Previous   |    Next

doi:10.2204/iodp.proc.347.103.2015

Biostratigraphy

Diatoms

Holes M0059A and M0059C were analyzed for siliceous microfossils. Diatoms were identified to species level, and the occurrences of silicoflagellates, ebridians, and chrysophyte cysts were recorded. Qualitative analyses were carried out on every sample from the core section tops (~1.5 m between samples) to 58 mbsf and then every core top (~3.3 m between samples) to 84 mbsf in Hole M0059A and at every recovered core top between ~125 and 142 mbsf in Hole M0059C. Hole M0059C was almost barren (only a limited number of corroded diatom fragments were found, together with reworked Cretaceous coccoliths), and the siliceous microfossil record described in the rest of this section refers to Hole M0059A. The results of the qualitative diatom analysis for Hole M0059A are summarized in a graph showing the number of taxa found divided into different salinity affinities and life forms (planktonic/periphytic) (Fig. F5). A species list of all 144 recorded taxa from Hole M0059A is presented in Table T2. 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 (Table T3). Diatom preservation in Hole M0059A can be regarded as good to moderate based on the ratio of heavily to finely silicified taxa and the degree of fragmentation (e.g., Koç and Scherer, 1996). More detailed preservational data will be compiled onshore (e.g., Warnock et al., 2007).

The results of qualitative analysis give a good overview of the diatom assemblages and paleoenvironmental changes downcore. The results must, however, be interpreted with caution because the graph (Fig. F5) is based only on qualitative analyses. Because each species was counted as only present or absent, a species represented by a single valve carries as much weight as a species dominating the assemblage.

Sections 347-M0059A-1H-1 through 15H-1; 0–46.5 mbsf

A relatively diverse brackish-marine diatom assemblage with 15–54 taxa per sample is recorded at 0–46.5 mbsf. The dominating planktonic taxa are Thalassionema nitzschioides, Chaetoceros spp. resting spores, Thalassiosira oestrupii, Thalassiosira eccentrica, and periphytic taxa Paralia sulcata, Dimmeregramma minor, Hyalodiscus scoticus, Grammatophora oceanica, and Cocconeis scutellum. This assemblage resembles that recorded in the more marine phase of the Littorina Sea stage of Baltic Sea history (e.g., Westman and Sohlenius, 1999; Andrén et al., 2000a, 2000b). Below is a description of the environmental requirements and geographic distribution of some key diatom taxa found in the sequence.

The very fine silicified vegetative valves of Chaetoceros are rarely recovered in sediments but occur very frequently in the photic zone (Snoeijs et al., 1993–1998). Fossil occurrences of Chaetoceros are instead often recorded as heavily silicified resting spores, which is consistent with the record from Hole M0059A. However, it should be noted that partial vegetative valves of Chaetoceros were found in many samples, indicating very high quality siliceous microfossil preservation in some intervals. Resting spores are difficult to separate at species level but contribute information on surface water salinity and nutrient availability. In the oceans, Chaetoceros resting spore sedimentation occurs in response to nutrient depletion at the termination of a phytoplankton bloom (Grimm and Gill, 1994). Leventer et al. (1996) concluded that environmental stress in the form of nitrogen deficiency results in the production of a Chaetoceros resting spore stage in the Antarctic region, and this conclusion has been further supported by experimental data (e.g., Oku and Kamatani, 1997). In the Baltic Sea, high resting spore abundance is recorded during the Holocene thermal maximum, Medieval Climate Anomaly, and the last ~50 y and used as an indicator of high-productivity events (Andrén et al., 2000a).

P. sulcata is a robust brackish-marine diatom with a wide ecological range. It is believed to live on sandy sediment bottoms and is easily lifted up into the plankton of the coastal waters (tychoplanktonic), particularly after winter storms (Hendey, 1964; Zong, 1997). The salinity reported is very broad, ranging from entirely marine conditions down to a salinity of 5, and P. sulcata seems to be very tolerant of fluctuating water salinities when compared with other taxa (Zong, 1997). P. sulcata is a neritic species commonly found in near-continental marine sediments (e.g., on the continental margins of Norway) and is advantaged by warm Atlantic surface waters (Koç Karpuz and Schrader, 1990). The valves of P. sulcata are heavily silicified and resistant to breakage and dissolution, and it is often found as the dominant species in fossil assemblages (Stabell, 1985), which is consistent with the record in Hole M0059A to 46.5 mbsf.

