Lovisa Zillen

Abstract Numerous hydro-acoustic studies of the seabed of the Baltic Sea have revealed the unusual occurrence of sediment contourite drifts and re-suspension at greater water depths. In addition, radiocarbon dating of bulk sediments indicates significant age reversals. We present new geophysical, sediment proxy data (including extensive radiocarbon dating) and hydrographic measurements, which are combined with results of numerous marine geological studies performed during the last decades. These data indicate that a deep-water formation process significantly affected the seabed dynamics during regional climatically cold phases during the last c. 7,000 years. We propose that, during the colder periods (e.g. the Little Ice Age), newly formed bottom waters likely caused widespread re-suspension of organic carbon-rich laminated sediments that were deposited during the preceding warm periods in shallower areas, and this material was transported to and re-deposited in the deeper parts of the Baltic Sea sub-basins. In our scenario, a topographic feature, known as the Baltic Sea Klint, acted as a hydrographic barrier for deep-water formed in the northern Baltic. Thus, during the cold periods increased lateral matter influx from the northern Baltic led to the accumulation of much thicker macroscopically homogenous clayey sediments in sub-basins north of the Klint. Moreover, deep-water formation produced bottom currents that led to the formation of sediment contourite drifts at water depths of >200 m in the Bothnian Sea, the Aland Deep and northern central Baltic Sea sub-basins. Bottom water ventilation in the Baltic Sea is generally assumed to be determined solely by the inflow of oxygen-rich, saline water from the North Sea, but we challenge this assumption and postulate that deep-water formation is a key process that ventilates the bottom waters of the Baltic Sea during climatically cold periods with substantial implications for its sedimentary archive.

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.

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 e...

Predefined classification schemes and fixed geographic scales are often used to simplify and cost-effectively map the spatial complexity of nature. These simplifications can however limit the usefulness of the mapping effort for users who need information across a different range of thematic and spatial resolutions. We demonstrate how substrate and biological information from point samples and photos, combined with continuous multibeam data, can be modeled to predictively map percentage cover conforming with multiple existing classification schemes (i.e., HELCOM HUB; Natura 2000), while also providing high-resolution (5 m) maps of individual substrate and biological components across a 1344 km2 offshore bank in the Baltic Sea. Data for substrate and epibenthic organisms were obtained from high-resolution photo mosaics, sediment grab samples, legacy data and expert annotations. Environmental variables included pixel and object based metrics at multiple scales (0.5 m–2 km), which impr...

The Baltic Sea has undergone large environmental changes since the retreat of the Weischselian Ice-sheet. In the Late Glacial Period and the early Holocene these changes were most likely caused by natural environmental changes (i.e. changes in the morphology and depths of the Baltic basin and the sills). In more recent time anthropogenic impacts have become more important as a

One of the largest impacts on the Baltic Sea ecosystem health is eutrophication, which causes hypoxia (< 2mg/l dissolved oxygen). It is estimated that the hypoxic zone in the Baltic Sea has increased about four times in area since 1960 due to surplus loads of waterborne and airborne nutrients (N and P) from anthropogenic sources. Hypoxia has barren vast areas

