| 1. |
- Sander, Mikkel
(författare)
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Climatic signals and frequencies in the Swedish Time Scale, River Ångermanälven, Central Sweden
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Any future climate variation forced by human activities will be superimposed on the background of natural climate variation. Therefore, before interpreting the present climate and addressing future climate scenarios some knowledge of past climate is necessary. This thesis offers a rare glimpse into a long record of fluvial activity in Central Sweden and illuminates some of the possible forcing agent behind past (and future) discharge variation. Along the Swedish East Coast varved deposits of sand silt and clay couplets make up a chronology, which extend from the present into the Late Glacial. This chronology is known as the Swedish Time Scale (STS) and the c. 8000 varves were deposited in River Ångermanälven, Central Sweden. Of these varves, the last c. 2000 years are considered secure in terms of coherent chronology and internal thickness variation. A 2000 year long geometric mean varve thickness series was calculated to account for the internal thickness variation, which is postulated to form a proxy for fluvial sediment transport. Geometric mean varve thickness was compared to observed maximum daily annual discharge Qmax (1909-1971 AD) and the relationship expressed in a power equation. Thus, a reconstruction of past discharge for the last 2000 years could be produced. Extreme reconstructed discharge events were shown to be reasonable, considering the range of the observed discharge. Observed Qmax normally occurs during the snowmelt flood. Thus it is reasonable to attribute the variation in reconstructed Qmax to the snowmelt flood and, therefore, to melt water generation. Accumulated observed winter precipitation data from eleven meteorological stations from within and in the vicinity of the Ångermanälven catchment were compared to Qmax. Nine time series shared variation with Qmax and were complied into an average accumulated winter precipitation series. This series shares c. 40% of its variation with Qmax (observed and reconstructed) and it is reasonable to attribute some of the variation in the reconstructed Qmax to snow accumulation changes. The reconstructed Qmax series holds a centennial length waveform, which is interpreted in terms of the late Holocene climate succession, i.e. the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and Contemporary Warm Period. Non-stationary frequencies in the sub decadal, decadal and centennial bands have been detected in the 2000 year long geometric mean varve series. Sub decadal frequencies are tentatively interpreted as forced by the North Atlantic Oscillation.. Decadal frequencies are interpreted as sunspot forcing. Centennial frequencies are tentative interpreted as Suess cycle forcing. Missing varves in the STS (minimum 500 years) have previously been postulated on the grounds of an independent calibrated radiocarbon calendar age obtained from varve deposits and its correlation to the GRIP Ice Core. However, an evaluation of the STS by cross correlation does not support the presence of a centennial error in the post glacial part of STS. Additional analyses on the relationship between reconstructed discharge and other climate (-derived) proxies have been tested, including temperature, tree rings, atmospheric indices, sea-ice extent, C-14 production variation, global temperature reconstructions and volcanic eruptions. These analyses were mostly internally contradictive and non-conclusive.
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| 2. |
- Albrecht, Joachim
(författare)
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Marginal behaviour of the last Scandinavian Ice Sheet during its final termination and deglaciation over Northeastern Germany
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents results from studies on geomorphology, lithostratigraphy and ice flow patterns during the final termination and deglaciation of the last Scandinavian Ice Sheet in NE Germany. Ice flow patterns derived from till fabrics and glaciotectonical data indicate that short-lived marginal domes had formed during the advance and the retreat of the last Scandinavian Ice Sheet in NE Germany. This interpretation is supported by several events with ice flow directions, which deviate from the otherwise prevailing movement direction from the NNE through NE. The area north of the Pomeranian terminal moraine is divided into a hummocky moraine terrain in the SW and a cover moraine terrain in the NE. A regional lithostratigraphy has been established, which is based on lithological, sedimentological and glaciotectonical data from open sections. One Weichselian basal till, overlying glaciotectonically deformed outwash sediments, has been found in the cover moraine area. Till fabrics imply an initial ice movement direction from the SSE through E. Glaciotectonically dislocated sediments, eg. rafts of Tertiary clay, Tertiary sand, pre-Weichselian till and Eemian peat, occur occasionally in the entire study area, obscuring the stratigraphical record. These sediments must therefore be handled with caution when used as stratigraphical marker horizons. The sediments in the hummocky moraine area consist of out-wash and mass movement deposits related to the deglaciation after the Pomeranian stage. Some of the sites in the hummocky terrain also show implications of active ice movement from the SSE and SSW during dead-ice decay. An event-stratigraphical model is erected, which is tested against existing numerical and qualitative models. The existence of marginal domes is confirmed by several numerical models, most qualitative models are ambiguous to dome-build up, however. The marginal dome theory is controversial, although it provides an opportunity to explain frequently occurring deviating ice flow directions in NE Germany.
