Retention of mercury in sediments of the Baltic Sea near Oulu,Finland

Several cores were compared to elucidate the grade of mercury retention in sediments of the Baltic Sea near Oulu, Finland. The known history of the mercury discharge to waste water from a chlor-alkali plant, the primary pollution source, was compared to that in the sedimentary record. One core was dated by lead-210 and varve counts. During the past thirty years, the rate of sedimentation was up to 10 mm per year corresponding to a dry matter accumulation rate of about 3 kg m^–2a^–1. Under these conditions, most of the mercury reaching the sediment was retained. Correspondingly, the mercury levels in fish were considerably lower, with a delay of some five years.The present mercury content of the upper layers is some 1/4 or 1/5 that of the late 1970's, but it is still some twentyfold compared to levels at the beginning of the century. The extent to which the present mercury level in the sediment represents the various phases of industrial emission remains unsolved due to the origin of mercury in this sea area from several other sources and due to difficulties in exact core correlations from different years.     

An overview on lagoons in the Polish coastal area of the Baltic Sea

Coastal lagoons in Poland arebiologically diverse and very productive areas, andhave a high economic importance as waterways, astourist resorts and through fisheries. On the otherhand, they remain under strong antrophogenic pressuredue to the high load of nutrients from agriculture,discharge of large amounts of municipal sewage andindustrial wastes. Three important Polish lagoons,namely the Odra/Szczecin, the Vistula, the PuckLagoons, are briefly described and environmentalproblems to be considered for sustainable managementare discussed.     

Holocene diatom stratigraphy in the Archipelago Sea,northern Baltic Sea

Two sediment cores from the Archipelago Sea in the northern Baltic Sea were examined for their siliceous microfossils in order to study the Holocene palaeoenvironmental history of the area. The diatom record was divided into local diatom assemblage zones (LDAZ). An age model was constructed using independent palaeomagnetic and AMS-¹⁴C methods. The early history of the Archipelago Sea was freshwater. Initial brackish-water influence is observed at 7,950 ± 80 cal. BP (LDAZ4), but fully brackish conditions were established at 7,700 ± 80 cal. BP (LDAZ5). Diatom assemblages indicate increasing salinity, warming climate, and possible increasing trophic state during the transition from lacustrine to brackish-water conditions. The decreasing abundance of Pseudosolenia calcar-avis (Schultze) Sundström and the increasing abundance of the ice-cover indicator species Pauliella taeniata (Grunow) Round and Basson indicate reduced salinity and climatic cooling after ～5,000 cal. BP. LDAZ boundaries do not always correlate with changes in the sediment appearance, which underlines the importance of defining biostratigraphic boundaries independently to the sediment visual character, in contrast with the conventional practice for classifying the Baltic Sea sediments.     

Assessing Recent Eutrophication in Coastal Waters of the Gulf of Finland (Baltic Sea) using Subfossil Diatoms

Marine eutrophication of estuaries and coastal waters is considered to be a significant problem worldwide. In the semi-enclosed Baltic Sea, where the nutrient load has strongly increased from its natural level, this has led to marked changes in the coastal ecosystems. A key to successful management of coastal waters is reliable scientific evidence of their past state. The palaeolimnological record of subfossil diatoms was used to study the rate and magnitude of eutrophication over the last ca. 200 years in two urban and three rural sites. The urban sites showed marked increases in the percentage abundance of planktonic diatoms (from <50 to ca. 90% and from <5 to ca. 70%) and diatom-inferred total dissolved nitrogen (from <800 to ca. 3000 μg l⁻¹ and from <400 to ca. 800 μg l⁻¹), and a decrease in species richness starting in the 19th – early 20th century with increased urbanisation. At both sites a clear recovery was observed after the cessation of waste water loading by the mid 1980s. The present planktonic diatom assemblages of these embayments, however, show no change back to the pre-disturbance diverse benthic communities. In contrast, the changes observed in the rural sites were only moderate and occurred later starting in the 1940s. No marked increases in diatom-inferred total dissolved nitrogen were seen, however, all sites showed an increase in small planktonic taxa (from ca. 1–6% to 8–36%) indicating increased nutrient enrichment and turbidity. These small floristic changes could be seen as an early warning signal despite little change in the inferred nutrient concentrations. The results have implications for the European Water Framework Directive, which requires European surface waters to be of good ecological status, defined both by biological and chemical quality elements.     

