Minna Väliranta - Academia.edu (original) (raw)

Papers by Minna Väliranta

Research paper thumbnail of High-resolution reconstruction of wetness dynamics in a southern boreal raised bog, Finland, during the late Holocene: a quantitative approach

The Holocene, 2007

Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/17/ 8/1093 The online... more Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/17/ 8/1093 The online version of this article can be found at: DOI: 10.1177/ 0959683607082550 2007 17: 1093 The Holocene Minna Väliranta, Atte ...

Research paper thumbnail of The importance of northern peatland expansion to the late-Holocene rise of atmospheric methane

Quaternary Science Reviews, 2010

Wetlands have been considered as the most important natural source of the atmospheric methane con... more Wetlands have been considered as the most important natural source of the atmospheric methane concentration (AMC) prior to anthropogenic influences. According to ice cores, AMC varied significantly during the Holocene, the causes of which are not completely understood. In particular, the reasons for the increased AMC during the late Holocene (from 5 ka onwards) have been debated widely, including an anthropogenic explanation. Initially, this increase was associated with increased emissions from northern wetlands, but estimated peat initiation rates seem not to support the conclusion. Based on a new data set of 954 basal peat radiocarbon dates that accounts more properly for the horizontal growth dynamics of northern peatlands (by containing only sites with multiple basal dates per site), we show here that the most extensive lateral expansion of high-latitude peatlands occurred only after 5 ka, parallel with the rise of CH 4 in the ice cores. Because this explosive increase in the extent of peatlands resulted in the formation of moist minerotrophic fen ecosystems that emit high amounts of CH 4 for a long time since their formation, and because many Arctic peatlands have remained minerotrophic throughout their development, northern peatlands cannot be neglected when seeking cause(s) for the late-Holocene rise in CH 4 . A similar event in future could enhance climate change by causing a rapid shift in atmospheric greenhouse gas concentrations.

Research paper thumbnail of The distribution of late-Quaternary woody taxa in northern Eurasia: evidence from a new macrofossil database

Quaternary Science Reviews, 2009

Research paper thumbnail of Holocene climate and landscape evolution East of the Pechora Delta, East-European Russian Arctic

Quaternary Research, 2003

This study presents a multiproxy record of Holocene environmental change in the region East of th... more This study presents a multiproxy record of Holocene environmental change in the region East of the Pechora Delta. A peat plateau profile (Ortino II) is analyzed for plant macrofossils, sediment type, loss on ignition, and radiocarbon dating. A paleosol profile (Ortino III) is described and radiocarbon dated. A previously published peat plateau profile (Ortino I) was analyzed for pollen and conifer stomata, loss on ignition, and radiocarbon dating. The interpretation of the latter site is reassessed in view of new evidence. Spruce immigrated to the study area at about 8900 14 C yr B.P. Peatland development started at approximately the same time. During the Early Holocene Hypsithermal taiga forests occupied most of the present East-European tundra and peatlands were permafrost free. Cooling started after 5000 14 C yr B.P., resulting in a retreat of forests and permafrost aggradation. Remaining forests disappeared from the study area around 3000 14 C yr B.P., coinciding with more permafrost aggradation. The retreat of forests resulted in landscape instability and the redistribution of sand by eolian activity. The displacement of the Arctic forest line and permafrost zones indicates a warming of at least 2-3°C in mean July and annual temperatures during the Early Holocene. At least two cooling periods can be recognized for the second half of the Holocene, starting at about 4800 and 3000 14 C yr B.P.

Research paper thumbnail of The Holocene thermal maximum and late-Holocene cooling in the tundra of NE European Russia

Quaternary Research, 2011

To investigate the Holocene climate and treeline dynamics in the European Russian Arctic, we anal... more To investigate the Holocene climate and treeline dynamics in the European Russian Arctic, we analysed sediment pollen, conifer stomata, and plant macrofossils from Lake Kharinei, a tundra lake near the treeline in the Pechora area. We present quantitative summer temperature reconstructions from Lake Kharinei and Lake Tumbulovaty, a previously studied lake in the same region, using a pollen-climate transfer function based on a new calibration set from northern European Russia. Our records suggest that the early-Holocene summer temperatures from 11,500 cal yr BP onwards were already slightly higher than at present, followed by a stable Holocene Thermal Maximum (HTM) at 8000-3500 cal yr BP when summer temperatures in the tundra were ca. 3°C above present-day values. A Picea forest surrounded Lake Kharinei during the HTM, reaching 150 km north of the present taiga limit. The HTM ended with a temperature drop at 3500-2500 cal yr BP associated with permafrost initiation in the region. Mixed spruce forest began to disappear around Lake Kharinei at ca. 3500 cal yr BP, with the last tree macrofossils recorded at ca. 2500 cal yr BP, suggesting that the present wide tundra zone in the Pechora region formed during the last ca. 3500 yr.