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| 3. |
- Alexanderson, Helena
(författare)
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Glacial geology and palaeo-ice dynamics of two ice-sheet margins, Taymyr Peninsula, Siberia and Jameson Land, East Greenland
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The North Taymyr ice-marginal zone (NTZ) on the Taymyr Peninsula, Arctic Siberia and the Ugleelv Valley on Jameson Land, East Greenland, have been investigated with the aim of reconstructing the glacial history, including depositional processes and environments. Geomorphological, sedimentological, stratigraphical and remote sensing methods have been combined to give a comprehensive view of developments in the two areas. Optically stimulated luminescence (OSL) and radiocarbon (14C) dates provide the chronological control. The Kara Sea shelf was glaciated three times during the Weichselian, each time with a smaller ice cover than before. The ice sheets caused a reversal of the fluvial drainage towards the south on the Taymyr Peninsula and, during the Early-Middle Weichselian, also the damming of proglacial lakes. The youngest ice-advance, but probably also the older ones, was warm-based and 'surge-like'. After it had reached its maximum position, the margin froze to its base and compressional flow took place there. The North Taymyr ice-marginal zone (NTZ) was initiated during an Early Weichselian retreat stage (c. 80 ka BP) and added to during the Middle (c. 65 ka BP) and Late Weichselian (<20 ka BP) ice advances, thus revealing a complex history. It comprises ice-marginal and supraglacial landsystems dominated by 2-3 km wide thrust-block moraines. Large areas are still underlain by remnant glacier ice and a supraglacial landscape with numerous ice-walled lakes and kames is forming even today. The proglacial landsystem is characterised by subaqueous or terrestrial environments, depending on altitude and time of formation. The sedimentary succession in the Ugleelv area comprises three tills, glaciolacustrine, glaciofluvial and aeolian sediments. The depositional history started in the early Saalian with a prograding delta in a lake dammed by an outlet glacier in Scoresby Sund. Soon thereafter, and also once during the Weichselian, Jameson Land was inundated by glaciers emanating from Liverpool Land in the east. These glaciers were warm-based and deposited glaciofluvial sediments, local tills and small end moraines. Conditions were less dynamic later in the glacial cycles. During most of the Weichselian the Ugleelv area was ice free and aeolian activity took place, but in the late Saalian the Greenland ice sheet expanded eastwards over the area. This ice was mainly cold-based but in places temporarily warm-based, where a lodgement till was deposited. During deglaciation there was substantial glaciofluvial erosion. The Kara Sea ice sheet and the Greenland ice sheet behave differently through a glacial cycle. The shelf-centred Kara Sea ice experiences large shifts in areal extent and disappears completely during interglacials. 'Individual' ice advances seem to be gover-ned by internal ice dynamics rather than by climatic changes. Contrary to this, the Greenland ice sheet is relatively stable and the climatically driven expansion or retreat of its outlet glaciers through the fjords represents the major changes.
