Aarno Kotilainen

Sediment Bacterial Communities Reflect the History of a Sea Basin

How entire microbial communities are structured across stratified sediments from the historical standpoint is unknown. The Baltic Sea is an ideal research object for historical reconstruction, since it has experienced many fresh- and brackish water periods and is depleted of dissolved oxygen, which increases the sediments preservation potential. We investigated the bacterial communities, chemical elements (e.g. Cr, Pb Na, P, Sr and U) and sediment composition in a stratified sediment core dated by radiocarbon and spanning 8000 years of Baltic Sea history, using up-to-date multivariate statistics. The communities were analysed by 16S rRNA gene terminal restriction fragment length polymorphism. The communities of the deep Early Litorina and surface Late Litorina Sea laminae were separated from the communities of the middle Litorina Sea laminae, which were associated with elevated concentrations of U and Sr trace elements, palaeo-oxygen and palaeosalinity proxies. Thus, the Litorina Sea laminae were characterized by past oxygen deficiency and salinity increase. The communities of the laminae, bioturbated and homogeneous sediments were differentiated, based on the same historical sea phases, with correct classifications of 90%. Palaeosalinity was one of the major parameters that separated the bacterial communities of the stratified sediments. A discontinuous spatial structure with a surprising increase in community heterogeneity was detected in Litorina Sea sediments from 388 to 422 cm deep, which suggests that a salinity maximum occurred in the central Gulf of Finland app. 6200–6600 years ago. The community heterogeneity decreased from the surface down to 306 cm, which reflected downcore mineralization. The plateau of the decrease was in the app. 2000-year-old sediment layers. Bacterial community data may be used as an additional tool in ocean-drilling projects, in which it is important to detect mineralization plateaus both to determine historically comparable portions of sediment samples and historical events, such as sea-level rise culminations.

Iron isotope heterogeneity in pyrite fillings of Holocene worm burrows

We present 54Fe and 56Fe data on pyrite from burrow-like and irregularly shaped pyrite concretions from Holocene postglacial lacustrine clays in the northern Baltic Sea collected using a secondary ion mass spectrometry (SIMS) microanalysis technique. The burrow-like concretions were formed in reducing, mucus-coated worm burrows in oxic surface sediments. Framboidal pyrite in the cores of the burrow-like concretions shows extensively fractionated δ56Fe values down to −3.1‰. The framboids are cemented by poorly crystalline FeS2 with δ56Fe values between −2.1‰ and +1.4‰. The irregularly shaped concretions with microcrystalline textures were formed in organic-poor sediment pore spaces, and display a wide spread of δ56Fe values up to +4.1‰. The measured δ56Fe values reflect the preferential capture of 54Fe to pyrite in the diagenetic sequence and the 56Fe enrichment of remaining pore water. The diagenetic sequence of the pyrite materials is supported by previous petrographical study and 34S/32S microanalysis of the same samples. Our results demonstrate substantial early-diagenetic δ56Fe and δ34S heterogeneity within individual pyrite grains, underlining the necessity of high spatial resolution measurements in studying biological and abiological isotopic signatures.

Climate induced human demographic and cultural change in northern Europe during the mid-Holocene

The transition from hunter-gatherer-fisher groups to agrarian societies is arguably the most significant change in human prehistory. In the European plain there is evidence for fully developed agrarian societies by 7,500 cal. yr BP, yet a well-established agrarian society does not appear in the north until 6,000 cal. yr BP for unknown reasons. Here we show a sudden increase in summer temperature at 6,000 cal. yr BP in northern Europe using a well-dated, high resolution record of sea surface temperature (SST) from the Baltic Sea. This temperature rise resulted in hypoxic conditions across the entire Baltic sea as revealed by multiple sedimentary records and supported by marine ecosystem modeling. Comparison with summed probability distributions of radiocarbon dates from archaeological sites indicate that this temperature rise coincided with both the introduction of farming, and a dramatic population increase. The evidence supports the hypothesis that the boundary of farming rapidly extended north at 6,000 cal. yr BP because terrestrial conditions in a previously marginal region improved.