Research paper thumbnail of Reconstructing peatland water tables using transfer functions for plant macrofossils and testate amoebae: A methodological comparison

Quaternary International, 2012

Article history: Available online xxx a b s t r a c t Relatively seldom is the same parameter rec... more Article history: Available online xxx a b s t r a c t Relatively seldom is the same parameter reconstructed from the same site using different proxies, resulting in a persistent problem for palaeoecological studies whereby a reconstruction based on a single-proxy may provide an unequivocal view of the changes in past conditions. Plant macrofossils and testate amoebae are commonly used proxies to reconstruct past changes in peatland surface moisture conditions, but there are no comparisons between quantitative reconstructions based on both techniques. This paper compares two high-resolution late-Holocene quantitative water table (WT) reconstructions based on transfer functions for plant macrofossils and testate amoebae from two boreal peatland sites in Finland and Estonia. The reconstructed WT variation patterns during the last ca. 5000 years are almost identical in directions for both proxies. However, both bog records contain one inconsistent episode when the two proxies indicate different hydrological conditions. In both cases, the testate amoebae reconstruction shows wetter than the average conditions, whereas the plant-based reconstruction indicates drier than the average conditions. Several, possibly simultaneously affecting, reasons can be suggested for mismatches between proxies: 1) the proxies have different response times and sensitivities to hydrological changes, 2) the species-ecology is inadequately known, 3) the modern analogues are poor, 4) the microhabitat dynamics are unpredictable, and 5) the modern data set is too small. Divergences between the proxy records emphasize the fact that single-proxy reconstructions are subject to larger uncertainties than those based on two or more methods.

Research paper thumbnail of Conservative composition of n-alkane biomarkers in Sphagnum species: Implications for palaeoclimate reconstruction in ombrotrophic peat bogs

Organic Geochemistry, 2010

... Peat deposits accumulate at a high rate on a decadal scale, such that high resolution, palaeo... more ... Peat deposits accumulate at a high rate on a decadal scale, such that high resolution, palaeoclimate proxy records can be obtained, based on macrofossils and pollen ([Barber et al., 1994] and [Väliranta et al., 2007]), humification indices (Blackford and Chambers, 1993), testate ...

Research paper thumbnail of Fire history and vegetation recovery in two raised bogs at the Baltic Sea

Journal of Vegetation Science, 2011

Cal. yr BP = calibrated years before present (counted back from 1950); vol % = volume percentage,... more Cal. yr BP = calibrated years before present (counted back from 1950); vol % = volume percentage, the proportion of (moss) species in total plant macrofossils; n = number of (charcoal) particles Abstract Questions: What were the bog fire patterns and frequencies in two boreal peatlands during the last 5000 years? What is the nature and time-scale of postfire vegetation successions? Were fire events related to climate? Location: Männikjärve bog, central east Estonia; Kontolanrahka bog, southwest Finland.

Research paper thumbnail of Palaeoecological evidence of changes in vegetation and climate during the Holocene in the pre-Polar Urals, northeast European Russia

Journal of Quaternary Science, 2003

Väliranta, M.; Sarmaja-Korjonen, K.; Solovieva, N.; Virtanen, T.; Kauppila, T.; Eronen, M. Citation

Research paper thumbnail of The influence of Holocene tree-line advance and retreat on an arctic lake ecosystem: a multi-proxy study from Kharinei Lake, North Eastern European Russia

Journal of Paleolimnology, 2011

abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder ... more abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder climate resulted in a shorter ice-free period, the lake was less productive and there was a loss of aquatic macrophytes. Increased wind-induced mixing following forest retreat had a greater influence on the lake ecosystem than the effects of decreasing organic matter concentration and increased light penetration.