T. nitzschioides is a common widely distributed marine neritic diatom species that often has massive blooms in the northern Atlantic area (Hendey, 1964; Hustedt, 1930). It is currently found from the Kattegat Sea to the southwest Baltic Sea around Bornholm in waters of salinity ranging between ~8 and 34 (Snoeijs and Vilbaste, 1994). In the Baltic Basin, this species is mainly recorded in the most marine phase of the Littorina Sea stage of Baltic Sea history (e.g., Witkowski, 1994; Sohlenius et al., 1996; Westman and Sohlenius, 1999; Andrén et al., 2000a, 2000b).

T. oestrupii is considered a cosmopolitan oceanic species that is occasionally found in coastal waters (Fryxell and Hasle, 1980). It does not live in the modern Baltic Sea (Snoeijs and Kasperoviciene, 1996), but it has been found in Littorina Sea stage sediments (e.g., Westman and Sohlenius, 1999; Andrén 2000a, 2000b). Koç Karpuz and Schrader (1990) found T. oestrupii and T. nitzschioides to be indicators of warm Atlantic surface water in the Greenland, Iceland, and Norwegian Seas.

Cyclotella choctawhatcheeana is a small pelagic taxon that is very common in the Baltic proper today (Håkansson et al., 1993; Snoeijs, 1993–1998). It blooms occasionally in the Baltic Sea in the summer when there is an upwelling event near the east coast of Sweden (Hajdu and Larsson, 1990). C. choctawhatcheeana is the most abundant taxon in Chesapeake Bay (USA) and is thought to be an indicator of anthropogenic perturbation in brackish water (Cooper, 1995). However, in a low-salinity environment such as the Gulf of Finland, there is no clear correlation between high nutrient availability and this taxon (Weckström and Juggins, 2006).

Besides a diverse diatom assemblage, the silicoflagellate Dictyocha speculum Ehrenberg was recorded at nearly all levels in the brackish-marine interval, and the ebridian Ebria tripartita (Schumann) Lemmermann was recorded at a few levels (Fig. F5).

Silicoflagellates are planktonic algae that are most abundant in areas of upwelling and in equatorial waters but are also abundant at high latitudes (McCartney, 1993). Silicoflagellates are exclusively marine, requiring a salinity of >20 (Tappan, 1980). However, they are found in the Straits of Denmark (Moestrup and Thomsen, 1990) and the Arkona Basin (Willén, 1995), and they live in the Black Sea, where salinities are as low as 10 (McCartney, 1993). The taxon found in Hole M0059A, D. speculum Ehrenberg, generally occurs in water cooler than 15°C and has an optimum temperature near 0°C (Tappan, 1980; McCartney, 1993).

Ebridians are marine planktonic organisms present in the neritic zone and near river mouths (Tappan, 1980). The ebridian recorded in Hole M0059A, E. tripartita (Schumann) Lemmermann, has an optimum temperature of ~10°C and is known in the North Sea and the Baltic Sea (Tappan, 1980; Willén, 1995). The presence of ebridians has been attributed to high availability of nutrients in studies of recent eutrophication in the Baltic Sea (e.g., Risberg, 1990; Witkowski and Pempkowiak, 1995; Andrén et al., 2000a).

Sections 347-M0059A-15H-2 through 16H-1; 48–51.3 mbsf

A lacustrine assemblage is recorded in this part of the core. The transition between the lacustrine and brackish-marine environment can be narrowed down to Section 347-M0059A-15H-1 between 46.5 and 48 mbsf. The assemblage indicates a large lake flora typical of the Ancylus Lake (e.g., Hedenström and Risberg, 1999; Ojala et al., 2005): Aulacoseira islandica, Stephanodiscus neoastraea, Karayevia clevei, Martyana martyi, Ellerbeckia arenaria, and Diploneis mauleri. In the lowermost sample at 51.3 mbsf, there are other taxa in addition to the large lake flora that indicate a small, more nutrient rich lake environment (Andrén, 1999; Ojala et al., 2005). This interval is dominated by Aulacoseira granulata and Aulacoseira ambigua.