A recently published geomagnetic field model (CALS7K.2, Korte and Constable, 2005) predicts inclination, declination and intensity values for the last 7000 years at any point on the Earth's surface. We compare this model's output for the Fennoscandian region to a series of seven high-resolution and independently dated records of geomagnetic field evolution derived from lake sediments. None of these records were used to constrain the constructed model, so differences between the modeled time series and the empirical data may indicate model deficiencies or poor reconstructions. Six of the sediment sequences have chronologies based on the counting of annual laminations (varves). Two of these varve chronologies are validated by radiocarbon dating and tephrochronology. The chronology of the seventh site is based on secular variation correlation, but with validation by radiocarbon dating. The seven records contain all of the directional features that were first reconstructed in a study of lake sediments located in the United Kingdom (Turner and Thompson, 1981) although discrepancies of the ages of curvature maximums can differ by as much as 400 years. We find that this difference has no relationship to the longitudes of the sites, which suggests that dating errors could be responsible instead of significant westwards drift of geomagnetic field features. As implied by other palaeomagnetic studies in high-latitudes on the northern hemisphere the most rapid change in field direction occurred at c. 3000 cal BP and was dominated by a change in declination. This feature is predicted by the CALS3K.1 model, but not the CALS7K.2 model, while an older predicted declination feature at c. 4000 cal BP is not indicated by our data compilation. Inclination differences are also greatest at c. 3000 cal BP. In general, the differences between predicted directions and our reconstructions increase further back in time. It is notable that the general trends in the relative paleointensity records obtained from the seven sites agree well with the modeled intensities. In fact, the overall differences are not significantly greater if the local RPI records are compared to the model's prediction of global dipole moment. This implies that paleointensity can form the basis of intra-hemispheric correlation if it can be reconstructed accurately.

Annually laminated (varved) lakes sediments are relatively common in Sweden because the distinct seasonality of climate induces winter ice cover and long periods of anoxia in the deeper parts of lake basins. The preserved annual cycle of sediment accumulation is characterized by a mineral rich layer, which is deposited during the spring snow melt, and an organic rich layer, which accumulates during the summer and winter. Mineral magnetic studies of several varved lake sediment sequences point to a common characteristic - high concentrations of a stable single domain magnetic mineral in the organic rich layers. In fact, in bulk sampled there is often a linear relationship between the amount of organic carbon and mass specific magnetic parameters that indicate the concentrations of ferrimagnetic minerals. The magnetic properties of catchment soils and sub-soils are quite different from the organic rich sediments and point to the production of magnetite in the lake. This magnetite is the carrier of an extremely stable natural remanent magnetization, which we have used to reconstruct the direction and intensity of the Earth's geomagnetic field over the past 9000 years, and it's concentrations in the sediment sequences also appear to be related to known climate changes during the Holocene. At this point of our studies, however, only one definite conclusion can be made: that the fine grained magnetite is produced within the lake, before or after sediment deposition. Where do we go from here? What methods can we use to test our hypothesis that magnetotactic bacteria are indeed responsible for the production of the single-domain magnetite? And how much secondary magnetite does a single lake contain? These are questions posed by this study. We attempt to answer the last one through using magnetic techniques and the known properties of magnetite. For the central parts of the lake basins we derive a conservative estimate in the range between 16 and 18 mg magnetite per square meter per year for the last 9000 years. This estimate implies that a cylinder of sediment with a radius of 50 m and thickness of 4-5 meters would contain approximately 1300 kg of secondary single domain magnetite.

The hypoxic zone in the Baltic Sea has increased in area by about four times since 1950. Widespread oxygen deficiency below the halocline has severely reduced macro benthic communities in the Baltic Proper and the Gulf of Finland over the past decades and negatively effected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. In addition, hypoxia

One of the largest impacts on the Baltic Sea ecosystem health is eutrophication, which causes hypoxia (< 2mg/l dissolved oxygen). It is estimated that the hypoxic zone in the Baltic Sea has increased about four times in area since 1960 due to surplus loads of waterborne and airborne nutrients (N and P) from anthropogenic sources. Hypoxia has barren vast areas

