Jens-Ove Näslund

New ways of studying ice sheet flow directions and glacial erosion by computer modelling—examples from Fennoscandia

A computer ice sheet model has been used to study regional ice flow directions and glacial erosion of the Weichselian ice sheet, adopting a new method of presenting modelled ice flow directions. Ice sheet model results from different time periods during the Weichselian were extracted for five regions and presented in rose diagrams. When comparing these computer-generated results with information on flow directions obtained from current conceptual geological models based on field data, large similarities were observed. In comparing the ice flow directions as such, the similarities were strikingly good. In many cases there was also a good agreement in the timing of the events, while in some cases a certain flow direction was assigned to a different time period by the ice sheet model than in the current interpretation of geological information. Nevertheless, the overall agreement between the data sets shows that results from ice sheet models can aid in placing geological information into a coarse timeframe and chronostratigraphic context, and also fill in time gaps in the glacial geological record where chronological control is sparse. Ice sheet model results thus constitute a new data set against which glacial geological information can be compared and tested. A new quantity, basal sliding distance, has also been calculated from the ice sheet model results, describing the over time accumulated length of ice that has passed over the landscape by basal sliding. The results show high basal sliding distance values in SW Sweden/SE Norway, in Skagerrak, and along the Gulf of Bothnia, implying relatively large amounts of glacial erosion in these regions. On elevated parts of the Scandinavian mountain range and on adjacent plains in the east the basal sliding distance values are low, implying weaker glacial erosion. This compares well with different types of geological and morphological data, suggesting that basal sliding distance is a useful entity for studying regional patterns of glacial erosion intensity.

Drumlin Formation Time: Evidence from Northern and Central Sweden

Abstract Large‐scale drumlins occur abundantly throughout central and northern Sweden. Whereas many drumlins in the north are an integral part of a relict glacial landscape >100,000 years old, those to the south are generally interpreted as of last deglaciation age. Typically, the latter ones have not been overprinted by younger glacial landforms. Despite this apparent difference in formation history, drumlins in both regions have similar directional and morphological characteristics. A systematic analysis of >3000 drumlins in (i) areas within relict landscapes, (ii) areas with an ambiguous deglaciation age assignment, and (iii) areas within deglacial landscapes, indicates that these latter deglaciation drumlins differ clearly in both shape and size from drumlins in the other two types of landscapes. In addition, numerical modelling indicates that basal melting conditions, a prerequisite for drumlin formation, prevailed only for a very limited time over much of northern Sweden during the last deglaciation, but lasted for longer periods of time during earlier stages of the Weichselian. A reconnaissance radionuclide bedrock exposure date from the crag of a large drumlin in the relict landscape indicates that glacial erosion, and presumably drumlin formation, at this location predated Marine Isotope Stage 7. We conclude, therefore, that the large‐scale drumlins of central and northern Sweden did not form during the last deglaciation, or during any other specific ice flow event. Instead, we suggest that they were formed by successive phases of erosion and deposition by ice sheets of similar magnitude and configuration.

Detailed spatially distributed geothermal heat-flow data for modeling of basal temperatures and meltwater production beneath the Fennoscandian ice sheet

Abstract Accurate modeling of ice sheets requires proper information on boundary conditions, including the geothermal heat flow (or heat-flow density (HFD)). Traditionally, one uniform HFD value is adopted for the entire modeled domain. We have calculated a distributed, high-resolution HFD dataset for an approximate core area (Sweden and Finland) of the Scandinavian ice sheet, and imbedded this within lower-resolution data published for surrounding regions. Within the Last Glacial Maximum ice margin, HFD varies with a factor of as much as 2.8 (HFD values ranging between 30 and 83 mWm–2), with an average of 49 mWm–2. This average value is 17% higher than 42 mWm–2, a common uniform value used in ice-sheet modeling studies of Fennoscandia. Using this new distributed dataset on HFD, instead of a traditional uniform value of 42 mWm–2, yields a 1.4 times larger total basal meltwater production for the last glacial cycle. Furthermore, using the new dataset in high-resolution modeling results in increased spatial thermal gradients at the bed. This enhances and introduces new local and regional effects on basal ice temperatures and melt rates. We observed significant strengthening of local ‘ice streaming’, which in one case correlates to an ice-flow event previously interpreted from geomorphology. Regional to local variations in geothermal heat flow need to be considered for proper identification and treatment of thermal and hydraulic bed conditions, most likely also when studying Laurentide, Greenland and Antarctic ice sheets.

