The Baltic Sea Basin

The Baltic Sea Basin serves as an example of a region where the use of natural resources and the need of environmental protection require a comprehensive and holistic approach in terms of geosciences, environmental sciences, and socio-economics. In this book, authors from countries around the Baltic Sea and overseas shed light on the Baltic Sea Basin with respect to (1) the formation of the Baltic Basin and its geological resources, (2) the stratigraphic record – mirror of climatic changes during the last glacial cycle, (3) coastal processes and sediment dynamics including aspects of coastal engineering, (4) interaction between socio-economic driving forces and the natural environment since the prehistoric colonization, (5) management of the marine ecosystem, and (6) monitoring strategies, respectively remote sensing. The editors intend not only to provide a record of the current state of the art in the investigation of the Baltic Sea Basin, but also to initiate innovative interdisciplinary and international research activities.

The Baltic Sea is a young geomorphologic feature that formed during Quaternary time. It covers the western and the central part of the Baltic sedimentary basin. The origin of the Baltic Sea and of the corresponding morphological low is still controversial, considered by some as an erosional structure and as a tectonic depression by others. The chapter gives a summary of the evolution and the known resources of the Baltic sedimentary basin focussing on its central part and thus tries to present new evidence for the origin of the Baltic Sea. The Baltic sedimentary basin was formed during Late Ediacaran–Early Cambrian time. Its formation was caused by the reactivation of the weakest lithospheric part of the East European craton. All the following stages of pronounced basin subsidence (major subsidence phase during Late Ordovician–Middle Silurian), including the recent tectonic stage, were dominated by extensional tectonics. However, the most intense structuring of the crust in the region took place in a compressional setting during Late Silurian and Early Devonian time. The NW–SE-directed compression was caused by the collision of Laurentia and Baltica. It caused the formation of an Early Palaeozoic thrust and fold belt at the margin of the East European craton and led to the formation of E–W and NE–SW striking faults in the Baltic basin northeast of the Danish–North German–Polish Caledonides during that time. Typical for the Permocarboniferous period are magmatic intrusions in the southern part of the Baltic Sea, in northern Poland, and in the area of the Rügen Island. Tectonic activities ceased within the Permian and only small amplitude faulting is detected in the Mesozoic. Later on, tectonic activities increased during the Cretaceous inversion in the southwestern part of the basin. The typical wrench-dominated faulting is related to the reactivation of Pre-Permian fault systems by Late Cretaceous inversions of the Mesozoic Danish and Polish basins. Large-scale ancient structures of the Baltic basin are reflected in the sea bottom morphology. Detailed analysis indicates that those morphological structures are mainly passive features related to selective glacial erosion, but some hints for neotectonic activities do also exist. Glacial erosional processes undoubtedly contributed to the shape and depth of the Baltic Sea. Evidences available today, however, suggest the existence of a pre-existing tectonic depression. Major resources of the deeper underground of the Baltic basin are oil, gas, geothermal energy and reservoir formations which can be used as storage sites (natural gas, CO₂, compressed air). Location of known oil and gas fields shows a strong relation to the major fault zones.

Plio-Pleistocene erosion and sedimentation significantly impact postglacial uplift. We estimate in the last glacial cycle sedimentation could produce up to 155 m of subsidence and erosion 32 m of uplift. To show this we determine the changes in surface load caused by glacial and postglacial erosion and sedimentation over 1,000 year time intervals (coarser intervals before 50,000 years) utilizing a largely automated interpretation of regional geological and geomorphological observations that is constrained by plausible bounds on the rate of erosion of various lithologies and the known general pattern and behavior of glacial ice (ice boundaries over time, the dendritic pattern of ice movement, geometry of fast-flowing ice streams, plausible changes in frozen-bed conditions, etc.). Mass balance between erosion and deposition is enforced at all times. The analysis is regional and obliged to agree with all known geological constraints. Although the focus is on the last glacial cycle, all previous cycles are considered. The analysis suggests that the first glaciations probably shaped the major overdeepened troughs, although it is possible that the deepening was distributed evenly over all the cycles. Younger glaciations mainly removed sediments left by their predecessors, decreasing the thickness of the Quaternary succession and only locally incising and changing the dip of the bedrock surface. Over the last glacial cycle, ~20–90 m of sediments (and locally more) was removed in the zones of most active erosion.

