The Sensitivity of Annually Laminated Lake Sediments in Central and Eastern Finland to the Late Holocene Climate

This dissertation discusses sedimentary varve formation and Late Holocene paleoenvironmental and climatic variation. Varved sediment records from five lakes in central and eastern Finland were studied. Three varve records, 3 000 - 4 000 years long, from the clastic-biogenic sediments of Lake Kalliojärvi and Lake Kuninkaisenlampi and biogenic sediments of Lake Kallio-Kourujärvi were reconstructed. In addition, two short records that cover the last 100 years were constructed from Lake Kantele and Lake Linnanlampi. These lakes were cored after an intensive search for previously unknown varved lakes. Piston and freeze cores of sedimentary sequences were obtained from each lake basin for varve, magnetic and chemical analyses. Sediments were impregnated in epoxy resin and the physical varve properties, which include total varve thickness and laminae thicknesses, were analyzed using either a stereographic microscope under dark field illumination or a digital image analysis technique. Paleomagnetic secular variation of the Earth’s magnetic field was measured from Lake Kallio-Kourujärvi, Kalliojärvi and Kuninkaisenlampi sediments and inclination and relative declination were obtained. The major paleomagnetic secular variation features of the records are in agreement with both North Karelian Stack and Fennostack and support the varve dating. The varve data were subjected to statistical analyses in order to study the relationship between hydroclimate and varve characteristics. The results highlight the influence of catchment dynamics in varve formation. Clastic lamina thickness has been previously related to spring flood intensity controlled by snow accumulation. However, this study shows an inverse relationship between snow accumulation and clastic lamina thickness in lakes located in the region of fine-grained tills. This could be explained by the frost sensitivity of fine-grained tills, where enhanced erosion on catchment is interpreted as a result of frost that prevents infiltration of melt waters into the ground. Consequently, in years with deeper ground frost the surface run-off during melting season is increased, thus increasing the amount of catchment derived mineral matter in the lake. The frost formation is strongest during winters with low snow accumulation. The clastic laminae of the Lake Kalliojärvi in central Finland, located in an area dominated by sand moraines, are positively correlated and sensitive to winter precipitation and snow accumulation. Biogenic laminae at the lakes with low trophic status were sensitive to precipitation. Enhanced precipitation not only increases the nutrient transport from the catchment into the lake, but also increases the transportation of terrigenous organic matter. Lake Kalliojärvi record reflects the North Atlantic Oscillation variability, very likely through increased snow accumulation during the positive NAO phase. The long varve record from Lake Kuninkaisenlampi reflects solar activity variation in detail and atmospheric blocking related to cold and less snowy winters would explain this link. Based on the results of this study the catchment characteristics and the region impose a strong control over which climatic forcing the lake is sensitive to.Siirretty Doriast

Between the North Atlantic Oscillation and the Siberian High: A 4000-year snow accumulation history inferred from varved lake sediments in Finland

Clastic-organic varved sediments from the boreal Lake Kalliojärvi, Central Finland, record changes in snow accumulation for more than 4000 years. The varve record was reconstructed using digital image analysis from 4132 varve yr BP to present with 2.2% counting error and is supported by paleomagnetic data. Two laminae are identified in a typical varve structure: (1) the minerogenic lamina, which accumulates during spring as a result of catchment erosion triggered by spring floods and (2) the organic lamina, which is composed of allochthonous and autochthonous organic matter that accumulates during summer, autumn, and winter. The minerogenic influx is related to variations in snow accumulation and follows the different phases of the North Atlantic Oscillation (NAO). Decreased snow accumulation is related to a weakened NAO phase. Thus, the minerogenic influx record provides additional information about NAO variation. The Fe/Mn ratio is related to changes in redox conditions at the lake floor. The oxygen availability in the lake floor depends on the duration of the ice cover during winter. Strengthened Siberian High (SH) causes colder autumn and winter temperatures and therefore leads to extended duration of ice cover. Fe/Mn can be considered as a proxy for SH. The sediment record suggests pronounced, generally positive but strongly fluctuating NAO phase, from ca. 4100 to 3200 varve yr BP. Periods of strengthened SH are observed at 3900–3600, 1900–1500, and 1200–750 varve yr BP. Our study suggests that NAO and SH operate individually, are not clearly linked, and do not entirely block each other.</p

Organic lacustrine sediment varves as indicators of past precipitation changes: a 3,000-year climate record from Central Finland