Research paper thumbnail of Holocene aquatic ecosystem change in the boreal vegetation zone of northern Finland

Journal of Paleolimnology, 2011

We studied multiple variables in a sediment core from Lake Kipojärvi, northern Finland, to invest... more We studied multiple variables in a sediment core from Lake Kipojärvi, northern Finland, to investigate Holocene ecosystem changes in relation to catchment characteristics and known climate variations. We focused on a forested catchment because previous paleolimnological studies conducted in Fennoscandia focused mainly on subarctic lakes within a range of shifting treeline(s). Data on aquatic macrophytes, diatoms, Cladocera, C:N ratio, organic

Research paper thumbnail of Palaeolimnological Development of Lake Njargajavri, Northern Finnish Lapland, in a Changing Holocene Climate and Environment

Journal of Paleolimnology, 2006

This study used palaeolimnological approaches to determine how Holocene climatic and environmenta... more This study used palaeolimnological approaches to determine how Holocene climatic and environmental changes affected aquatic assemblages in a subarctic lake. Sediments of the small Lake Njargajavri, in northern Finnish Lapland above the present treeline, were studied using multi-proxy methods. The palaeolimnological development of the lake was assessed by analyses of chironomids, Cladocera and diatoms. The lake was formed in the

Research paper thumbnail of Holocene development of aquatic vegetation in shallow Lake Njargajavri, Finnish Lapland, with evidence of water-level fluctuations and drying

Journal of Paleolimnology, 2005

Holocene development of aquatic plant communities in subarctic Lake Njargajavri, Finnish Lapland,... more Holocene development of aquatic plant communities in subarctic Lake Njargajavri, Finnish Lapland, was studied using plant macrofossil analysis. Sediment lithology, grain size, and C/N ratios showed distinct lithological phases, indicating past water-level fluctuations. The colonization of limnophytes took place right after the formation of the lake (after ca. 11,500 cal. BP). The earliest plant macrofossil assemblages indicate nutrient-rich conditions and a warmer

Research paper thumbnail of Mid-Holocene palaeoclimatic and palaeohydrological conditions in northeastern European Russia: a multi-proxy study of Lake Vankavad

Journal of Paleolimnology, 2000

Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sedime... more Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sediments of Lake Vankavad in the northern taiga of the Usa Basin, NE European Russia, through the analysis of pollen, plant macrofossils, Cladocera and diatoms. Lake Vankavad was probably formed at ca. 5000 BP (ca. 5600 cal. BP) and initially it was shallow with a littoral cladoceran fauna. Macrofossil and pollen results suggest that dense Betula-Picea forests grew in the vicinity and the shore was close to the sampling point. At ca. 4600 BP (ca. 5400 cal. BP) the water level rose coincident with the decrease in the density and area of forests, probably caused by cooling climate and accelerated spread of mires. There was also a further rise in the water level at ca. 3500 BP (ca. 3800 cal. BP). The initiation of the lake, followed by two periods of rising waterlevel, as well as the increase in mire formation, was a consequence of a rise in groundwater level. This probably reflects lower evapotranspiration in a cooling mid-Holocene climate and/or higher precipitation in the lowland area. Also the decreased forest density and area may have contributed to the lower evapotranspiration. It is also possible that permafrost aggradation or changes in peat ecosystems might have affected the hydrological conditions in the area.

Research paper thumbnail of Acknowledging the spatial heterogeneity in modelling/reconstructing carbon dioxide exchange in a northern aapa mire