During the Eemian interglacial 130–115 ka BP, the hydrology of the Baltic Sea was significantly different from the Holocene. A pathway between the Baltic basin and the Barents Sea through Karelia existed during the first ca. 2.5 ka of the interglacial. Both sea surface temperature and salinity of the SW Eemian Baltic Sea were much higher, ca. 6°C and 15‰, respectively, than at present. A first early Weichselian Scandinavian ice advance is recorded in NW Finland during marine isotope stage (MIS) 4 and the first Baltic ice lobe advance into SE Denmark is dated to 55–50 ka BP. From the last glacial maximum that was reached ca. 22 ka BP, the ice sheet retreated northward with a few still-stands and readvances; however, by ca. 10 ka BP the entire basin was deglaciated. Weak inflows of saline water were registered in the southern and central Baltic Sea ca. 9.8 ka BP with full brackish marine conditions reached at ca. 8 ka BP and the maximum Holocene salinity was recorded between 6 and 4 ka BP. The present Baltic Sea is characterized by a marked halocline preventing the vertical water exchange resulting in hypoxic bottom conditions in the deeper part of the basin.

Late Pleistocene to Holocene climate change of the Atlantic and the northern European realm is reflected by the facies of sediments in the Baltic Sea. The sedimentary sequence have been subdivided into zones reflecting the main postglacial stages of the Baltic Sea basin development according to sediment echosounder profiling and investigating sediment cores from the central Baltic. The changes in the environment of Baltic Sea bottom water is displayed by sediment physical, geochemical, and microfossil proxies. These proxies mark the main shift in the sedimentary facies of the Baltic Basin at 8.14 cal. years BP, from a freshwater to a brackish/marine environment due to the Littorina transgression of marine water masses from the North Sea. The downhole physical facies variation from the Eastern Gotland can be correlated basinwide. Thickness maps of the freshwater and the brackish sediments ascribe the general change in the hydrographic circulation from a coast-to-basin to a basin-to-basin system along with the Littorina transgression. Variations in the salinity of the brackish Littorina Baltic Basin are attributed to changes in the North Atlantic Oscillation (NAO) ascribing the wind forces driving the inflow of marine water into the Baltic Basin. Time series analysis of facies variation reveals distinct periodicities of 900 and 1,500 years. These periods can be compared with data from North Atlantic marine sediments and Greenland ice cores identifying global climate change effects in Baltic Basin sediments.

The Klaipėda Strait is located between the Curonian Spit and the mainland coast of Lithuania. It links the Curonian Lagoon with the Baltic Sea. The Quaternary sequence is represented here by Pleistocene sediments formed during a few glaciations and interglacials. Its uppermost part is composed of Late glacial and Holocene sediments originating from different stages of the Baltic Sea development. One of the main problems of Quaternary geology in the vicinities of the Klaipėda Strait, as well as in the whole Lithuanian Coastal Area, is the reliable geochronology and stratigraphic correlation of sediments. To contribute to the solution of this problem, the infrared optically stimulated luminescence (IR-OSL) dating of the lacustrine inter-till sandy sediments was done during the engineering geological mapping of the Klaipėda Strait. The absolute majority of the IR-OSL ages obtained for the investigated inter-till sediments fall within the age range of marine isotope stages (MIS) 5d-5a. The subsequent more detailed examination of geological setting of Quaternary sequence has led to the assumption that the sampled inter-till sediments occur not in situ, i.e. they are found as blocks (rafts) in a thick till bed that have been formed by the ice advance during the Weichselian early pleniglacial maximum (MIS 4). This conclusion does not support the former standpoint that the till beds beneath the bottom of the Klaipėda Strait were formed during the Warthanian (Medininkai, MIS 6) glaciation.