Annually laminated (varved) sediments from Lake Kallio-Kourujarvi, Central Finland, provide high-resolution sedimentological data for the last three millennia. These varves consist of two laminae that represent (1) deposition during the spring-to-autumn growing season, composed of degraded organic matter and a variety of microfossils, and (2) deposition during winter, composed of fine-grained homogenous organic matter. Because of the absence of a clastic lamina, these varves differ from the typical, well-described, clastic-organic varve sequences in Fennoscandian lakes. Such organic varves in Finnish lakes have not been studied in detail before. Three thousand varves were counted and their seasonal deposition was distinguished. Comparison of varve thickness with meteorological data revealed a positive correlation between organic varve thickness and precipitation. This suggests that catchment erosion processes and consequent organic matter and nutrient inputs are important factors in organic varve formation. The correlation between temperature and growing-season lamina thickness varied from insignificant, to positive, to negative during different time spans. This suggests that organic matter accumulation can sometimes have a significant, but unpredictable role in organic varve formation, via organic matter production and degradation, processes that are influenced strongly by water column temperature. The organic varves of Lake Kallio-Kourujarvi enable a unique, high-resolution approach for the study of past climate and environment. Our results suggest that decadal periods of increased precipitation occurred during BP 2150-2090, 1710-1620, 1410-1360, 920-870 (1030-1080 AD), and after 370 BP (1580 AD). Dryer intervals occurred during BP 2750-2720, 1900-1850, 1800-1740, 1600-1500, and 780-700 (1170-1250 AD), 590-520 (1360-1430 AD).</p

Launching a study of nanoplastics in the Baltic Sea, SW Finland - Picking up small debris

Developing of a methodology for measuring nanoplastic particles in environmental samples, and when done, using the developed methodology, conduct the first estimates of the amounts of nanoplastics in Finnish coastal waters</p

Sediment trapping - An attempt to monitor temporal variation of microplastic flux rates in aquatic systems

Sediment trapping as a tool to monitor microplastic influx was tested in an urban boreal lake basin. The one-year-long trap monitoring consisted of 5-month and 7-month periods representing growing season and winter season (including the spring flood event), respectively. Sediment accumulation rate (SAR), and organic content were determined, highest SAR - 14.5 g/m(2)/d - was measured during the winter period. Microplastics were extracted from the sediment applying heavy-liquid density separation method and collected under a microscope for further identification with FTIR spectroscopy. PE was identified as the most abundant synthetic polymer type, while PP and PET are also present. The annual microplastic flux rate is 32 400 pieces/m2/ year, and highest accumulation does not coincide with the highest SAR, but occurs during the growing season. Changes in the microplastic accumulation rates are related to seasonal conditions. Highest microplastic concentration with respect to dry sediment weight (10 200 pieces/kg) was observed in a growing season sample, while highest concentration with respect to sediment volume (1800 pieces/l) was observed during winter. This finding underlines the problems related to reporting microplastic concentrations in various units. The results highlight that sediment trap monitoring is an efficient tool for monitoring microplastic accumulation rate in aquatic environments and provides an opportunity to better understand and define processes controlling microplastic accumulation. (C) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND licensePeer reviewe

Human actions were responsible for both initiation and termination of varve preservation in Lake Vesijärvi, southern Finland

The influence of lake restoration efforts on lake bottom-water conditions and varve preservation is not well known. We studied varved sediments deposited during the last 80 years along a water-depth transect in the Enonsaari Deep, a deep-water area of the southernmost Enonselkä Basin, Lake Vesijärvi, southern Finland. For the last few decades, the Enonselkä Basin has been subject to ongoing restoration efforts. Varve, elemental, and diatom analyses were undertaken to explore how these actions and other human activities affected varve preservation in the Enonsaari Deep. In contrast to most varved Finnish lakes, whose water columns have a natural tendency to stratify, and possess varve records that span thousands of years, varve formation and preservation in Lake Vesijärvi was triggered by relatively recent anthropogenic stressors. The multi-core varve analysis revealed that sediment in the Enonsaari Deep was initially non-varved, but became fully varved in the late 1930s, a time of increasing anthropogenic influence on the lake. The largest spatial extent of varves occurred in the 1970s, which was followed by a period of less distinguishable varves, which coincided with diversion of sewage from the lake. Varve preservation weakened during subsequent decades and was terminated completely by lake aeration in the 2010s. Despite improvements in water quality, hypolimnetic oxygen depletion and varve preservation persisted beyond the reduction in sewage loading, initial aeration, and biomanipulation. These restoration efforts, however, along with other human actions such as harbor construction and dredging, did influence varve characteristics. Varves were also influenced by diatom responses to anthropogenic forcing, because diatoms form a substantial part of the varve structure. Of all the restoration efforts, a second episode of aeration seems to have had the single most dramatic impact on profundal conditions in the basin, resulting in replacement of a sediment accumulation zone by a transport or erosional zone in the Enonsaari Deep. We conclude that human activities in a lake and its catchment can alter lake hypolimnetic conditions, leading to shifts in lake bottom dynamics and changes in varve preservation.</p