Ecological Modelling, 2009

Chamber method is commonly used to measure the CO 2 exchange from plant communities. Due to low t... more Chamber method is commonly used to measure the CO 2 exchange from plant communities. Due to low time resolution, actual measurements reflect only momentary CO 2 exchange rates. Therefore, a common way to derive seasonal or annual estimates is to establish models describing the response of CO 2 exchange to environmental variables, and then to reconstruct the CO 2 exchange over the desired time period. There are several alternative ways to obtain the CO 2 balance for the entire mire: models can be parameterized by individual sample plots, plant communities or the entire site. Similarly, the CO 2 balance can be reconstructed by plots, plant communities or the entire site. We tested how the choice of the modelling and reconstruction approach influences the CO 2 exchange estimates for the entire mire and for individual sample plots and plant communities. We measured the CO 2 exchange in a spatially heterogeneous sedge-dominated northern aapa mire for two growing seasons. We observed high spatial variation in CO 2 balance between the plant communities. We noticed that when the CO 2 balances of individual sample plots or plant communities are of interest, using a model appropriate for the entire site may result in biased estimates. In worst case the different modelling approaches may turn the CO 2 balance of an individual sample plot from positive to negative. Further, while using the whole ecosystem approach in modelling, the superior ability of chamber method in acknowledging spatial variation is lost. While the modelled growing season CO 2 balance of the mire ranged from 232 to 625 g CO 2 m −2 depending on the chosen modelling and reconstruction approach, the average estimates still remained within the uncertainty range of one another. Acknowledgement of the spatial variation in plant community level makes the areal estimate more robust to varying weather conditions. Further, the reliability of estimates is improved by explicit formulation of the choices behind the modelling and reconstruction units reflecting the study objectives.

Research paper thumbnail of Carbon Dioxide and Methane Dynamics Trough Fen-Stream-Lake Continuum in North Boreal Zone, Finnish Lapland

This study aims to determine spatial and temporal variation in carbon dioxide (CO2) and methane (... more This study aims to determine spatial and temporal variation in carbon dioxide (CO2) and methane (CH4) dynamics through a fen-lake continuum. Study was carried out in a catchment dominated by aapa mire over the period from spring 2005 to autumn 2006 in north boreal zone in Finnish Lapland. The study region characteristically has very high lake and mire density. To

Research paper thumbnail of Two different hydrological proxies-replicate reconstructions?

ABSTRACT Bog plants and testate amoebae are perhaps the most commonly used proxies to reconstruct... more ABSTRACT Bog plants and testate amoebae are perhaps the most commonly used proxies to reconstruct past changes in mire surface moisture conditions. In bog environment, distribution and occurrence of these organisms is known to be mainly regulated by water table depth - in minerotrophic fens this relationship is more complex. Establishment of modern testate amoebae training sets has enabled quantitative water table (WT) reconstructions whereas plant macrofossil records have traditionally only provided qualitative information of past changes in hydrological regimes. Recent study from one Finnish bog, however, suggests that, by utilising modern calibration data set, fossil plant communities can be equally well applied for quantitative WT reconstructions. Very seldom plant macrofossil and testate amoebae data are available from the same study site and no previous evaluations exist where quantitatively reconstructed local hydrological signals are produced and compared with each other. The persisting question in palaeoecological studies is whether any reconstruction based on one single proxy will/can result in an unambiguous picture of the changes in past conditions. Here we set against two late-Holocene quantitative hydrological reconstructions, one based on variations in mire plants and the other changes in testate amoebae assemblages. By using WA-PLS- technique, we transformed species data to quantitative measures (cm) of water table and then calculated the historical WT changes as deviations of the mean value. Same procedure was applied for two different boreal bog peat sections: Kontolanrahka in Finland and Männikjärve in Estonia (only ombrotrophic part of the peat column was included). The reconstructed WT variations during the last ca. 5000 years seem to be replicated by both proxies nearly identical. However, both study sites also include mismatching periods during which proxies suggest deviating moisture conditions when compared to mean WT value.

Research paper thumbnail of Plant macrofossil evidence for an early onset of the Holocene summer thermal maximum in northernmost Europe

Nature communications, 2015

Holocene summer temperature reconstructions from northern Europe based on sedimentary pollen reco... more Holocene summer temperature reconstructions from northern Europe based on sedimentary pollen records suggest an onset of peak summer warmth around 9,000 years ago. However, pollen-based temperature reconstructions are largely driven by changes in the proportions of tree taxa, and thus the early-Holocene warming signal may be delayed due to the geographical disequilibrium between climate and tree populations. Here we show that quantitative summer-temperature estimates in northern Europe based on macrofossils of aquatic plants are in many cases ca. 2 °C warmer in the early Holocene (11,700-7,500 years ago) than reconstructions based on pollen data. When the lag in potential tree establishment becomes imperceptible in the mid-Holocene (7,500 years ago), the reconstructed temperatures converge at all study sites. We demonstrate that aquatic plant macrofossil records can provide additional and informative insights into early-Holocene temperature evolution in northernmost Europe and sugge...