Diatom stratigraphy in five offshore sediment cores from the northwestern Baltic proper implying large scale circulation changes during the last 8500 years

Five Holocene sediment cores from the northwestern Baltic proper were analysed for lithology, siliceous microfossil assemblages and geochemical parameters. The data indicate that surface water salinity and redox conditions below the halocline have changed drastically at least four times since the Baltic Sea changed from a fresh water lake (the Ancylus Lake) to a semi-enclosed brackish water sea (the Litorina Sea) c. 8500 yrs BP. These variations appear to be mainly effects of changes in water depth at the thresholds of inlet areas. Based on these changes, and earlier studies of the shoreline displacement in the inlet areas, we propose a tentative model for changes of large scale water circulation in the Baltic Sea during the last c. 8500 yrs.At the transition from fresh to brackish water 8500 ¹⁴C yrs BP, upwelling of nutrient rich bottom water started to occur, causing a slight increase in primary production. Diatom assemblages in sediments indicate a slow rise in surface water salinity during this period.At 7000-6500 ¹⁴C yrs BP, surface water salinity and primary production simultaneously increased, as anoxic bottom conditions were established at depth below the halocline. We suggest that high primary production was caused by increased input of oceanic water, leading to increased upwelling of nutrient rich bottom water. At the anoxic bottoms laminated sediments formed until 5000-4500 ¹⁴C yrs BP. This period (c. 7000-4500 ¹⁴C BP) was contemporaneous with the post-glacial transgression maximum in Öresund, and we suggest it represents the most saline phase of the Baltic Sea post-glacial history.Due to a regression in Öresund starting 4500 ¹⁴C yrs BP, upwelling decreased and the halocline was lowered, resulting in decreased primary productivity and hence oxic deep water conditions. The diatom assemblages of the sediments indicate a lowering of salinity at the beginning of this period.We suggest that the second period of anoxic bottom conditions c. 2000-1500 ¹⁴C yrs BP was caused by a change of dominating inflows from the Öresund to the Belt Sea. This resulted in decreased salinity of the inflowing water which did not penetrate to the deepest parts of the basin as frequently as before. The diatom record indicates both a second lowering of salinity and a change in the large scale water circulation at the beginning of this period.     

Cyanobacteria as a trigger for increased primary productivity during sapropel formation in the Baltic Sea—a study of the Ancylus/Litorina transition

A sediment sequence from the Gotland Basin, representing the Ancylus Lake/Litorina Sea transition, was analysed for pigments, stable isotopes (δ¹³C and δ¹⁵N), diatoms and carbon and nitrogen content. The negative correlation between the cyanobacterial specific pigment zeaxanthin and δ¹⁵N supports the hypothesis that cyanobacteria in the early Litorina Sea indeed were nitrogen fixers. Their incorporation of nitrogen could have acted as a trigger for eutrophication during the Litorina Sea stage of the Baltic Sea. As cyanobacteria normally flourish in eutrophicated waters, the increasing concentrations of zeaxanthin during early Litorina Sea also corroborate that high primary production was an important part of the formation of organic rich (sapropel) sediments in the Baltic Sea. The first occurrence pre-dates the formation of laminated sediments, but the peaks of both zeaxanthin and organic carbon are within laminated sequences. This implies that the oxygen conditions of the bottom water also play a major role in the formation of sapropel sediments.     

Upper-mantle structure of the Baltic Shield below the Swedish National Seismological Network (SNSN) resolved by teleseismic tomography

Upper-mantle structure under the Baltic Shield is studied using non-linear high resolution teleseismic P -phase tomography. Observed relative arrival-time residuals from 52 teleseismic earthquakes recorded by the Swedish National Seismological Network (SNSN) are inverted to delineate the structure of the upper mantle. The network consists of 47 (currently working) three-component broad-band stations located in an area about 450 km wide and 1450 km long. In order to reduce complications due to possible significant three-dimensionality of Earth structure, events chosen for this study lay close to in-line with the long-axis of the array  (±30°)  . Results indicate P -wave velocity perturbations of ±3 per cent down to at least 470 km below the network. The size of the array allows inversion for structures even at greater depths, and lateral variations of velocity at depths of up to 680 km appear to be resolved. Below the central part of the array (60°–64° N), where ray coverage is best, the data reveals a large region of relatively low velocity at depths of over about 300 km. At depths less than about 250–300 km, the models include a number of features, including an apparent slab-like structure dipping gently towards the north.     

Conductivity anomalies in the Baltic Shield in Finland

In 1981–84 six arrays of 30 or 31 magnetometers were operated on the Baitic Shield in central and southern Finland to deduce information about the electrical conductivity within the Earth's crust. The magnetometer sites cover the area with 20–60 km spacing allowing the identification of large-scale electrical structures within the crust. Each array recorded the three orthogonal magnetic field components on analogue film for about two months simultaneously at all the stations. Altogether 17 magnetic variation events of 2–6hr length have been digitized and analysed.
This paper includes results of the last array in south-western Finland, a comparison of two methods of induction vector estimation and combination of results from all the arrays. Four zones of anomalously high electrical conductivity have been observed. They are all interpreted as lying at depths greater than 5 km. The 'Oulu Anomaly' is wider than the others and rather short, about 100 km. The 'Southern Finland Anomaly' is probably a 500 km-long narrow zone. This and the 'Ostrobothnian Anomaly' surround three sides of the large Central Finland Granite area. The 'Outokumpu Anomaly' is probably deeper lying than the others. Blocks separated by these anomalies are presented. The blocks have different average resistivities.     