Relative sea level change is, besides the geological buildup and hydrographic parameters, the main controlling factor in shaping the coastlines on the centennial timescale and beyond. Vertical displacement of the earth’s crust and eustasy serve as main components driving the relative sea level (RSL) change during the Quaternary. Whereas the eustatic change mirrors mainly climatic factors, the vertical displacement of the earth’s crust has to be regarded in former glaciated areas as a result of glacio-isostatic adjustment superimposed by the regional tectonic regime or land subsidence due to local factors. A simple model is applied to reconstruct the palaeogeographic development of a coastal area and to generate future projections as coastline scenarios. For the hindcast relative sea level curves have to be compared with recent digital elevation models. For future projections data of vertical crustal displacement received from gauge measurements and eustatic changes based on climate scenarios have to be superimposed. The model has been applied to the Baltic Basin, considered as a natural laboratory for coastal research as it is extending from the uplifting Fennoscandian Shield to the subsiding southern Baltic lowlands. Subsidence, climatically driven sea level rise, and meteorologically induced coastal flooding provoke permanent coastal retreat at the southern sinking coasts. Predictions of coastal hazards are made with the model by using neotectonical data and long-term sea level change data superimposed with extreme sea level data measured during the storm surge in November 1872.

The authors combined geological, geodetic and archaeological shore displacement evidence to create a temporal and spatial water-level change model for the SW Estonian coast of the Baltic Sea since 13,300 cal. years BP. The Baltic Sea shoreline database for Estonian territory was used for the modelling work and contained about 1,200 sites from the Baltic Ice Lake, Ancylus Lake and Littorina Sea stages. This database was combined with a shore displacement curve from the Pärnu area (in SW Estonia) and with geodetic relative sea-level data for the last century. The curve was reconstructed on the basis of palaeocoastline elevations and radiocarbon-dated peat and soil sequences and ecofacts from archaeological sites recording three regressive phases of the past Baltic Sea, interrupted by Ancylus Lake and Littorina Sea transgressions with magnitudes of 12 and 10 m, respectively. A water-level change model was applied together with a digital terrain model in order to reconstruct coastline change in the region and to examine the relationships between coastline change and displacement of the Stone Age human settlements that moved in connection with transgressions and regressions on the shifting coastline of the Baltic Sea.

A GIS-based palaeogeographic reconstruction of the development of the Baltic Ice Lake (BIL) in the eastern Baltic during the deglaciation of the Scandinavian Ice Sheet is presented. A Late Glacial shoreline database containing sites from Finland, NW Russia, Estonia, Latvia and modern digital terrain models was used for palaeoreconstructions. The study shows that at about 13,300 cal. years BP the BIL extended to the ice-free areas of Latvia, Estonia and NW Russia, represented by the highest shoreline in this region. Reconstructions demonstrate that BIL initially had the same water level as the Glacial Lakes Peipsi and Võrtsjärv because these water bodies were connected via strait systems in central and northeast Estonia. These strait systems were gradually closed at about 12,700–11,700 cal. years BP due to isostatic uplift, prior to the final drainage of the BIL. Glacial Lake Võrtsjärv isolated from the BIL at about 12,400–12,000 cal. years BP. Exact timing of Glacial Lake Peipsi isolation is not clear, but according to the altitude of the threshold in northeast Estonia and shore displacement data, it was completed at about 12,400–11,700 cal. years BP.

As the existing data on the location, number, and age of submerged Holocene wave-cut cliffs (submerged coastlines) in the Gulf of Gdańsk (SE Baltic Sea) are rather conflicting, earlier data were reanalysed and compared with recent information. Digital bathymetric and slope angle maps were developed from the modern 1:25,000, 1:50,000, and 1:100,000 nautical charts. The maximum slope lines were assumed to correspond to wave-cut cliff axes. A total of five axial lines of post-glacial wave-cut cliffs were identified: two dated to the Yoldia Sea (58–45 and 52–40 m), one assigned to the Ancylus Lake (38 m), and two dated to the Littorina Sea (29 and 21 m).