Research paper thumbnail of High-resolution reconstruction of wetness dynamics in a southern boreal raised bog, Finland, during the late Holocene: a quantitative approach

The Holocene, 2007

Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/17/ 8/1093 The online... more Page 1. http://hol.sagepub.com/ The Holocene http://hol.sagepub.com/content/17/ 8/1093 The online version of this article can be found at: DOI: 10.1177/ 0959683607082550 2007 17: 1093 The Holocene Minna Väliranta, Atte ...

Research paper thumbnail of The importance of northern peatland expansion to the late-Holocene rise of atmospheric methane

Quaternary Science Reviews, 2010

Wetlands have been considered as the most important natural source of the atmospheric methane con... more Wetlands have been considered as the most important natural source of the atmospheric methane concentration (AMC) prior to anthropogenic influences. According to ice cores, AMC varied significantly during the Holocene, the causes of which are not completely understood. In particular, the reasons for the increased AMC during the late Holocene (from 5 ka onwards) have been debated widely, including an anthropogenic explanation. Initially, this increase was associated with increased emissions from northern wetlands, but estimated peat initiation rates seem not to support the conclusion. Based on a new data set of 954 basal peat radiocarbon dates that accounts more properly for the horizontal growth dynamics of northern peatlands (by containing only sites with multiple basal dates per site), we show here that the most extensive lateral expansion of high-latitude peatlands occurred only after 5 ka, parallel with the rise of CH 4 in the ice cores. Because this explosive increase in the extent of peatlands resulted in the formation of moist minerotrophic fen ecosystems that emit high amounts of CH 4 for a long time since their formation, and because many Arctic peatlands have remained minerotrophic throughout their development, northern peatlands cannot be neglected when seeking cause(s) for the late-Holocene rise in CH 4 . A similar event in future could enhance climate change by causing a rapid shift in atmospheric greenhouse gas concentrations.

Research paper thumbnail of The distribution of late-Quaternary woody taxa in northern Eurasia: evidence from a new macrofossil database

Quaternary Science Reviews, 2009

Research paper thumbnail of Holocene climate and landscape evolution East of the Pechora Delta, East-European Russian Arctic

Quaternary Research, 2003

This study presents a multiproxy record of Holocene environmental change in the region East of th... more This study presents a multiproxy record of Holocene environmental change in the region East of the Pechora Delta. A peat plateau profile (Ortino II) is analyzed for plant macrofossils, sediment type, loss on ignition, and radiocarbon dating. A paleosol profile (Ortino III) is described and radiocarbon dated. A previously published peat plateau profile (Ortino I) was analyzed for pollen and conifer stomata, loss on ignition, and radiocarbon dating. The interpretation of the latter site is reassessed in view of new evidence. Spruce immigrated to the study area at about 8900 14 C yr B.P. Peatland development started at approximately the same time. During the Early Holocene Hypsithermal taiga forests occupied most of the present East-European tundra and peatlands were permafrost free. Cooling started after 5000 14 C yr B.P., resulting in a retreat of forests and permafrost aggradation. Remaining forests disappeared from the study area around 3000 14 C yr B.P., coinciding with more permafrost aggradation. The retreat of forests resulted in landscape instability and the redistribution of sand by eolian activity. The displacement of the Arctic forest line and permafrost zones indicates a warming of at least 2-3°C in mean July and annual temperatures during the Early Holocene. At least two cooling periods can be recognized for the second half of the Holocene, starting at about 4800 and 3000 14 C yr B.P.

Research paper thumbnail of The Holocene thermal maximum and late-Holocene cooling in the tundra of NE European Russia

Quaternary Research, 2011

To investigate the Holocene climate and treeline dynamics in the European Russian Arctic, we anal... more To investigate the Holocene climate and treeline dynamics in the European Russian Arctic, we analysed sediment pollen, conifer stomata, and plant macrofossils from Lake Kharinei, a tundra lake near the treeline in the Pechora area. We present quantitative summer temperature reconstructions from Lake Kharinei and Lake Tumbulovaty, a previously studied lake in the same region, using a pollen-climate transfer function based on a new calibration set from northern European Russia. Our records suggest that the early-Holocene summer temperatures from 11,500 cal yr BP onwards were already slightly higher than at present, followed by a stable Holocene Thermal Maximum (HTM) at 8000-3500 cal yr BP when summer temperatures in the tundra were ca. 3°C above present-day values. A Picea forest surrounded Lake Kharinei during the HTM, reaching 150 km north of the present taiga limit. The HTM ended with a temperature drop at 3500-2500 cal yr BP associated with permafrost initiation in the region. Mixed spruce forest began to disappear around Lake Kharinei at ca. 3500 cal yr BP, with the last tree macrofossils recorded at ca. 2500 cal yr BP, suggesting that the present wide tundra zone in the Pechora region formed during the last ca. 3500 yr.