Paleomagnetic analyses were carried out on five annually laminated (varved) Holocene lake sediment sequences in two different areas of Sweden. Three of the lakes (Furskogstjärnet, Mötterudstjärnet and Kälksjön) are situated in Värmland, west central Sweden, and the other two (Sarsjön and Frängsjön) in Västerbotten, northern Sweden. Tephra shards of Icelandic origin found in the Värmland sites form isochrons that can be used as an independent synchronisation tool and radiocarbon dates also support the Värmland sites varve chronologies. Mineral magnetic measurements indicate that autochthonous single-domain (SD) magnetite, most likely of bacterial origin, is the dominant carrier of an extremely stable natural remanent magnetisation (NRM) in all sediment sequences, while allochthonous multi-domain (MD) magnetite contributes little to the NRM. Declination, inclination and palaeointensity data were derived from multiple cores obtained from each site and these were stacked according to their varve ages to produce two local palaeomagnetic secular variation (PSV) curves for Västerbotten and Värmland, respectively. These PSV records show directional trends that are identical in form to the UK Holocene PSV master curve constructed by Turner and Thompson (1981) and Finnish data (Ojala and Saarinen, 2002). Differences in the ages of PSV features of up to 500 years do exist, which may be caused by dating errors, unknown lock-in effects or true drift of non-dipole field features. The sediment properties are ideal for palaeointensity studies, and by assuming a dominant dipole field a cosmogenic nuclide production rate curve has been derived from two of the sites. Comparison of this curve with the tree-ring derived radiocarbon data empirically demonstrates the dominant modulation of cosmogenic nuclide production by dipole-moment between 5000 BC and AD 1500 (Snowball and Sandgren, 2002), with significant changes at the multi-centennial to millennial time-scale. Current work aims (i) to increase the resolution of the PSV records; (ii) to extend varve-based records into the late-Pleistocene using Danish and Polish varve sequences; and (iii) to refine the North-European tephrochronology using varved sediments.

The hypoxic zone in the Baltic Sea has increased in area about four times since 1960 and widespread oxygen deficiency has severely reduced macro benthic communities below the halocline in the Baltic Proper and the Gulf of Finland, which in turn has affected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. The cause of increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of nutrients, such as nitrogen and phosphorus. However, the spatial variability of hypoxia on long time-scales is poorly known: and so are the driving mechanisms. We review the occurrence of hypoxia in modern time (last c. 50 years), modern historical time (AD 1950–1800) and during the more distant past (the last c. 10 000 years) and explore the role of climate variability, environmental change and human impact. We present a compilation of proxy records of hypoxia (laminated sediments) based on long sediment cores from the Baltic Sea. The cumulated results show that the deeper depressions of the Baltic Sea have experienced intermittent hypoxia during most of the Holocene and that regular laminations started to form c. 8500–7800 cal. yr BP ago, in association with the formation of a permanent halocline at the transition between the Early Littorina Sea and the Littorina Sea s. str. Laminated sediments were deposited during three main periods (i.e. between c. 8000–4000, 2000–800 cal. yr BP and subsequent to AD 1800) which overlap the Holocene Thermal Maximum (c. 9000–5000 cal. yr BP), the Medieval Warm Period (c. AD 750–1200) and the modern historical period (AD 1800 to present) and coincide with intervals of high surface salinity (at least during the Littorina s. str.) and high total organic carbon content. This study implies that there may be a correlation between climate variability in the past and the state of the marine environment, where milder and dryer periods with less freshwater run-off correspond to increased salinities and higher accumulation of organic carbon resulting in amplified hypoxia and enlarged distribution of laminated sediments. We suggest that hydrology changes in the drainage area on long time-scales have, as well as the inflow of saltier North Sea waters, controlled the deep oxic conditions in the Baltic Sea and that such changes have followed the general Holocene climate development in Northwest Europe. Increased hypoxia during the Medieval Warm Period also correlates with large-scale changes in land use that occurred in much of the Baltic Sea watershed during the early-medieval expansion. We suggest that hypoxia during this period in the Baltic Sea was not only caused by climate, but increased human impact was most likely an additional trigger. Large areas of the Baltic Sea have experienced intermittent hypoxic from at least AD 1900 with laminated sediments present in the Gotland Basin in the Baltic Proper since then and up to present time. This period coincides with the industrial revolution in Northwestern Europe which started around AD 1850, when population grew, cutting of drainage ditches intensified, and agricultural and forest industry expanded extensively.

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.