day regional vegetation in the area is dominated by pine (Pinus), birch (Betula), and spruce (Picea) trees.

Hole M0062A

From Hole M0062A, 10 sediment samples (from Cores 347-M0062A-3H through 8H, two sections from Core 10H, and Cores 11H and 13H) were analyzed. Only the uppermost two samples contain enough palynomorphs to produce statistically relevant results (90,000 to 120,000 grains/cm3). Samples from Cores 5H and 6H contain pollen in very low concentrations. Samples from Cores 7H, 8H, 10H, 11H, and 13H are virtually barren of palynomorphs. In the following sections, results for the samples from Cores 3H and 4H are described and discussed in combination with results from neighboring Hole M0062B.

Hole M0062B

From Hole M0062B, six sediment samples (Cores 347-M0062B-2H through 7H) were analyzed. The uppermost four samples contain enough palynomorphs to generate statistically relevant results. Cores 2H through 4H (1.23–7.70 mbsf), in particular, contain very well preserved palynomorphs, mainly of terrestrial origin (Figs. F4, F5), whereas marine palynomorphs are rare. The pollen concentration varies between 100,000 and 240,000 grains/cm3 for Cores 2H through 4H; in the sample from Core 5H (11.23 mbsf), the concentration is only ~6400 grains/cm3.

1.23–14.35 mbsf

The pollen diagram (Fig. F6) from Site M0062 (Holes M0062A and M0062B spliced) is divided into two phases, with five pollen spectra from 1.23 to 7.70 mbsf reflecting the Holocene and only one sample (11.23 mbsf) representing the onset of the Holocene. The most characteristic element of the pollen spectra from the uppermost three samples investigated (samples at 4.39, 4.22, and 1.23 mbsf) is the occurrence of Picea pollen. The percentage of the Picea pollen in these samples amounts to 13%, 16%, and 9%, respectively. In northern central Sweden, such high amounts of Picea pollen are noted between 3000 and 2500 cal y BP in lacustrine sediments (Antonsson et al., 2006; Wallin, 1996). In pollen spectra at 7.45 and 7.70 mbsf, the amounts of Pinus sylvestris type pollen are only 31%–41%, whereas those of Betula alba type increase to 40%–42%. Also, the percentage of Alnus glutinosa type pollen increases to 12.5%. Pollen of other thermophilous deciduous trees also occur, with Quercus and Ulmus pollen percentages reaching 4%. Fraxinus pollen are present, too. All the above may suggest a mid-Holocene age for the sediment interval.

For nonpollen palynomorphs, the uppermost sample is characterized by particularly frequent Cosmarium freshwater algae. This genus also occurs in other samples from Site M0062 and, together with regular occurrences of Botryococcus and Pediastrum, indicates strong freshwater input or lacustrine conditions. Lacustrine conditions are also implied by occurrences of aquatic insect larvae in Cores 347-M0062B-3H and 5H (4.34 and 11.23 mbsf). Two jaws could be identified as chironomid remains (one from a member of Tanytarsini and one from Corynoneura; Fig. F4). Cores 347-M0062B-2H through 4H (1.23–7.70 mbsf) also contain several Thecamoebae remains.

The oldest level characterized by the pollen spectra is represented by a single pollen spectrum at 11.23 mbsf. This pollen spectrum is dominated by pioneer tree species Pinus sylvestris type (74%) and Betula alba type (14%). Among pollen from other trees, pollen of Alnus glutinosa type (5.5%) and Ulmus (1%) was noted. Nonarboreal pollen is represented by Artemisia (2%), Poaceae (1%), and Chenopodiaceae (1%). Compared to the high-resolution pollen record from Lake Gilltjärnen in northern central Sweden (Antonsson et al., 2006), this pollen spectrum may be provisionally ascribed to the onset of the Holocene. This suggestion may be endorsed by the presence of the above-mentioned herb pollen (i.e., Artemisia and Chenopodiaceae). Pollen of these taxa are related to open plant communities, most of which were probably of importance during the early Holocene.

Marine dinocysts and other marine palynomorphs are extremely rare in all samples from Site M0062, except for a few occurrences of Operculodinium centrocarpum/Protoceratium reticulatum with short processes. This underlines the similarity to the palynomorph record from Site M0061. Tintinnid remains were encountered in Core 347-M0062B-2H (Fig. F4).