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| 4. |
- Andersson, Gunnar
(författare)
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Deglaciation pattern and dynamics in the Bolmen area, southwestern Sweden
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Late Weichselian deglaciation of an area in southwestern Sweden was reconstructed by geomorphological, sedimentological and lithostratigraphical methods. The morainic deposits were mapped and classified into three landsystems: Streamlined terrain, hummocky moraine and cover moraine. Streamlined terrain was assumed to have been formed subglacially by a wet-based ice sheet. Cover moraine was interpreted to consist of redeposited sediments which have melted out supraglacially from a debris-rich ice. Hummocky moraine was formed by supraglacial melt-out and flowage of glacial debris into depressions and subsequent inversion of the landscape due to melting of ice cores. A model is presented where difference in morphology between hummocky and cover moraine is explained by differences in distribution of glacial debris in the stagnant ice. This distribution depends on the flow regime of the ice before stagnating. Hummocky moraine developed where the ice had compressive flow before stagnating and glacial debris was elevated to high levels in the ice by thrusting and folding of debris-rich basal ice. Cover moraine was formed where the ice had an extending or steady state flow before stagnating and the debris-rich ice was concentrated near the base of the ice. An east-west trending end moraine was mapped and sedimentologically investigated. The moraine was found to be mainly composed of ”ice slope colluvium” deposited in front of an active ice front which had readvanced from the north. The moraine was connected to the west to a previously known end moraine named the Torup moraine and the combined moraine was named the Torup-Unnaryd moraine. A 40 km long southward continuation of the Gothenburg moraine on the Swedish west coast was found and based on geomorphological evidences it was correlated with the Torup-Unnaryd moraine. The total extension of this recession line supported by firm geomorphological evidences is 145 km.
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| 5. |
- Anjar, Johanna
(författare)
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The Weichselian in southern Sweden and southwestern Baltic Sea : glacial stratigraphy, palaeoenvironments and deglaciation chronology
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis the Weichselian glaciation history of southernmost Sweden and the southwestern Baltic Basin is discussed, with special emphasis on Middle and Late Weichselian ice advances and subsequent deglaciations. The main study area was Kriegers Flak in the southwestern Baltic Sea where pre-Late Weichselian sediments were identified. We suggest that the lowermost till on Kriegers Flak, dated to the Early or Middle Weichselian, was deposited during the Ristinge advance, which implies an age of c. 55-50 ka. Following the deglaciation after this advance, isostatic depression enabled a marine influence in the southern Baltic Basin. During this time Kriegers Flak unit A, with a low-diversity benthic foraminifera fauna indicative of cold water and low salinities, was deposited. This was followed by a forced regression on Kriegers Flak, probably caused by a combination of isostatic rebound and a falling global sea level. Between 42 and 36 cal. ka BP wetlands and shallow lakes existed on Kriegers Flak (unit B). Macrofossil and pollen from this unit suggest tundra-like, or forest tundra-like vegetation, possibly with birch and pine in sheltered locations. From 28.5 to 26 ka, a thick succession of glaciolacustrine clay, unit C, was deposited at Kriegers Flak indicating a damming the Baltic Basin by an ice advance into Kattegat. The upper part of the stratigraphy has been reconstructed from published and new terrestrial sections in Skåne. Here three tills are identified. The lowermost, the Allarp Till, was deposited after the damming of the Baltic Basin at c. 30 ka. It was followed, after a deglaciation, by deposition of the Dalby Till, representing the Last Glacial Maximum advances including an early advance from southeast, the main advance from northeast and the first Young Baltic advances from southeast. The uppermost till, the Lund Till was deposited by the Öresund advance after a deglaciation. The deglaciation of southern Sweden was dated using cosmogenic nuclide surface exposure dating. We suggest that central Skåne was deglaciated between 17 and 16 ka. The deglaciation in southern Småland was dated to 15.6±0.8 and 16.6±0.9 ka, while a site in northern Småland was dated to 14.6±1.0 ka. Our northernmost site, situated only 25 km south of the Younger Dryas Moraine, was dated to c. 13.8±0.8 ka while cosmogenic surface exposure ages from Gotland suggest a deglaciation before 13.0±0.8 ka.