Research paper thumbnail of Reconstructing peatland water tables using transfer functions for plant macrofossils and testate amoebae: A methodological comparison

Quaternary International, 2012

Article history: Available online xxx a b s t r a c t Relatively seldom is the same parameter rec... more Article history: Available online xxx a b s t r a c t Relatively seldom is the same parameter reconstructed from the same site using different proxies, resulting in a persistent problem for palaeoecological studies whereby a reconstruction based on a single-proxy may provide an unequivocal view of the changes in past conditions. Plant macrofossils and testate amoebae are commonly used proxies to reconstruct past changes in peatland surface moisture conditions, but there are no comparisons between quantitative reconstructions based on both techniques. This paper compares two high-resolution late-Holocene quantitative water table (WT) reconstructions based on transfer functions for plant macrofossils and testate amoebae from two boreal peatland sites in Finland and Estonia. The reconstructed WT variation patterns during the last ca. 5000 years are almost identical in directions for both proxies. However, both bog records contain one inconsistent episode when the two proxies indicate different hydrological conditions. In both cases, the testate amoebae reconstruction shows wetter than the average conditions, whereas the plant-based reconstruction indicates drier than the average conditions. Several, possibly simultaneously affecting, reasons can be suggested for mismatches between proxies: 1) the proxies have different response times and sensitivities to hydrological changes, 2) the species-ecology is inadequately known, 3) the modern analogues are poor, 4) the microhabitat dynamics are unpredictable, and 5) the modern data set is too small. Divergences between the proxy records emphasize the fact that single-proxy reconstructions are subject to larger uncertainties than those based on two or more methods.

Research paper thumbnail of Conservative composition of n-alkane biomarkers in Sphagnum species: Implications for palaeoclimate reconstruction in ombrotrophic peat bogs

Organic Geochemistry, 2010

... Peat deposits accumulate at a high rate on a decadal scale, such that high resolution, palaeo... more ... Peat deposits accumulate at a high rate on a decadal scale, such that high resolution, palaeoclimate proxy records can be obtained, based on macrofossils and pollen ([Barber et al., 1994] and [Väliranta et al., 2007]), humification indices (Blackford and Chambers, 1993), testate ...

Research paper thumbnail of Fire history and vegetation recovery in two raised bogs at the Baltic Sea

Journal of Vegetation Science, 2011

Cal. yr BP = calibrated years before present (counted back from 1950); vol % = volume percentage,... more Cal. yr BP = calibrated years before present (counted back from 1950); vol % = volume percentage, the proportion of (moss) species in total plant macrofossils; n = number of (charcoal) particles Abstract Questions: What were the bog fire patterns and frequencies in two boreal peatlands during the last 5000 years? What is the nature and time-scale of postfire vegetation successions? Were fire events related to climate? Location: Männikjärve bog, central east Estonia; Kontolanrahka bog, southwest Finland.

Research paper thumbnail of Palaeoecological evidence of changes in vegetation and climate during the Holocene in the pre-Polar Urals, northeast European Russia

Journal of Quaternary Science, 2003

Väliranta, M.; Sarmaja-Korjonen, K.; Solovieva, N.; Virtanen, T.; Kauppila, T.; Eronen, M. Citation

Research paper thumbnail of The influence of Holocene tree-line advance and retreat on an arctic lake ecosystem: a multi-proxy study from Kharinei Lake, North Eastern European Russia

Journal of Paleolimnology, 2011

abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder ... more abundance of diatom phytoplankton, especially heavily silicified Aulocoseira species. The colder climate resulted in a shorter ice-free period, the lake was less productive and there was a loss of aquatic macrophytes. Increased wind-induced mixing following forest retreat had a greater influence on the lake ecosystem than the effects of decreasing organic matter concentration and increased light penetration.