Sediment trapping – An attempt to monitor temporal variation of microplastic flux rates in aquatic systems

ediment trapping as a tool to monitor microplastic influx was tested in an urban boreal lake basin. The one-year-long trap monitoring consisted of 5-month and 7-month periods representing growing season and winter season (including the spring flood event), respectively. Sediment accumulation rate (SAR), and organic content were determined, highest SAR – 14.5 g/m2/d – was measured during the winter period. Microplastics were extracted from the sediment applying heavy-liquid density separation method and collected under a microscope for further identification with FTIR spectroscopy. PE was identified as the most abundant synthetic polymer type, while PP and PET are also present. The annual microplastic flux rate is 32 400 pieces/m2/year, and highest accumulation does not coincide with the highest SAR, but occurs during the growing season. Changes in the microplastic accumulation rates are related to seasonal conditions. Highest microplastic concentration with respect to dry sediment weight (10 200 pieces/kg) was observed in a growing season sample, while highest concentration with respect to sediment volume (1800 pieces/l) was observed during winter. This finding underlines the problems related to reporting microplastic concentrations in various units. The results highlight that sediment trap monitoring is an efficient tool for monitoring microplastic accumulation rate in aquatic environments and provides an opportunity to better understand and define processes controlling microplastic accumulation.</p

Comparison of cold season sedimentation dynamics in the non-tidal estuary of the Northern Baltic Sea

In this study, we investigated sedimentation differences between two distinctive cold seasons, in terms of hydrometeorological and hydrodynamic conditions, in a coastal area of the Northern Baltic Sea in 2018–2020. A combination of sediment trap data, hydrometeorological data and hydrodynamic modelling provided a unique set-up to discover differences in sedimentation rates and compositions. Our study shows that the averaged sedimentation accumulation rate (SAR) was nearly three times higher during warmer cold season (30.9 g m−2 day−1), characterised by higher precipitation, especially rain and discharge, as well as snowless and open water conditions, compared to regular cold season (10.6 g m−2 day−1). While sedimentation was higher during the warmer season, the mean sediment grain size (D50) was higher during the regular cold season with permanent snow and ice cover. Similarly, while sediments of the regular cold season were organically rich, the total amount of organic matter accumulation was larger during the warmer cold season. Sediments consisted mostly of clastic matter (85–89%), of which the mean grain size varied from clay to fine silt (0.3–3.0 μm). Sedimentation differences between the cold seasons can be explained by differences in precipitation, river flow, wind-induced resuspension and a low air pressure system forcing sea level changes. Sedimentation differences along the study bay were found to be connected to channel cross-sectional area and flow conditions caused by river input and sea level changes.</p

Impacts of a deep reactive layer on sedimentary phosphorus dynamics in a boreal lake recovering from eutrophication

Using biogeochemical analyses of sediments and porewaters, we investigate the legacy of a brief, intense period of eutrophication on sedimentary phosphorus (P) cycling in a boreal lake (Enonselka basin, Lake Vesijarvi, Finland). Point-source sewage inputs in the twentieth century caused deoxygenation of the lake and accelerated the focusing of iron (Fe) and manganese (Mn) oxides into deeper areas. Early diagenesis under Fe-Mn-rich conditions now favors rapid burial of P in these areas, likely as a combination of both oxide-bound P phases and authigenic manganous vivianite. A new P budget for Enonselka basin shows that P burial causes an annual drawdown of 1.2% (+/- 0.2%) of the surface sediment P inventory, supporting a long-term trend towards recovery since the construction of a wastewater treatment plant in the mid-1970s. However, remineralization of organic matter and associated dissolution of Fe-Mn oxides continues to regenerate P from a deep reactive layer (20-60 cm depth) deposited at the height of past eutrophication, leading to an upwards diffusive flux of dissolved phosphate towards the surface sediments. The magnitude of this flux is similar to that of external P loading to the lake. The combined incoming fluxes of P are likely to retard the complete recovery from eutrophication by decades, despite ongoing restoration actions.Peer reviewe

Long-Term Consequences of Water Pumping on the Ecosystem Functioning of Lake Sekšu, Latvia

Cultural eutrophication, the process by which pollution due to human activity speeds up natural eutrophication, is a widespread and consequential issue. Here, we present the 85-year history of a small, initially Lobelia–Isoëtes dominated lake. The lake’s ecological deterioration was intensified by water pumping station activities when it received replenishment water for more than 10 years from a eutrophic lake through a pipe. In this study, we performed a paleolimnological assessment to determine how the lake’s ecosystem functioning changed over time. A multi-proxy (pollen, Cladocera, diatoms, and Chironomidae) approach was applied alongside a quantitative reconstruction of total phosphorus using diatom and hypolimnetic dissolved oxygen with chironomid-based transfer functions. The results of the biotic proxy were supplemented with a geochemical analysis. The results demonstrated significant changes in the lake community’s structure, its sediment composition, and its redox conditions due to increased eutrophication, water level fluctuations, and erosion. The additional nutrient load, particularly phosphorus, increased the abundance of planktonic eutrophic–hypereutrophic diatoms, the lake water’s transparency decreased, and hypolimnetic anoxia occurred. Cladocera, Chironomidae, and diatoms species indicated a community shift towards eutrophy, while the low trophy species were suppressed or disappeared