In addition to the diverse freshwater diatom assemblage, chrysophyte cysts are recorded in this sequence. Chrysophyte cysts occur in both marine and freshwater environments (Lipps and McCartney, 1993; Duff et al., 1995; Wilkinson et al., 2001). The factors involved in inducing encystment are poorly understood, but freezing and ice formation and changes in nutrient stress have been implicated (Mitchell and Silver, 1982; Duff and Smol, 1988; Smol, 1988). An attempt to assign the cysts found in the Littorina Sea stage sediments to different salinity ranges has been made (Westman and Sohlenius, 1996), but in Hole M0059A, cysts were only recorded in the lacustrine environment (Fig. F5).

Sections 347-M0059A-16H-2 through 25H-1; 52.8–78.4 mbsf

The glacial clays deposited beneath the sandy layer at 51.7 mbsf are barren of siliceous microfossils. It has been suggested that the very low primary production in the ice lake dammed in front of the receding Scandinavian Ice Sheet was caused by poor light conditions in the water due to the heavy load of sediments discharging from the melting ice (Winterhalter, 1992). This environment with extremely high sedimentation rates would further dilute any traces of diatoms in the sediment.

Sections 347-M0059A-26H-1 through 28H-1; 81.7–84 mbsf

Below the glacial clays, the two samples from core tops contain diatoms and chrysophyte cysts. The assemblage is a mixture of freshwater and brackish-marine taxa with both pelagic and periphytic life forms.

Foraminifers

For foraminifers, results are summarized for samples taken offshore and onshore (i.e., samples taken from core catchers and sections, respectively). A total of 196 samples were processed and analyzed for abundance and diversity of benthic foraminiferal fauna (>125 µm size fraction) (Table T4). The low diversity of species at this site indicates that conditions were never fully marine in Little Belt, as they are today in the Kattegat/Skagerrak area with bottom water salinity >30. The abundance of foraminifers, however, varies significantly downcore, suggesting that large variations between nearly fresh and marine-brackish conditions occurred (Fig. F6). However, abundances can also be influenced by the large variations in lithology and/or sedimentation rates, which could have led to dilution of foraminifers with high sediment volume. For postcruise investigations, the volume of sediment used will need to be taken into account for quantifying number of specimens per unit of sediment.

Downcore abundance of foraminifers at Site M0059 can be divided into five major intervals.

In the first interval (0–48 mbsf; Cores 1H through 15H in Holes M0059A, M0059C, and M0059D), foraminifers continuously occur in relatively high abundances and decrease with depth (Fig. F6). Species diversity is low in the entire interval (Fig. F7). The assemblage is always dominated by Elphidium spp., especially Elphidium excavatum clavatum (60%–95%), along with Elphidium excavatum selseyensis, Elphidium albiumbilicatum, and Elphidium williamsoni. Ammonia beccarii, often the first foraminiferal species to appear in low-salinity environments (Kristensen et al., 2000), occurs occasionally in the deeper part. The predominance of E. excavatum clavatum indicates mainly brackish conditions (salinity < 22) with slightly fresher conditions occurring around 40 mbsf as indicated by the slight increase of E. albiumbilicatum, a low-salinity tolerant species (Kristensen et al., 2000).

The interval from 48 to 60 mbsf is barren with respect to foraminifers.

The third interval, 60 to ~87 mbsf, has a clear maximum in foraminifers between 81 and 86.5 mbsf (Cores 347-M0059A-25H through 27H, 347-M0059C-25H through 27H, and 347-M0059B-7H through 8H). E. excavatum clavatum and E. albiumbilicatum (up to 80%) are the predominant species in this interval, with the peak occurrence around 81 mbsf. Findings of E. albiumbilicatum and occasional tests of Bucella frigida indicate lower salinity and possibly colder conditions (Madsen and Knudsen, 1994). However, colder conditions in the shallow-marine setting can be caused by a relative sea level rise leading to increased water depth and thus lower bottom water temperature.

Between ~87 and 160 mbsf, the sediments do not contain Quaternary foraminifers.

The fifth interval covers the lowermost part of Site M0059 from 160 to 210 mbsf. Because of lower recovery in this interval, it was not possible to determine if a continuous record of foraminifers is present. The analyzed samples (Cores 347-M0059C-80S, 347-M0059C-84S, 347-M0059B-27X, and 347-M0059B-29P) only contained reworked foraminifers. Examples of reworked species include Parasubbotina sp. and Globalomalina sp., which are known from the Maastrichtian/Danian boundary at Stevns Klint (Rasmussen et al., 2005). Reworked foraminifers were identified as species that do not occur during the Quaternary and have a “frosty” appearance (Sexton et al., 2006). The sample from Core 347-M0059C-84S also included several agglutinated foraminifers.