Niobium in boreal stream waters and brackish-water sediments

The focus is on ‘non-detrital’ Nb in boreal stream waters (dissolved fraction and acid-available particulate fraction) and brackish-water/lacustrine sediments (aqua regia extractable fraction). Spatial patterns, temporal trends and speciation experiments all point to dissolved humic substances and colloidal Fe as the main control of Nb concentrations in stream waters. In addition, clay-silt deposits and/or ore deposits may be responsible for producing local streamwater Nb anomalies. In groundwater in overburden (glacial till) overlying Proterozoic granitoids, dissolved Nb concentrations were about an order of magnitude higher than in stream waters and strongly correlated with dissolved Fe. In the brackish-water sediments, the Nb concentrations (1.33–4.20 ppm) were higher than in the lacustrine ones (0.25–0.53 ppm). To explain this, we assessed the potential role of organic material, biological processes, sulphide mineralogy, silicate mineralogy and input factors. However, none of them could satisfactorily explain the observed Nb geochemical features. Although Nb can be considered relatively immobile, the data presented in this paper show that under certain conditions its abundance in the aquatic environment increases.

The undatables: Quantifying uncertainty in a highly expanded Late Glacial-Holocene sediment sequence recovered from the deepest Baltic Sea basin-IODP Site M0063

Laminated, organic-rich silts and clays with high dissolved gas content characterize sediments at IODP Site M0063 in the Landsort Deep, which at 459 m is the deepest basin in the Baltic Sea. Cores ...

Submarine De Geer Moraines in the Kvarken Archipelago, the Baltic Sea

Publisher Summary The Kvarken Archipelago is located in the European epicontinental basin, the Baltic Sea. The Baltic Sea is one of the largest brackish water bodies in the world. The Kvarken area, between Finland and Sweden, is the narrowest part of the Gulf of Bothnia. It forms a submarine sill (25 m) that separates the Bothnian Sea in the south from the Bothnian Bay in the north. The majority of the Kvarken Archipelago is very shallow (0–25 m) and shoaly, with ∼7,000 islands and islets. Glacioisotatic land uplift rate is ∼8.0–8.5 mm/year. The boulder-rich De Geer moraines are the most characteristic geomorphic features within the area, creating a unique, washboard-like submarine landscape. Because the area is a transition zone of salinity levels at critical levels to both marine and limnic species, the diversity of marine life is poor. The local bladder wrack, Fucus radicans, is characteristic of hard bottoms in the shallow areas with salinity up to 4.5‰. The region has undergone several glaciations during the past approximately 3 million years, which has resulted in periods of repeated glacial erosion and sediment accumulation. During the latest glaciation, the Kvarken Archipelago was located close to the center of the Weichselian ice sheet, which reached a thickness of approximately 3 km during the glacial maximum. The seafloor morphology of the Kvarken area is characterized by broad-scale geomorphic features including mounds, plains, basins, and some sea valleys (holes). However, the seafloor bathymetry follows mainly the surface of the bedrock, and the relief of the area is relatively low.

Holocene stratigraphy of the Ångermanälven River estuary, Bothnian Sea

This study explores the Holocene depositional succession at the IODP Expedition 347 sites M0061 and M0062 in the vicinity of the Ångermanälven River estuary in the Bothnian Sea sector of the Baltic Sea in northern Scandinavia. Site M0061 is located in a coastal offshore setting (87.9 m water depth), whereas site M0062 is fully estuarine (69.3 m water depth). The dataset comprises acoustic profiles and sediment cores collected in 2007 and late 2013 respectively. Three acoustic units (AUs) were recognized. Lowermost AU1 is interpreted as a poorly to discontinuous stratified glaciofluvial deposit, AU2 as a stratified conformable drape of glaciolacustrine origin, and AU3 as a poorly stratified to stratified mud drift. A strong truncating reflector separates AU2 and AU3. Three lithological units (LUs) were defined in the sediment cores. LU1 consists of glaciofluvial sand and silt gradating into LU2, which consists of glaciolacustrine varves. A sharp contact interpreted as a major unconformity separates LU2 from the overlying LU3 (brackish-water mud). In the basal part of LU3, one debrite (site M0061) or two debrites (site M0062) were recognized. Information yielded from sediment physical properties (magnetic susceptibility, natural gamma ray, dry bulk density), geochemistry (total carbon, total organic carbon, total inorganic carbon and nitrogen), and grain size support the LU division. The depositional succession was formally subdivided into two alloformations: the Utansjö Alloformation and overlying Hemsön Alloformation; the Utansjö Alloformation was further subdivided into two lithostratigraphic formations: the Storfjärden and Åbordsön formations. The Storfjärden (sandy outwash) and Åbordsön (glaciolacustrine rhythmite) formations represent a glacial retreat systems tract, which started at ca. 10.6 kyr BP. Their deposition was mainly controlled by meltwater from the retreating ice margin, glacio-isostatic land uplift and the regressive (glacial) lake level. The Hemsön Alloformation (organic-rich brackish-water mud) represents a period of forced regression, starting possibly at ca. 9.5 kyr BP. At about 7 kyr BP, brackish water reached the study area as a result of the mid-Holocene marine flooding of the Baltic Sea Basin, but the rapid land uplift soon surpassed the associated (Littorina) transgression. Changed near-bottom current patterns, caused by the establishment of a permanent halocline, and the reduced sediment consistency caused by increased organic deposition resulted in a sharp and erosional base of the brackish-water mud. Estuarine processes and salinity stratification at site M0062 started to play a more important role. This study applies a combined allostratigraphic and lithostratigraphic approach over the conventional Baltic Sea stages. This approach makes it more straightforward to study this Baltic Sea deglaciation–postglacial sequence and compare it to other formerly glaciated shallow sea estuaries.

