| 1. |
- Sturkell, Erik, 1962, et al.
(författare)
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Concentric impact structures in the Palaeozoic of Sweden - the Lockne and Siljan craters
- 2010
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 132:1, s. 65-70
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Tidskriftsartikel (refereegranskat)abstract
- The Ordovician age Lockne and Devonian Siljan craters are two of the largest impact structures in northern Europe. Both formed in targets with a thick, low-strength upper layer. This target configuration is known to generate concentric crater structures with an outer, shallow crater in the low-strength layer, surrounding a central, deeper crater in the more resistant substrate. The concentric craters of Lockne and Siljan are excellent models for studies of similar concentric craters on Earth and elsewhere in the Solar system. Several structural issues remain, and drilling through the craters within the Swedish Deep Drilling Program intends to address the following: the extent of the craters with respect to the time of impact; the effects of cratering on the basement; and the role of basement structure for the crater formation. A problem for the Lockne crater is the relation to the Caledonian orogeny and the lateral extension of the ejecta blanket - the rim is interrupted by a radial depression that has been interpreted both as primary and secondary, tectonically induced. A second feature to study is the deeper and older (1.82-1.80 Ga) NNW-SSE shear zones that cut the basement. In the Siljan area the development of mega block associations comprising the infilling of the graben is disputed. The blocks may either be formed by sagging of peripheral parts of the fault blocks or alternatively by major radial movement involving kilometre long transport.
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| 2. |
- Stockmann, Gabrielle, et al.
(författare)
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Carbon mineral storage in seawater:Ikaite(CaCO3·6H2O)columns in Greenland.
- 2018
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Ingår i: Energy Procedia. - : Elsevier BV. - 1876-6102. ; 146, s. 59-67
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Tidskriftsartikel (refereegranskat)abstract
- The marine Ikka Fjord in Greenland is well known for its remarkable submarine columns made of the cold-carbonate ikaite (CaCO3·6H2O). Here, natural processes lead to fast-precipitating ikaite at low temperatures (< 10°C) when carbonate-bearing groundwater seeps through fractures in Ikka Fjord and mixes with seawater. Within an area of 0.75 km2, 678 columns of 1 – 20 meters height have been registered, continuously growing at rates measured at 50 cm per year. Understanding this natural system is of importance for carbon capture and storage efforts as it represents a very efficient method for carbon mineral storage in cold seawater.
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| 3. |
- Sturkell, Erik, 1962, et al.
(författare)
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The Proximal Ejecta Around the Marine-Target Lockne Impact Structure, Sweden
- 2023
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS. - 2169-9097 .- 2169-9100. ; 128:7
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Tidskriftsartikel (refereegranskat)abstract
- Very few impact craters on Earth have preserved proximal ejecta (ejecta blanket), which when present help us to better understand the cratering processes when asteroid hits Earth. The 458 Ma old Lockne impact structure consists of a 7.5-km wide nested crater in the crystalline basement surrounded by an approximately 3-km wide brim developed in the upper sedimentary target. The asteroid struck a marine environment with 500 m sea water, 50-m lithified limestone, and 30 m of Cambrian clay covering a peneplainized crystalline basement. The transient crater that developed in rock and water obtained a "soup-plate" shape and reached about 7 km from the impact crater center, the farthest on the down-range side. The brim of the soup-plate was partially stripped of Ordovician limestone and water before the emplacement of inner impact crater ejecta. Most of the ejecta rest upon the Cambrian clay (today shale). The asteroid struck obliquely from the east, which is reflected in the ejecta distribution. The proximal ejecta field is divided into two crescent-shaped areas to the northwest and southwest of the nested crater and covers 26 km(2). Resistivity profiles, mapping, and core drilling show that the thickness of the ejecta masses range between 30 and 50 m with a total volume of about 1 km(3). They were not re-worked by the resurge. They represent roughly 26 vol% of the calculated excavated volume of crystalline rocks. Thus, it can be concluded that the Lockne impact crater has a well-preserved ejecta blanket.