The volume and paleobathymetry of glacial Lake Agassiz

During the last retreat of the Laurentide Ice Sheet in North America, many proglacial lakes formed as continental drainage was impounded against the southern and western ice margin. Lake Agassiz was the largest of these lakes. The bathymetry of Lake Agassiz at the Herman and Upper Campbell beach levels – formed at about 11.5–11.0 ka and 9.9–9.5 ka, respectively – was computer modelled in this study by first collecting data for the isostatically-deformed paleowater planes of the two lake levels (derived from isobase lines constructed from beach elevations), and then subtracting these from the modern topography of the former lake floor. Pixels with dimensions of 1/30 × 1/30 of a degree were used in the model. Using these data, the area and volume of the lake were also calculated: at the Herman level these were 152 500 km2 and 13 100 km3 respectively; at the Upper Campbell level these were 350 400 km2 and 38 700 km3. Contour maps showing the paleobathymetry of both periods in the lake's history were also constructed. Determining the paleobathymetry and volume of Lake Agassiz is an important step in understanding the impact that the lake had on its surrounding environment and on the rivers, lakes, and oceans into which it flowed.     

The role of biogeochemical cycling for the formation and preservation of varved sediments in Soppensee (Switzerland)

We analyzed seasonally aggregated observations of temperature, conductivity, dissolved oxygen and dissolved inorganic carbon from Soppensee (District of Lucerne, Switzerland) for the yrs 1980 to 1993. Holomictic Soppensee is characterized by a strong summer stratification with a thin epilimnion separated from an anoxic hypolimnion by a strong pycnocline formed by thermal and chemical gradients. A vertical one-dimensional model was developed to simulate the observed seasonal cycles of carbon and oxygen. The processes of net community production, mineralization of organic matter, precipitation and dissolution of calcite, gas exchange, in- and outflow, sedimentation and vertical eddy diffusion are included. According to the model, the annual net community production is estimated to about 110 g C m^-2 yr^-1 and the annual net primary production to about 330 to 440 g C m^-2 yr^-1, which is a typical value for eutrophic lakes. A mass balance of the carbon cycle indicates that most of the inflow comes from groundwater which is super-saturated with respect to atmospheric CO₂. Therefore the surface waters exhibit a large capacity for calcite precipitation. The results of the model are used to constrain the conditions that favor the formation of varved sediments in Soppensee during thousands of yrs. Model calculations show that the deep waters would still turn anoxic even if the sedimentation rate of organic matter were decreased to 25%. Several physical factors such as biogenic stabilization of the deep waters due to calcite dissolution and low input of wind energy are responsible for the long term anoxia in Soppensee.     

Variation among fossil chironomid assemblages in surficial sediments of Bodensee-Untersee (SW-Germany): implications for paleolimnological interpretation

In paleolimnology, subfossil head capsules of chironomids play an important role as ecological indicators of lake history. It is important to determine, therefore, whether fossil assemblages are representative of former biocoenoses. There is evidence that headcapsules washed in from other places can make up a significant percentage of the total. As interpretations are usually drawn from the examination of a single core, it is of special interest to know whether a fossil assemblage of a single site properly reflects limnological conditions of the whole lake. This study examined the taxonomic distribution of subfossil chironomids in the surficial sediments of the Bodensee-Untersee, with the aim of assessing the variability in chironomid assemblages. Apparently, most of the head capsules of the profundal fossil assemblages in the Untersee had been washed in from the littoral zone or from the slope. Although the Bodensee-Untersee is a rather large lake, variability is surprisingly low among all samples. Therefore a correct interpretation from a single core may be possible.     

Freeze coring of soft surface sediments at a water depth of several hundred meters

A freeze corer was developed specifically for the sampling of unconsolidated surface sediments by in-situ freezing. The new device which is presented here is designed to be used to a water depth of up to 1000 meters and was successfully tested at a depth of 200 m. The corer consists of a high pressure resistant housing with a vacuum insulated jacket for the ethanol which can be chilled by separate cooling agents, adjustable stabilizing legs, an electrical pump and a freezing wedge of 100 cm length. The freeze coring can be performed from a platform or a boat by using only a single cable to pull it up or to lower it. All other equipment is inside of the resistant housing. The core sample can easily be cut into two distinct slabs by use of the freeze-protectors on the small sides of the freeze wedge.