Base of brackish-water mud as key regional stratigraphic marker of mid-Holocene marine flooding of the Baltic Sea Basin

Many modern epicontinental seas were dry land before their marine flooding by the mid-Holocene glacioeustatic sea-level rise, whereas the Baltic Sea Basin was covered by a huge postglacial lake. This change from a postglacial lake to the present-day semi-enclosed brackish-water sea is studied here in sediment cores and acoustic profiles from the Baltic Sea major sub-basins, based on novel datasets combined with information extracted from earlier publications. In shallow areas (<50m water depth), the base of the brackish-water mud is erosional and covered by a patchy, thin, transgressive silt-sand sheet resulting from decreased sediment supply, winnowing and the redistribution of material from local coarse-grained deposits during transgression. This erosional marine flooding surface becomes sharp and possibly erosional in deep areas (>50m water depth), where it may be locally less clearly expressed due to reworking and bioturbation. Both in the shallow and deep areas, the brackish-water mud is strongly enriched in organic matter compared to underlying sediments. Bioturbation type changes at the flooding surface in response to the increased sedimentary organic content, but no firm-ground ichnofacies were developed because of low erosion. It is concluded that the base of the brackish-water mud is a robust allostratigraphic bounding surface that is identifiable by the lithologic examination of cores over the Baltic Sea. The surface is a distinct reflector in seismic-acoustic profiles, which facilitates mapping and basin-wide stratigraphic subdivision. Detailed geochronologic studies are required to confirm if sediments immediately overlying the erosional flooding surface in shallow areas are younger than the basal part of the brackish-water mud in deep areas that is predicted to be time-equivalent to the erosion.

Seafloor Desertification – A Future Scenario for the Gulf of Finland?

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.

Assessment of the Influence of Dredge Spoil Dumping on the Seafloor Geological Integrity

The European Marine Strategy Framework Directive requires the development of suitable indicators for regular reporting on the environmental state and achievement of a good environmental status of EU’s marine waters by 2020. The development of indicators for determining seafloor integrity and its possible disturbance by human activities have so far largely ignored the geological properties of seafloor. This paper presents a study of Vuosaari and Uusikaupunki-D offshore dumping sites in Finland, the northern Baltic Sea. Full coverage multibeam bathymetry and relative backscatter data, and a number of sediment cores were collected over the sites. The areas covered by dumped dredge spoil stand out in the multibeam images because of their irregular surface and elevated backscatter. The short gravity cores were studied for lithology, and in 1-cm slices for 137Cs activity, organic content, and grain size distribution. The dumped material is represented in the cores by the gravelly mud lithofacies with massive texture and angular coarse particles. The dumped material is coarser, less sorted and has higher kurtosis compared to natural sediment due to the admixing of blasted rock during the dredging activities, and limited sorting during fall through the water column upon dumping. Dispersed dredge spoil, which was suspended in the water column during the dumping activities or reworked from the dumped material mounds and redistributed along the seafloor soon thereafter, was deposited over a wide area as a thin layer that is not necessarily readily identifiable by visual inspection in the cores. Cesium activity helped distinguish the dumped material from the 137Cs-enriched natural sediments deposited after the 1986 Chernobyl disaster. Considering that the dumped material at many of the coring sites in the Vuosaari dumping area is covered by natural sediment, it probably is largely stable. In contrast, dumped material at the shallower Uusikaupunki-D site has slumped down to an adjacent channel and is likely being redistributed by near-bottom currents. Based on the findings of the study, a protocol for the assessment of the geological integrity of seafloor, its anthropogenic change due to dumping, and its potential recovery is proposed, as required by the Marine Strategy Framework Directive.