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| 4. |
- Alwmark, C., et al.
(författare)
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Impact origin for the Hummeln structure (Sweden) and its link to the Ordovician disruption of the L chondrite parent body
- 2015
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Ingår i: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 43:4, s. 279-282
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Tidskriftsartikel (refereegranskat)abstract
- Several studies of meteorites show that a large disruption of an asteroid occurred ca. 470 Ma in our solar system's asteroid belt. As a consequence, a large number of meteorite impacts occurred on Earth during the following few million years. The finding and characterization, for the first time, of planar deformation features in quartz grains from rocks collected at the Middle Ordovician Hummeln structure (Sweden) prove the hypervelocity impact origin of the structure. The unambiguous shock features allow us to close an similar to 200-yr-old discussion about its origin, and further the hypothesis of enhanced asteroid bombardment during the Middle Ordovician, adding an impact crater to the increasing number confirmed and properly dated from this period. Despite its relatively small size (similar to 1.2 km in diameter), similar to the young Meteor Crater (Arizona, USA), and its old age, the Hummeln structure is remarkably well preserved, contradicting the general assumption that small craters are not preserved on Earth for more than a few tens of thousands to a couple of million years.
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| 5. |
- Alwmark, Carl, et al.
(författare)
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Shocked quartz grains from the Malingen structure, Sweden-Evidence for a twin crater of the Lockne impact structure
- 2014
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 49:6, s. 1076-1082
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Tidskriftsartikel (refereegranskat)abstract
- The Malingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features (PDFs) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock-produced PDFs indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Malingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Malingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne-Malingen pair).
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| 6. |
- Andren, M., et al.
(författare)
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Coupling between mineral reactions, chemical changes in groundwater, and earthquakes in Iceland
- 2016
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Ingår i: Journal of Geophysical Research-Solid Earth. - : American Geophysical Union (AGU). - 2169-9313. ; 121:4, s. 2315-2337
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Tidskriftsartikel (refereegranskat)abstract
- Chemical analysis of groundwater samples collected from a borehole at Hafralaekur, northern Iceland, from October 2008 to June 2015 revealed (1) a long-term decrease in concentration of Si and Na and (2) an abrupt increase in concentration of Na before each of two consecutive M 5 earthquakes which occurred in 2012 and 2013, both 76km from Hafralaekur. Based on a geochemical (major elements and stable isotopes), petrological, and mineralogical study of drill cuttings taken from an adjacent borehole, we are able to show that (1) the long-term decrease in concentration of Si and Na was caused by constant volume replacement of labradorite by analcime coupled with precipitation of zeolites in vesicles and along fractures and (2) the abrupt increase of Na concentration before the first earthquake records a switchover to nonstoichiometric dissolution of analcime with preferential release of Na into groundwater. We attribute decay of the Na peaks, which followed and coincided with each earthquake to uptake of Na along fractured or porous boundaries between labradorite and analcime crystals. Possible causes of these Na peaks are an increase of reactive surface area caused by fracturing or a shift from chemical equilibrium caused by mixing between groundwater components. Both could have been triggered by preseismic dilation, which was also inferred in a previous study by Skelton et al. (2014). The mechanism behind preseismic dilation so far from the focus of an earthquake remains unknown.
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| 7. |
- Aradottir, N., et al.