Research paper thumbnail of Holocene aquatic ecosystem change in the boreal vegetation zone of northern Finland

Journal of Paleolimnology, 2011

We studied multiple variables in a sediment core from Lake Kipojärvi, northern Finland, to invest... more We studied multiple variables in a sediment core from Lake Kipojärvi, northern Finland, to investigate Holocene ecosystem changes in relation to catchment characteristics and known climate variations. We focused on a forested catchment because previous paleolimnological studies conducted in Fennoscandia focused mainly on subarctic lakes within a range of shifting treeline(s). Data on aquatic macrophytes, diatoms, Cladocera, C:N ratio, organic

Research paper thumbnail of Palaeolimnological Development of Lake Njargajavri, Northern Finnish Lapland, in a Changing Holocene Climate and Environment

Journal of Paleolimnology, 2006

This study used palaeolimnological approaches to determine how Holocene climatic and environmenta... more This study used palaeolimnological approaches to determine how Holocene climatic and environmental changes affected aquatic assemblages in a subarctic lake. Sediments of the small Lake Njargajavri, in northern Finnish Lapland above the present treeline, were studied using multi-proxy methods. The palaeolimnological development of the lake was assessed by analyses of chironomids, Cladocera and diatoms. The lake was formed in the

Research paper thumbnail of Holocene development of aquatic vegetation in shallow Lake Njargajavri, Finnish Lapland, with evidence of water-level fluctuations and drying

Journal of Paleolimnology, 2005

Holocene development of aquatic plant communities in subarctic Lake Njargajavri, Finnish Lapland,... more Holocene development of aquatic plant communities in subarctic Lake Njargajavri, Finnish Lapland, was studied using plant macrofossil analysis. Sediment lithology, grain size, and C/N ratios showed distinct lithological phases, indicating past water-level fluctuations. The colonization of limnophytes took place right after the formation of the lake (after ca. 11,500 cal. BP). The earliest plant macrofossil assemblages indicate nutrient-rich conditions and a warmer

Research paper thumbnail of Mid-Holocene palaeoclimatic and palaeohydrological conditions in northeastern European Russia: a multi-proxy study of Lake Vankavad

Journal of Paleolimnology, 2000

Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sedime... more Mid-Holocene changes in vegetation, palaeohydrology and climate were investigated from the sediments of Lake Vankavad in the northern taiga of the Usa Basin, NE European Russia, through the analysis of pollen, plant macrofossils, Cladocera and diatoms. Lake Vankavad was probably formed at ca. 5000 BP (ca. 5600 cal. BP) and initially it was shallow with a littoral cladoceran fauna. Macrofossil and pollen results suggest that dense Betula-Picea forests grew in the vicinity and the shore was close to the sampling point. At ca. 4600 BP (ca. 5400 cal. BP) the water level rose coincident with the decrease in the density and area of forests, probably caused by cooling climate and accelerated spread of mires. There was also a further rise in the water level at ca. 3500 BP (ca. 3800 cal. BP). The initiation of the lake, followed by two periods of rising waterlevel, as well as the increase in mire formation, was a consequence of a rise in groundwater level. This probably reflects lower evapotranspiration in a cooling mid-Holocene climate and/or higher precipitation in the lowland area. Also the decreased forest density and area may have contributed to the lower evapotranspiration. It is also possible that permafrost aggradation or changes in peat ecosystems might have affected the hydrological conditions in the area.

Research paper thumbnail of Acknowledging the spatial heterogeneity in modelling/reconstructing carbon dioxide exchange in a northern aapa mire