Ostracods

Ostracods were examined from 195 samples (including 119 core catchers) from Site M0059 during the onshore phase of Expedition 347 at the Bremen Core Repository. Samples were studied entirely in the >125 µm fraction in order to achieve a higher number of ostracods. Ostracods were found in 62 samples (Table T5).

Ostracod abundance from the onshore samples was calculated per sediment volume and is similar among the different holes at the site (Fig. F8). Ostracods were recorded from 2 to 60 mbsf (Holes M0059A and M0059D) and from 76 to 86 mbsf (Holes M0059A, M0059C, and M0059D). Ostracod abundance varies markedly but remains relatively low in most samples (10–20 valves/20 cm³). Peaks of up to 40–50 valves/20 cm³ are observed at 23 mbsf in Hole M0059D, at 41.5 mbsf in Hole M0059A, and at 52 mbsf in Hole M0059E (Fig. F8).

Preliminary taxonomic identifications revealed at least three different assemblages of ostracods. The upper ~50 m (Holes M0059A, M0059C, M0059D, and M0059E) contains marine and brackish-water ostracod taxa. A group of very shallow brackish-water and marine species is dominated by Leptocythere spp., Cythere lutea, Hirshmania viridis, and Cytherura spp. These taxa can tolerate decreased salinity to 6.5–10 and inhabit the shallow sublittoral zone (Frenzel et al., 2010). Two peaks in abundance are observed at 23 mbsf (Hole M0059A) and 41.5 mbsf (Hole M0059D). Both peaks are characterized by an increase in deeper water marine ostracods. The second group of marine species includes Cytheropteron latissimum, Palmoconcha spp., Robertsonites tuberculatus, and Sarsicytheridea bradii. C. latissimum, R. tuberculatus, and S. bradii are typical of open-sea conditions and are commonly found on Arctic shelves (e.g., Stepanova et al., 2007; Frenzel et al., 2010). In the Baltic Sea, R. tuberculatus and S. bradii inhabit the deeper parts (18–32 m) of the southern Baltic region where salinity is higher (Rosenfield, 1977).

At ~50 mbsf (Holes M0059A, M0059C, M0059D, and M0059E), the assemblage comprises the freshwater species Cytherissa lacustris and Candona spp. C. lacustris is commonly found in cold and well-oxygenated waters with salinity <1.5 (Frenzel et al., 2010). This interval is also characterized by a peak in abundance and a high juvenile to adult ratio, altogether indicating an in situ assemblage and a freshwater environment.

At ~80 mbsf (in Holes M0059A, M0059B, and M0059D), the assemblage comprises different ecological groups: freshwater, oligohaline, shallow-water marine, and redeposited pre-Quaternary valves. Together with the low percentage of juvenile valves, it likely indicates a high-energy environment in a very shallow near-shore location.

Palynological results

For Site M0059, palynological analyses focused on Holes M0059A and M0059C (lower part). Resolution of one sample per ~1.5 cores (2–5 m between samples) was used for pollen and one per two cores for dinoflagellate cysts. The majority of samples analyzed contained enough palynomorphs to generate statistically relevant results for low-resolution pollen profiles (Figs. F9, F10). For Site M0059, bisaccate pollen was included in the reference sum because it does not appear to be overrepresented as a result of transportation bias. The dinocyst content was variable depending on the type of sediment. Sediments with marine components (e.g., foraminifers) contained significant numbers of dinoflagellate cysts. Lacustrine sediments could be identified by the presence of freshwater algae and insect remains and the absence of marine dinocysts.

Hole M0059A

For Hole M0059A, 28 sediment samples were prepared for pollen analysis.