This chapter presents a newly discovered locality of tree stumps occurring in situ at the bottom of the Gulf of Gdańsk. It focuses in particular on the age of the stumps and characterization of the palaeoenvironment, i.e. the nature of the plant communities in which the trees grew and also on their position in relation to the palaeo sea level. Tree stumps occurring in situ on the sea floor along with peat deposits are the most reliable indicators of sea level changes. The site is located about 6–7 km NE of the entrance to the Gdańsk harbour, in water depth of 16–17 m. The thickness of marine sand at the site is from a few to a dozen centimetres. Below the sand, gyttja with peat intercalations and wood fragments occur. Sixteen fragments of alder trunks and one oak trunk’s fragment were extracted. Radiocarbon ages of the tree trunk fragments are 7,920 ± 50 BP, 7,940 ± 40 BP, 7,960 ± 40 BP and 8,000 ± 50 BP. The age of gyttja, according to pollen analyses, is of early Atlantic period. The characteristic forest composition of that time was the broad deciduous forest with oak (Quercus), elm (Ulmus) and lime (Tilia). The climate was characterized by good thermal and moisture conditions, which is confirmed by the presence of pollen grains of mistletoe (Viscum) and ivy (Hedera). The obtained data about the time of accumulation of the investigated sediments indicate that the sea level at that time was about 19–20 m lower than at present.

Coastal barriers and spits develop when the accumulation space available in the coastal sea for sediment deposition decreases and partly fills up. The accommodation space increases when sea level rises and decreases when sediment accumulates. In addition to the coastal relief prior to the sea-level rise, which determines the potential accommodation, the evolution depends on the volume and rate of sediment supply. The example from the north-eastern German Baltic coast shows how the course of Holocene sea-level rise (Littorina transgression) varied due to glacio-isostatic uplift of different coastal sections and thus the growth of accommodation space. Further, the role of the sediments which built up the shoreface and the coastal landforms is discussed. We also examine the influence of the main inclination of pre-transgressional relief on the development, aggradation and progradation of beach ridges, spits and barriers. The determination of the volume of the present barriers allows rough estimations regarding the volume of sediment supplied from eroding cliffs. In a final synopsis, the interplay of all factors is discussed, explaining the distribution, volume and stability of the barriers along the German Baltic coast.

Beaches along the northern coast of Estonia form an interesting class of almost equilibrium, bayhead beaches located in bays deeply cut into the mainland in an essentially non-tidal, highly compartmentalised coastal landscape, and that develop mostly under the influence of wave action. These beaches, although often suffering from a certain sediment deficit, are stabilised by the postglacial land uplift. We describe the basic features of their appearance and functioning from the viewpoint of sediment transport processes. Wave action normally impacts a relatively narrow nearshore band and additionally stabilises the beaches through littoral drift of sandy sediment and gravel towards the bayheads. Eolian transport and fluvial sediment supply have typically very modest magnitude. Such beaches, in general, evolve quite slowly and may represent an almost equilibrium stage, even when the active sand mass is very limited. The concept of the equilibrium beach profile is an adequate tool for their analysis. As an example, its parameters and longshore transport patterns are evaluated for Pirita Beach based on a granulometric survey and long-term simulation of wave climate. It is demonstrated that net sand changes for such beaches can be estimated directly from the properties of the equilibrium profile, land uplift rate, and loss or gain of the dry beach area. Another type of highly dynamic equilibrium exists owing to interplay of the effects of river flow and wave action at the mouths of large rivers such as the Narva River.

Coastlines do not change because of sea level variation alone. Instead, the changes are the result of a complex interaction between climate and geologically controlled processes. Especially on a local scale, sedimentary dynamics play an important role. Even with a rising sea level, concurrent sediment accumulation may prevent coastline retreat. On the other hand, erosion may accelerate marine transgressions remarkably. The southern coast of the Baltic Sea is an impressive example for the impact of erosion, transport, and accumulation of sediments to coastline change during the Holocene. Since the end of the Littorina transgression the coastline morphology has been shaped here mainly by longshore sediment transport controlled by the geological situation and glacioisostatic influence. The longshore sediment transport is driven by wind and consequently waves shaping young Holocene structures like the Darss-Zingst peninsula. In order to model these processes, Sedsim (SEDimentary Basin SIMulation), a stratigraphic forward modelling software, has been applied for the Darss-Zingst peninsula on a centennial time scale. In Sedsim, the sedimentary dynamics are modelled by an approximation to the Navier–Stokes equation. Using high-resolution digital elevation data, information about the local wave characteristics, geology, estimates of sea level rise, and experimental scenarios for the development of the Darss-Zingst peninsula through the coming 840 years are presented. The results of the experiments show possible implications to the area of investigation and may serve as a basis for decision makers in coastal zone management.