Subglacial preservation of valley morphology at Amundsenisen, western Dronning Maud Land, Antarctica

On the high altitude polar plateau of Amundsenisen, western Dronning Maud Land, East Antarctica, a subglacial valley, with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta, was studied by radio echo sounding. In addition, a small, probably glacial, valley was mapped within the same subglacial massif. Basal ice temperatures were calculated using field data on precipitation, air temperature and ice sheet thickness. Discoveries of old landforms which have been preserved more or less intact beneath the former Fennoscandian and Laurentide ice sheets have received increasing attention during the last decade. The aim of this study is to investigate whether preservation of landforms occurs under the East Antarctic Ice Sheet, and to discuss under that climatological and glaciological circumstances preservation may take place. The results show that the ice sheet covering the investigated localities is frozen to bed, and therefore has an insignificant erosional capability. The observations suggest that a large-scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold-based ice sheet marginal zone for a long time period. The process of glacial preservation, recognized for bedrock features and tentatively observed for sediment accumulations, should act on similar large-scale landforms under any cold-based ice sheet, present or past. On the basis of existing studies of the age and stability of the East Antarctic Ice Sheet, a Middle Pliocene age is suggested for the preserved landforms. The presence of the presumed sediment-filled valley further indicates that no prolonged periods of basal melting have occurred at the Amundsenisen study area during the ice sheet history, which includes the Quaternary glaciation periods. Finally, calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation.

Landscape development in western and central Dronning Maud Land, East Antarctica

Large-scale bedrock morphology and relief of two key areas, the Jutulsessen Nunatak and the Jutulstraumen ice stream are used to discuss glascial history and landscape development in western and central Dronning Maud Land, Antarctica. Two main landform components were identified: well-defined summit plateau surfaces and a typical alpine glacial landscape. The flat, high-elevation plateau surfaces previously were part of one or several continuous regional planation surfaces. In western Dronning Maud Land, overlying cover rocks of late Palaeozoic age show that the planation surface(s) existed in the early Permian, prior to the break-up of Gondwana. A well-develoment escarpment, a mega landform typical for passive continental margins, bounds the palaeosurface remnants to the north for a distance of at least 700 km. The Cenozoic glacial landscape, incised in the palaeosurface and escarpment, is exemplified by Jutulsessen Nunatak, where a c. 1.2 km deep glacial valley system is developed. However, the prominent Penck-Jutul Trough represents some of the deepest dissection of the palaeosurface. This originally tectonic feature is today occupied by the Jutulstraumen ice stream. New topographic data show that the bed of the Penck-Jutul Trough is situated 1.9±1.1 km below sea level, and that the total landscape relief is at least 4.2 km. Todays relief is a result of several processes, including tectonic faulting, subaerial weathering, fluvial erosion, and glacial erosion. It is probable that erosion by ice streams has deepened the tectonic troughs of Dronning Maud Land since the onset of ice sheet glaciation in the Oligocene, and continues today. An attempt is made to identify major events in the long-term landscape development of Dronning Maud Land, since the break-up of the Gondwana continent.

Radar surveys on Scandinavian glaciers, in search of useful climate archives

Ice cores obtained from cold glaciers are known to contain detailed archives on past climates. So far, no such data-set has been retrieved from Scandinavia, in part because most Scandinavian glaciers are largely of a temperate type, thus having a less well preserved climatic signal. However, within the more continental parts of the Scandinavian mountain range, glaciers often have a substantial cold surface layer in the ablation area. Past climate variations may be reconstructed from studies of crystallography and stratigraphy of ice from the cold surface layer. The thicknesses of such cold layers at two closely situated sites in northern Sweden have been mapped by a ground based high resolution radar. The best results were obtained by using the frequency interval 700-900 MHz. The two investigated glaciers Storglaciaren and Marmaglaciaren are situated 20 km from each other in an area with a strong east-west climatic gradient. The annual average air temperature is similar at the two sites, whilst the precipitation rate on Storglaciaren is almost double that of Marmaglaciaren. The radar soundings show that Marmaglaciaren has a significantly thicker cold surface layer, 65 m on average compared with 30 m on Storglaciaren. The difference is caused by less snow cover insulation from winter cold and less net ablation on Marmaglaciaren. The measured thicknesses of the cold surface layer therefore seem to correlate well with the degree of continentality at the sites. Radar sounding is a highly useful tool for locating glaciers containing a potential relatively well preserved climatic signal. Such glaciers may be found within the more continental parts of the Scandinavian mountain range.