Ecological Modelling, 2009

Chamber method is commonly used to measure the CO 2 exchange from plant communities. Due to low t... more Chamber method is commonly used to measure the CO 2 exchange from plant communities. Due to low time resolution, actual measurements reflect only momentary CO 2 exchange rates. Therefore, a common way to derive seasonal or annual estimates is to establish models describing the response of CO 2 exchange to environmental variables, and then to reconstruct the CO 2 exchange over the desired time period. There are several alternative ways to obtain the CO 2 balance for the entire mire: models can be parameterized by individual sample plots, plant communities or the entire site. Similarly, the CO 2 balance can be reconstructed by plots, plant communities or the entire site. We tested how the choice of the modelling and reconstruction approach influences the CO 2 exchange estimates for the entire mire and for individual sample plots and plant communities. We measured the CO 2 exchange in a spatially heterogeneous sedge-dominated northern aapa mire for two growing seasons. We observed high spatial variation in CO 2 balance between the plant communities. We noticed that when the CO 2 balances of individual sample plots or plant communities are of interest, using a model appropriate for the entire site may result in biased estimates. In worst case the different modelling approaches may turn the CO 2 balance of an individual sample plot from positive to negative. Further, while using the whole ecosystem approach in modelling, the superior ability of chamber method in acknowledging spatial variation is lost. While the modelled growing season CO 2 balance of the mire ranged from 232 to 625 g CO 2 m −2 depending on the chosen modelling and reconstruction approach, the average estimates still remained within the uncertainty range of one another. Acknowledgement of the spatial variation in plant community level makes the areal estimate more robust to varying weather conditions. Further, the reliability of estimates is improved by explicit formulation of the choices behind the modelling and reconstruction units reflecting the study objectives.

Research paper thumbnail of Carbon Dioxide and Methane Dynamics Trough Fen-Stream-Lake Continuum in North Boreal Zone, Finnish Lapland

This study aims to determine spatial and temporal variation in carbon dioxide (CO2) and methane (... more This study aims to determine spatial and temporal variation in carbon dioxide (CO2) and methane (CH4) dynamics through a fen-lake continuum. Study was carried out in a catchment dominated by aapa mire over the period from spring 2005 to autumn 2006 in north boreal zone in Finnish Lapland. The study region characteristically has very high lake and mire density. To

Research paper thumbnail of Two different hydrological proxies-replicate reconstructions?

ABSTRACT Bog plants and testate amoebae are perhaps the most commonly used proxies to reconstruct... more ABSTRACT Bog plants and testate amoebae are perhaps the most commonly used proxies to reconstruct past changes in mire surface moisture conditions. In bog environment, distribution and occurrence of these organisms is known to be mainly regulated by water table depth - in minerotrophic fens this relationship is more complex. Establishment of modern testate amoebae training sets has enabled quantitative water table (WT) reconstructions whereas plant macrofossil records have traditionally only provided qualitative information of past changes in hydrological regimes. Recent study from one Finnish bog, however, suggests that, by utilising modern calibration data set, fossil plant communities can be equally well applied for quantitative WT reconstructions. Very seldom plant macrofossil and testate amoebae data are available from the same study site and no previous evaluations exist where quantitatively reconstructed local hydrological signals are produced and compared with each other. The persisting question in palaeoecological studies is whether any reconstruction based on one single proxy will/can result in an unambiguous picture of the changes in past conditions. Here we set against two late-Holocene quantitative hydrological reconstructions, one based on variations in mire plants and the other changes in testate amoebae assemblages. By using WA-PLS- technique, we transformed species data to quantitative measures (cm) of water table and then calculated the historical WT changes as deviations of the mean value. Same procedure was applied for two different boreal bog peat sections: Kontolanrahka in Finland and Männikjärve in Estonia (only ombrotrophic part of the peat column was included). The reconstructed WT variations during the last ca. 5000 years seem to be replicated by both proxies nearly identical. However, both study sites also include mismatching periods during which proxies suggest deviating moisture conditions when compared to mean WT value.

Research paper thumbnail of Plant macrofossil evidence for an early onset of the Holocene summer thermal maximum in northernmost Europe

Nature communications, 2015

Holocene summer temperature reconstructions from northern Europe based on sedimentary pollen reco... more Holocene summer temperature reconstructions from northern Europe based on sedimentary pollen records suggest an onset of peak summer warmth around 9,000 years ago. However, pollen-based temperature reconstructions are largely driven by changes in the proportions of tree taxa, and thus the early-Holocene warming signal may be delayed due to the geographical disequilibrium between climate and tree populations. Here we show that quantitative summer-temperature estimates in northern Europe based on macrofossils of aquatic plants are in many cases ca. 2 °C warmer in the early Holocene (11,700-7,500 years ago) than reconstructions based on pollen data. When the lag in potential tree establishment becomes imperceptible in the mid-Holocene (7,500 years ago), the reconstructed temperatures converge at all study sites. We demonstrate that aquatic plant macrofossil records can provide additional and informative insights into early-Holocene temperature evolution in northernmost Europe and sugge...