The maritime zone of the Baltic basin, in all the phases of its settlement history, was of special importance to the people living there. Only there did they have access to marine resources and to the transportation and communication routes. The Baltic shore was therefore utilized, occupied, settled and even modified by humans, despite the unstable environmental conditions due to the isostatic rebound and the eustatic rise in sea level, which made it necessary to constantly adapt to a changing environment. Changes in sea level and the shoreline are generally investigated by the earth sciences. The resulting data form the base for the calculation of sea-level curves and shore-displacement models. Especially in areas with high rates of shore displacement, the data and models can then be used to reconstruct environmental conditions and to date prehistoric coastal sites. Conversely, well-excavated and dated archaeological sites that were originally located on the shore can provide detailed information about the sea level at the time of their occupation and can be used as sea-level index points. In this chapter, the opportunities and problems arising from the use of shore-displacement models for the interpretation of archaeological sites are discussed, as is the utilization of data extracted from archaeological investigations. Both models and sites are introduced in case studies that represent not only the different areas and localities but also the different stages in the development of the Baltic Sea.

Geological hazards may threaten human life, may result in serious property damage, and may significantly influence normal development of biota. They are caused by natural endogenic and exogenic driving forces or generated by anthropogenic activities. An interaction of geological processes and intense anthropogenic activities, e.g., construction of buildings, harbors, oil and gas pipelines, hydroengineering facilities, and land reclamation, has resulted in hazard potential, especially for the densely populated areas of the Russian Baltic coastal zone. These hazards may in addition be harmful for the sensitive ecosystem of the Baltic Sea. Mapping and assessment of the geological hazard potential should be the main objectives of an integrated management program for the protection of coastal zones. This study documents the first step in that process for the Russian sector of the Baltic Sea and its coastal zone. A major part of endogenic hazard potential both in the Kaliningrad area and in the eastern Gulf of Finland remains at low- or medium-risk levels, but analysis of the recent environmental conditions at the seabed of the Russian sector of the Baltic Sea and, especially, within its coastal zone shows that during the last years the activity of exogenic geological processes has increased significantly. The highest risk within both studied areas has been caused by coastal and bottom erosion. In addition, in shallow area near the shore bottom of the eastern Gulf of Finland, “avalanche” sedimentation and sediment pollution can produce hazardous situations as well.

The Gulf of Finland is a shallow semi-enclosed sea area which due to strong anthropogenic pressure and poor water exchange is very sensitive to eutrophication. During its whole postglacial history, the seafloor of the gulf has been periodically anoxic, and anoxia below halocline can thus be seen as a natural phenomenon. During the last decades, however, this has been accompanied by a yearly repeated seasonal anoxia in the shallower basins above halocline. This yearly repeated shallower anoxia is triggered by substantial eutrophication of the sea and is a clear signal of anthropogenic pressure. The seasonal anoxia has during the last decades propagated to basins with water depths less than 20 m. The areal coverage of anoxia has thus expanded substantially. Phosphorus which is bound to oxic seafloor sediments is easily released during episodes of anoxia, which further intensifies eutrophication. It has been estimated that the concretion fields of the eastern Gulf of Finland, only, contain more than 330,000 tons of P₂O₅ which is equal to some 175,000 tons of elementary phosphorus. In case of shallowing of the area of permanent anoxia, these concretion fields would become anoxic, which would lead to rather rapid dissolution of the concretions and a release of a large amount of phosphorus together with the heavy metals which today are bound to the concretions.