5.62–10.45 mbsf

Pollen spectra of this interval are dominated by broad-leaved tree pollen. Among these, Fagus pollen is the most characteristic for the interval 5.62–10.45 mbsf, with values ranging from 4.5% to 19%. Among other tree taxa, fairly high amounts of pollen were noted for Quercus (maximum of 29%), Alnus glutinosa type (22.5%), Pinus sylvestris type (17.5%), Betula alba type (13.5%), and Carpinus (3%). The presence of Secale type and Plantago lanceolata pollen is noteworthy, as they are an indication of human activity. Pollen data from 5.62 to 45.07 mbsf reveal a succession that is very similar to that known from pollen records from Denmark and northern Germany and Poland (e.g., Dörfler et al., 2012; Apolinarska et al., 2012). The high percentages of Fagus in the uppermost samples (Fig. F9) indicate an age younger than ~2200 y BP (sub-Atlantic) as previously recorded in the high-resolution, well-dated pollen record from Lake Belau (~80 km south of Site M0059; Dörfler et al., 2012). The presence of numerous microreticulate cysts of Gymnodinium (probably G. nolleri; Figs. F10, F11) in the uppermost 38 m at this site indicates a sub-Atlantic age (e.g., Harland and Nordberg, 2011), which is in accordance with the findings from the pollen record. Gymnodinium cysts were excluded from the dinoflagellate percentage calculations because of their extraordinary high occurrences in some samples (Fig. F10).

15.57–45.07 mbsf

Pollen of Quercus (maximum of 35%) and A. glutinosa type (maximum of 34%) dominate this interval. Corylus avellana pollen percentages vary between 4% and 27% in this interval, reaching peak abundances at 24.27 and 41.57 mbsf. No reworked Tertiary pollen and no Pediastrum coenobia were found, only sporadic occurrences of Botryococcus colonies. Dinoflagellate cysts are present in high quantities. This indicates a marine environment during sedimentation, and the interval most possibly covers Atlantic and Subboreal phases.

49.16–50.65 mbsf

In pollen spectra of this interval, P. sylvestris type pollen dominates, reaching up to 42%. Quercus (21%) and C. avellana (15%) also occur frequently. Pollen of B. alba type, Ulmus, and Fraxinus are present. A characteristic feature of this interval is the absence of dinoflagellate cysts and Tertiary reworked pollen, whereas freshwater green algae Pediastrum and Botryococcus are present (3% and 6.5%, respectively). Presence of pollen originating from reed swamp plant communities (Phragmites type included in Poaceae pollen) as well as from Myriophyllum spicatum (a freshwater vascular plant) implies that a freshwater environment prevailed during this interval. This implication is also supported by more frequent findings of aquatic insect remains (Fig. F11). The high P. sylvestris type pollen amount may suggest an early Boreal age of the sediments (Apolinarska et al., 2012). An interval of particularly high percentages of Ulmus pollen between 36.03 and 49.16 mbsf can probably be correlated with a similar interval, dated between 9500 and 6000 y BP, in the Lake Belau pollen record from Northern Germany (Dörfler et al., 2012).

54.42–83.40 mbsf

Samples from the depth interval 54.42–83.40 mbsf have a very low frequency of pollen. Only in samples at 54.42, 80.10, and 83.40 mbsf was sporomorph frequency high enough to calculate percentage pollen spectra (Fig. F9). The pollen spectra are characterized by high amounts of reworked Tertiary pollen (Fig. F9). Among the observed Quaternary pollen taxa that are considered in situ, pollen of P. sylvestris type and B. alba type dominate, reaching >30%. Pollen of Picea is present with up to 10% and Juniperus with 5%. Among broad-leaved trees, the highest pollen percentages were noted for C. avellana and A. glutinosa type pollen. Very characteristic is a high amount of freshwater green algae Pediastrum and Botryococcus. Additionally, the presence of some organic-walled dinoflagellate cysts may point to a brackish environment. The reworked pollen of Tertiary origin most probably indicates meltwater inflow with redeposition of Tertiary sediments.

Hole M0059C

Six samples were macerated for pollen analysis. To complement the pollen record from Hole M0059A, the samples were selected from the depth interval 80.27–141.72 mbsf. Only the sample from 80.27 mbsf contains an appropriate pollen frequency for obtaining a reliable pollen spectrum. This spectrum correlates very well with the lower part of the pollen diagram from Hole M0059A (Figs. F9, F12). The rest of the results are given in Table T6 and PalyM0059.xls in PALYNOLOGY in “Supplementary material”). Dinocysts are frequently present in the analyzed samples, but many of the encountered specimens in the lower part of Hole M0059C appear to be reworked and probably belong to tertiary taxa. Based on pollen analysis results (mainly green algae), sediments from the interval 80.27–141.72 mbsf most likely have a brackish origin.

Top of page   |    Previous   |    Next