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| 6. |
- Barnekow, Lena
(författare)
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Holocene vegetation dynamics and climate changes in the Torneträsk area, northern Sweden
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this study was to reconstruct the Holocene vegetation history and the climate changes in a subarctic area. The results are based on pollen and macrofossil records from six lakes in the Torneträsk area. The lakes are situated at altitudes between 370 and 999 m a.s.l., along a transect from the boreal pine-birch forest to the alpine tundra with heath vegetation. The chronology in each lake is based on radiocarbon dated terrestrial plant macrofossils and combined with varve chronology in one of the lakes. Ages from radiocarbon dated plant macrofossils were compared with ages from radiocarbon dated bulk sediment samples in two of the lakes. The results show that radiocarbon dated bulk sediment samples give older ages compared to the macrofossil samples. The age differences varied between 550 and 2200 years in samples from a lake with calcareous rich sediment and between 250 and 800 years in samples from lake sediment with a low carbonate carbon content. Erroneous high ages from bulk sediment samples will not only effect the chronology, but will also lead to incorrect pollen-influx calculations. The results of this study also indicate that the tree-line, which consists of mountain birch (Betula pubescens ssp. tortuosa), was situated c. 300-400 m above the present tree-line during the early and the mid-Holocene. When land up-lift has been taken into account this higher tree-line corresponds to a summer temperature of 1.5-2°C higher than today. The climate was warm and moist during the early Holocene and warm and dry during the mid-Holocene. A pine-birch forest expanded first in the eastern part of the Torneträsk area at c. 8300 cal BP and about 700 years later to the western part at low altitudes. Optimal conditions for pine (Pinus sylvestris) occurred between 6300 and 4500 cal BP when the pine-birch forest reached its highest altitude at least 560 m a.s.l., i.e. 110 m above the present pine limit. During this time the pine limit reached 175 m above the present. Dry conditions during this interval are also supported from lake-level studies in one of the lakes, which show that the lake level was lowered c. 1-1.5 m. At 4500 cal BP the tree-line begun to descend and the lake-level rose again. The climate became both colder and more humid than before. As a result of this climate change the vegetation became more mosaic-like with sparse forests also at low altitudes and the heath vegetation increased.
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| 7. |
- Bergman, Jonas
(författare)
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Tree-limit ecotonal response to Holocene climate change in the Scandes Mountains of west-central Sweden
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this thesis was to reconstruct the Holocene vegetational and climatic development in the Sylarna-Storulvån area, western Jämtland, in the central Scandes Mountains. Temporal trends and fluctuations in the elevation and vegetational character of the tree-limit ecotone were studied mainly by means of pollen and plant macrofossil analysis of two lake sediment sequences (Lakes Stentjärn and Spåime), located above the present-day tree-limit. The lake sediments were also subjected to high-resolution elemental and mineral magnetic measurements, which contributed useful complementary information on the local environmental development. Plant macrofossil data indicate the presence of a short-lived deglaciation flora, dominated by light-demanding herbs and dwarf-shrubs, followed by the establishment of birch-pine forest. The vegetational data obtained were compared with previously published records of radiocarbon-dated subfossil wood remains (megafossils), collected mainly in the study area. A general conformity was revealed between the stratigraphic plant macrofossil data and pollen accumulation rates, and the comparison between the non-stratigraphic megafossil data and the pollen influx/plant macrofossil records also revealed a high level of consistency of the inferred tree-limit variations for Pinus sylvestris, Betula pubescens, and Alnus incana. Records of climatic humidity inferred from peat humification data (DOH) were obtained from two separate profiles at a nearby peat deposit (Klocka Bog), situated below the forest-limit. Evaluation of the DOH records exhibits millennial-scale trends, which are significantly correlated between profiles during the periods 6500-4000 cal yr BP and 2100-0 cal yr BP. Within these periods, the time between 5800 and 4800 cal yr BP, and 1800 cal yr BP until the present, are recognised as episodes of increasing climatic humidity. In general, the vegetational, geochemical and sedimentary records were shown to correlate with several Holocene climatic events and transitions, identified elsewhere in north-western Europe. The climatic forcing of some of these sub-Milankovitch scale perturbations is unclear, but a coupling to internal circulation dynamics of the North Atlantic Ocean is hypothesized. Chronologies of the geological archives studied within the project were based on radiocarbon dating and tephrochronology. At Lake Stentjärn, three Holocene cryptotephra horizons were detected, one of which was geochemically correlated with the Icelandic Askja-1875 eruption. At Klocka Bog, at least seven cryptotephra horizons were recorded in the two peat profiles, and five of the horizons were geochemically correlated with the Askja-1875, Hekla-3, Kebister, Hekla-4, and Lairg A tephras, respectively.