Today, phosphorus is regarded as the key nutrient for Baltic Sea eutrophication management. Major sources are large rivers like the Oder, Vistula and Daugava in the southern Baltic region. Before entering the Baltic Sea, these rivers discharge their nutrient load into coastal estuaries, bays and lagoons. The quantitative role of these coastal waters, with restricted water exchange, for Baltic Sea management is very important, but not well known. Taking the Oder/Odra estuary as an example, we analyse the long-term pollution history and the major sources for phosphorus and calculate a phosphorus budget, with special focus on anoxic phosphorus release from sediments. The budget shows that due to internal eutrophication in July 2000 the lagoon became a major temporary source of phosphorus for the Baltic Sea. A phosphorus emission reduction scenario, taking into account diffuse and point sources in the entire Oder/Odra river basin, is presented. Phosphorus load reductions have only limited effect on the eutrophic state of the lagoon. The lagoon is more sensitive to nitrogen load reductions. Therefore, both elements have to be taken into account in measures to reduce eutrophication.

The possible change in groundwater discharge from a medium-scale catchment to the Baltic is studied by means of a numerical groundwater flow model. The test area northeast of Wismar (Mecklenburg-Vorpommern, Germany) is built by quaternary glaciofluvial sands and intercalated tills. Today’s groundwater recharge is calculated as 24% of the recent average annual precipitation of 600 mm in the test area, and its submarine groundwater discharge is modelled to 14.3% of the precipitation. Based on climate scenarios calculated by the Swedish Meteorological and Hydrological Institute (SMHI) and the Hadley Centre (HC) three sea-level scenarios in combination with four precipitation scenarios are modelled for steady-state groundwater conditions in order to assess potential response in discharge. The temporal development is observed in a simplified schematic model for transient conditions. For the given conditions the influence of sea-level rise is almost not noticeable. However, the modelled scenarios indicate that changes in groundwater recharge as a consequence of climate-induced changes in precipitation lead to notable variations of submarine groundwater discharge.

This chapter focuses on recent advances in water quality monitoring of the Baltic Sea using remote sensing techniques in combination with optical in situ measurements. Here the Baltic Sea ecosystem is observed through its bio-optical properties, which are defined by the concentration of optical in-water constituents governing the spectral attenuation of light. In the introduction, typical geographical patterns and seasonal variations of optical properties and the cause of the mass occurrence of cyanobacterial blooms in summer are discussed. The optical characteristic of Baltic Sea waters is clearly dominated by a relatively high load of dissolved organic matter and, during the productive season, by phytoplankton growth, stimulated by nutrients mostly originating from land. In the coastal zone, inorganic suspended matter also has a significant effect on the light attenuation, which increases with proximity to land. The ecological status of the coastal zone may be synthesized using a bio-optical model, summarizing important ecosystem state variables such as terrestrial runoff and phytoplankton production. The optical properties can also be observed with visible, spectral satellite remote sensing providing repetitive and optically consistent data for the whole Baltic Sea basin. Such observations have already significantly influenced our understanding of Baltic Sea dynamics and provide us with a new look into this brackish ecosystem. The focus in this chapter is on the ocean colour sensor ‘Medium Resolution Imaging Spectrometer’ (MERIS), which since 2002 is flying continuously onboard the European Environmental Satellite ENVISAT, developed by the European Space Agency (ESA). The advantage of MERIS data is its good spatial resolution of 300 m allowing the analysis of coastal features and bays of the Baltic Sea. Algorithms to retrieve bio-optical parameters from MERIS data are continuously improved and extended. The MERIS mission will be continued with the Ocean and Land Colour Instrument (OLCI), an optically similar sensor which will be flown on SENTINEL-3, scheduled to be operational until 2023, to assure long-term monitoring of water quality from space. The wide aerial coverage, the frequent repetition and continuity of the satellite observations, the consistency of the measured data, and a relative cost-effectiveness clearly respond to the demands of a modern operational monitoring system, and the requirements of effective Baltic Sea management. An overview of existing monitoring approaches is given, and operational online systems that combine remote sensing and autonomous in situ measurements are discussed.