Glacial isostatic adjustment: implications for glacially induced faulting and nuclear waste repositories

Glacial isostatic adjustment: Implications for glacially induced faulting and nuclear waste repositories

Numerical modelling of the ice sheet in western Dronning Maud Land, East Antarctica: impacts of present, past and future climates

Time-dependent ice-sheet modelling of a 176 000 km2 area in western Dronning Maud Land, East Antarctica, provided information on the ice sheets response to six climate-change scenarios. Another experiment was done to study changes in ice thickness, flow and basal temperature conditions between the present ice configuration and a simulated maximum palaeo-ice sheet. The input to the model included new datasets of bed and surface topography compiled for this study. The results of the six climate-change experiments, including a 0.5°C per century global-warming scenario, show that the ice sheet has a robust behaviour with respect to the different climate changes. The maximum change in ice volume was

Deglaciation and shoreline displacement on Alexandra Land, Franz Josef Land

14C datings of driftwood from raised beaches on the island Alexandra Land, Franz Josef Land, Russia, indicate deglaciation by 6800 14C years BP. The age of a diamicton containing small shell fragments, found beneath a beach ridge formed c. 5000 BP, has been dated to 8265 ?90 t4C years BP (age after sea correction 7755 BP), suggesting that Alexandra Land was at least partly deglaciated by that time. The deposit containing shell fragments has been interpreted as a till which, if the interpretation is correct, indicates that a glacial advance occurred sometime between 7800 and 6800 BP. The highest shoreline, 23.5 m a.s.l., was formed either during a rapid deglaciation of the island or during a transgression. A shoreline displacement curve has been constructed for Alexandra Land. The rate of shore displacement since 6800 BP is calculated to 0.3 m/100 years. No pumice was found on Alexandra Land. Lunar Ice Cap on Alexandra Land was smaller than it is at present when the highest shoreline was formed, 6800 BP. The ice cap is believed to now be close to its Midto Late Holocene maximum, as almost all fluctuations of the Lunar Ice Cap since the deglaciation seem to have been within the present day glacier margin. A climatic deterioration c. 2000 BP is indicated by the formation of an ice/snow dammed lake. Evidence of former glacier movement directions, such as striae and chattermarks, were not found on Alexandra Land. However, the large scale bedrock topography and the orientation of elongated bedrock basins seem to indicate ice flow in a southeast to northwest direction.

Groundwater flow modeling of periods with periglacial and glacial climate conditions for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark, Sweden