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| 8. |
- Björkman, Leif
(författare)
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The Late Holocene history of beech Fagus sylvatica and Norway spruce Picea abies at stand-scale in southern Sweden
- 1996
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High resolution pollen analysis was carried out on five peat profiles from small forest hollows at four sites in southern Sweden. The general aim was to investigate the establishment of Fagus sylvatica and Picea abies at stand-scale. The sites used for this study were all located within the area where the present distribution limits of Fagus and Picea overlap each other. The results from the different sites are presented in more detail in separate papers (Appendices I-IV). Viewed on a continental scale the migration pattern of Fagus can be correlated with climate and its change over the millennia, but at finer scales such a correlation is weaker. At stand-scale there are factors other than climate that are crucial for the establishment of Fagus (e.g., disturbance, seed dispersal, human activities). The establishment of Fagus does not show a regional coherence in southern Sweden, and this may imply that climate was not the limiting factor for its establishment. The present day distribution of Fagus in southern Sweden suggests a migration with a discontinuous front with outlying populations, and this model probably applies to its past distribution. This type of migration means that the landscape becomes infilled by dispersal from outpost stands. The timing of stand-scale establishment is then largely influenced by site-specific factors and chance. Fagus may still be migrating northwards in Sweden. It grows well in its outpost area, and it seems that present day land-use, not climate, is the limiting factor for local Fagus expansion. Fagus seeds are highly dependent on ground disturbance for successful establishment, and an undisturbed forest then consequently would be able to resist Fagus invasion for some time. A semi-open cultural landscape may be optimal for Fagus establishment, as cultural activities may create conditions particularly suitable for its regeneration. Picea invaded southern Sweden from the north during a period when the cultural landscape had already been evolving for some time. Picea is a dominant tree with an effective seed dispersal, and the relatively open and probably grazed forests in the area were not particular resistant to Picea invasion. An intensive grazing regime may not affect Picea, as grazing animals normally avoid Picea. The timing of local Picea establishment seems to be mostly controlled by its migration, i.e., it became established when its front reached the studied sites.
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| 9. |
- Broström, Anna
(författare)
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Estimating source area of pollen and pollen productivity in the cultural landscapes of southern Sweden - developing a palynological tool for quantifying past plant cover
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fossil pollen records retrieved from peat and lake sediments have great potential for quantifying past plant cover. This thesis is a contribution to the development of a palynological interpretation tool for reconstructing past cultural landscapes in terms of plant abundance and distribution. The aims of this thesis are (1) to explore pollen-vegetation relationships in the ancient, traditional cultural landscape of southern Sweden, (2) to test the potentials and limits of using mechanistic models of pollen dispersal and deposition to reconstruct past cultural landscapes from fossil pollen assemblages, (3) to estimate pollen productivity for the most common plant taxa characteristic of the past cultural landscapes of southern Sweden, and (4) to define and estimate the spatial scale of the cultural landscape reflected by pollen assemblages in moss polsters and small lakes. Data collection included sampling moss polsters and lake sediments for pollen and making vegetation surveys. The compilation of vegetation data was performed using GIS software. Pollen/vegetation relationships were explored using numerical techniques (PCA, RDA, Monte Carlo permutation test). Pollen productivity was estimated using Extended R-Value (ERV) -models. These models imply that vegetation is distance-weighted, which required the development of an appropriate design of vegetation survey in the field. The spatial scale of the landscape reflected by pollen assemblages is defined as the area beyond which the correlation of pollen and vegetation does not improve, and is named "relevant source area of pollen" (RSAP). RSAP was estimated using the maximum likelihood method. For hypothesis testing, a computer simulation model (POLLSCAPE) developed for forest vegetation was used to simulate pollen dispersal and deposition in cultural landscapes. The empirical pollen/vegetation data shows a major difference between the two contrasting vegetation regions of southern Sweden in terms of the relationship between NAP (herb pollen) percentages and open-land percentage cover. The relationship between NAP % and open-land % cover was also simulated using POLLSCAPE. The simulated and empirical relationships are very similar, which demonstrates that POLLSCAPE can be used for semi-open and open landscapes. Pollen productivity estimates (PPE) were obtained for 25 herb and tree taxa. RSAP for small lakes and moss polsters was estimated to be 800-1000 m and 400 m, respectively. Empirical data and simulations show that random site selection is required in order to obtain reliable estimates of RSAP, and that the spatial resolution of the vegetation survey close to the pollen sample, i.e. the size distribution of the vegetation mosaic, plays a critical role in RSAP estimates. Major implications of the results are (1) the cover of open land is likely to be strongly underestimated on the basis of NAP percentages alone in a mosaic semi-open landscape, (2) provided that past and present landscapes have a similar vegetation structure, a quantitative reconstruction of the past landscape would be possible within 1000 m radius around small lakes and 400 m radius around soil profiles, (3) the availability of PPE for critical herb taxa and the demonstration that POLLSCAPE may be used to study open and semi-open landscapes provide a reliable tool for proceeding with the study of pollen dispersal and deposition in cultural landscapes.
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| 10. |
- de Jong, Rixt
(författare)
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Stormy records from peat bogs in south-west Sweden : implications for regional climatic variability and vegetation changes during the past 6500 years
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis aims to reconstruct and explain variations in storm activity, humidity, vegetation composition and land cover in south-west Sweden during the past 6500 years. Two ombrotrophic bogs, Store Mosse and Undarsmosse, situated on the coastal plain of Halland, were selected for this purpose. The coastal plain of Halland is positioned in the cyclone path and is therefore particularly sensitive to changing cyclone frequencies and associated storms. Cyclones have a strong control on the climate in this region, by moderating summer and winter temperatures and being the main source of precipitation. Aeolian sediment influx (ASl) was used as a proxy for storm activity. Bog surface wetness reconstructions were based on organic bulk density values from both study sites, whereas testate amoebae analysis was applied on cores from Undarsmosse bog only. Pollen analysis was carried out to reconstruct vegetation and land use changes. In addition the hypothetical regional scale vegetation composition around the Store Mosse bog was reconstructed by applying the REVEALS model, which corrects for differences in pollen productivity and dispersal between plant taxa. The land use reconstructions were also used to discriminate between climatic and human induced variations in ASI values, since increased sediment availability resulting from human activity may have affected sand influx into the bogs. The reconstructions of storm activity are similar at both study sites, with peaks of storminess around 4700, 2800-2000, 1500, 1100, 700 and 400-50 cal. yrs BP. These time periods are in good agreement with, for example, dune re-activation phases in Denmark. A comparison to land use in the region shows that ASI values are highest when the indications for cultivated fields are low, which implies that land use cannot explain the occurrence of ASI peaks. These are therefore most likely causally related to regional scale climatic changes. Humidity shifts were recorded around 4800, 2000 and 400 cal. yrs BP at both sites, which concurs with lake chemistry and lake level reconstructions from southern Sweden. These results were used to propose a regional scale reconstruction of climatic change, and may be used to infer variations in past atmospheric circulation patterns. In addition, a detailed comparison between ASI values and testare amoebae based wetness reconstructions has revealed an enigmatic pattern of increased storm activity during shifts in humidity during the past 1700 years.
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