The impact of periglacial and glacial climate conditions on groundwater flow in fractured crystalline rock is studied by means of groundwater flow modeling of the Forsmark site, which was recently proposed as a repository site for the disposal of spent high-level nuclear fuel in Sweden. The employed model uses a thermal-hydraulically coupled approach for permafrost modeling and discusses changes in groundwater flow implied by the climate conditions found over northern Europe at different times during the last glacial cycle (Weichselian glaciation). It is concluded that discharge of particles released at repository depth occurs very close to the ice-sheet margin in the absence of permafrost. If permafrost is included, the greater part discharges into taliks in the periglacial area. During a glacial cycle, hydraulic gradients at repository depth reach their maximum values when the ice-sheet margin passes over the site; at this time, also, the interface between fresh and saline waters is distorted the most. The combined effect of advances and retreats during several glaciations has not been studied in the present work; however, the results indicate that hydrochemical conditions at depth in the groundwater flow model are almost restored after a single event of ice-sheet advance and retreat.RésuméL’impact des conditions climatiques périglaciaires et glaciaires sur les écoulements d’eau souterraine dans un aquifère cristallin fissuré est étudié à l’aide d’une modélisation des écoulements souterrains du site de Forsmark, qui a été récemment proposé comme site de dépôt pour le stockage des combustibles radioactifs de haut niveau en Suède. Le modèle employé utilise une approche couplée thermo hydraulique pour la modélisation du pergélisol et discute des changements des écoulements souterrains dus aux conditions climatiques de l’Europe du Nord lors de différents moments au cours de la dernière période glaciaire (glaciation du Weichselien). La conclusion est que la décharge de particules libérées au niveau de la profondeur du site de dépôt prend place à très grande proximité de la marge de la calotte glaciaire en l’absence de pergélisol. Si le pergélisol est inclus, la plus grande partie se déverse dans des zones non gelées (taliks) dans la zone périglaciaire. Pendant un cycle glaciaire, les gradients hydrauliques à la profondeur du dépôt atteignent leurs valeurs maximales, lorsque la marge de la calotte glaciaire recouvre le site; à cette époque, aussi, l’interface entre les eaux douces et les eaux salées est la plus déformée. L’effet combiné des progressions et reculs au cours de plusieurs glaciations n’a pas été étudié dans le cadre de ce travail; cependant, les résultats indiquent que les conditions hydrochimiques en profondeur dans le modèle d’écoulements des eaux souterraines sont presque rétablies après un seul événement de progression et de recul de la calotte glaciaire.ResumenSe estudia el impacto de las condiciones climáticas glaciales y periglaciales en el flujo de agua subterránea en rocas cristalinas fracturadas por medio de modelado de flujo de agua subterránea del sitio Forsmark, que fue recientemente propuesto como un sitio repositorio para la disposición de combustible nuclear de alta actividad usado en Suecia. El modelo empleado usa un enfoque de acoplamiento térmico hidráulico para modelar el permafrost y discute cambios en el flujo de agua subterránea dado por las condiciones climáticas encontradas sobre Europa del Norte en diferentes épocas durante el último ciclo glacial (glaciación Weichselian). Se concluye que la descarga de partículas liberadas en la profundidad del repositorio ocurre muy cerca del margen de la capa de hielo en la ausencia de permafrost. Si el permafrost está incluido, la mayor parte descarga en los taliks en el área periglacial. Durante un ciclo glacial, los gradientes hidráulicos en la profundidad del repositorio alcanzan sus máximos valores cuando el margen de la capa de hielo sobrepasa el sitio; en este tiempo, también, la interfase entre el agua salina y el agua dulce está mayormente distorsionada. El efecto combinado de avances y retrocesos durante varias glaciaciones no han sido estudiados en el presenta trabajo; sin embargo, los resultados indican que las condiciones hidroquímicas en profundidad en el modelo de flujo de agua subterránea son casi restauradas después de un solo evento de avance y retroceso de la capa de hielo.摘要通过模拟福什马克储藏地的地下水流研究了冰缘和冰川气候条件对断裂结晶岩中地下水流的影响,该场地最近拟定为处理瑞典废弃高强度核燃料的储藏地。所采用的模型使用了热工-水力耦合方法对永久冻土进行模拟,探讨了最后一次冰川回旋(Weichselian冰期)期间不同时期北欧气候条件给地下水流带来的变化。结果是,在无永久冻土的情况下,储藏地深部释放的粒子排泄到冰盖边缘很近的 地方。如果存在着永久冻土,大部分排泄到冰缘地区的融区中。冰川回旋期间,当冰盖边缘越过储藏地时,储藏地深部的水力梯度达到最大值;在此时期,淡水和咸水界面同样扭曲最严重。目前的研究工作没有包括若干个冰期间的冰进和冰退的综合影响;然而,研究结果表明,地下水流模型中深部的水文化学条件在冰盖进退的单个事件之后几乎完全恢复。ResumoO impacte das condições climáticas periglaciais e glaciais no fluxo de água subterrânea em rochas cristalinas fraturadas é estudado através da modelação de fluxo de águas subterrâneas na zona de Forsmark, que foi recentemente proposta como um depósito de armazenamento de combustível nuclear usado de alta atividade, na Suécia. O modelo utilizado usa uma abordagem termo-hidráulica acoplada para a modelação de permafrost e discute mudanças no fluxo das águas subterrâneas sugeridas pelas condições climáticas encontradas no norte da Europa em momentos diferentes durante o último ciclo glacial (glaciação Weichselian). Conclui-se que a descarga de partículas libertada à profundidade do depósito ocorre muito perto da margem do manto de gelo, na ausência de permafrost. Se o permafrost está incluído, a maior parte da descarga ocorre para dentro dos taliks na área periglacial. Durante um ciclo glacial, os gradientes hidráulicos à profundidade do reservatório atingem os valores máximos quando a margem do manto de gelo passa sobre o local; por esta altura, também, a interface entre a água doce e a água salgada é mais distorcida. O efeito combinado de avanços e recuos durante várias glaciações não foi estudado no presente trabalho; no entanto, os resultados indicam que as condições hidroquímicas em profundidade no modelo de fluxo de águas subterrâneas são quase restauradas após um único evento de avanço e recuo do manto de gelo.