(författare)
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Drumlin formation within the Bustarfell drumlin field, northeast Iceland: integrating sedimentological and ground-penetrating radar data
- 2022
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 38:3, s. 386-402
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Tidskriftsartikel (refereegranskat)abstract
- Drumlins are important bedforms of former glaciated landscapes as they demonstrate past ice-flow directions and elucidate processes that operated at the ice/bed interface. Recently mapped drumlins and other streamlined subglacial bedforms in northeast Iceland reveal the flow-sets of cross-cutting palaeo-ice streams that were active within the Iceland Ice Sheet (IIS) during and following the Last Glacial Maximum. Here we study the Bustarfell drumlin field within the Vopnafjorour-Jokuldalsheioi flow-set. The internal architecture of two drumlins was investigated using sedimentological analysis and ground-penetrating radar (GPR, 50 and 100 MHz) to illuminate subglacial processes that contributed to drumlin formation, as well as the history and dynamics of the IIS. On the stoss side of one of the drumlins, two subglacial traction till units were identified, separated by a thick unit of deformed glaciofluvial sand and gravel. The core of glaciofluvial material suggests that the drumlin formed around well-drained patches (sticky spots) in the subglacial bed that retarded the ice flow locally through increased basal drag and encouraged till deposition. Furthermore, our GPR data indicate a combination of erosional and depositional processes. We suggest that the glaciofluvial sediments were deposited as small ice-marginal fans on the Bustarfell plateau, possibly during the Bolling-Allerod interstadial, and that the drumlins were formed around these fans during a subsequent readvance during the Younger Dryas.
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| 8. |
- Banzhaf, Stefan, 1980, et al.
(författare)
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Hydrogeological and geophysical investigations to evaluate groundwater influences on GHG emissions at the national research site Skogaryd
- 2015
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Ingår i: EGU General Assembly 2015. ; 17
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The overall objective of the presented study is to explore the impact of groundwater fluctuations on greenhouse gas (GHG) emissions from peatlands and in particular from drained organic soils. The hypothesis is that drained organic soils react sensitively to changing water content, i.e. that frequent changes of groundwater level enhance the emissions of GHG from these soils and thus contribute significantly to global warming. The area under investigation is based at the Skogaryd Research Catchment (within Swedish Infrastructure for Ecosystem Sciences, SITES) in western Sweden (Meyer, et al., 2013), which was recently assigned the status of a national research site by the Swedish research council (www.vr.se). Skogaryd is a unique place in Sweden for doing research on organic soils as the area was simultaneously afforested in the 1960s and the drained fertile soils have a different land-use history. The ditching for drainage purposes throughout the entire area has had and still has a huge influence on groundwater level, which in turn is assumed to trigger GHG emissions from the organic soils at Skogaryd. To address the influence of groundwater dynamics on GHG emissions in this system, a characterisation of the subsurface using electrical resistivity and Ground Penetrating Radar (GPR) measurements was carried out. These geophysical measurements were combined with drilling along them to allow for ground truthing. An average peat thickness of around 3 m was estimated for the field site. Below the peat follows a fine sand layer, which reaches a maximum thickness of around 1.0 m right at the valley borders and thins out significantly towards the middle of the valley. Below the fine sand layer follows a layer of marine clay, which extends down to the bedrock at depths between 12 and 15 m below ground surface. The results show that the peat layer in Skogaryd forms an isolated hydraulic system without interaction with deeper or regional groundwater systems. The continuously extending clay layer hinders water moving downward or upward crossing the border of peat and clay. The peat layer is a fast reacting hydraulic system that shows immediate reaction to precipitation or drought and is independent from regional groundwater levels. The study of groundwater controls on the GHG from the drained organic soils at Skogaryd can thus focus on the fast reacting peat layer. Future investigations will show if this conclusion can be generalized for similar situations in Sweden. Geophysical measurements have proved to be a valuable method for estimating the peat thickness over a large area. Meyer A, Tarvainen L, Nousratpour A, Björk RG, Ernfors M, Grelle A, Kasimir Klemedtsson Å, Lindroth A, Räntfors M, Rütting T, Wallin G, Weslien P, Klemedtsson L (2013) A fertile peatland forest does not constitute a major greenhouse gas sink. Biogeosciences 10: 7739-7758 DOI 10.5194/bg-10-7739-2013
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| 9. |
- Broman, Curt, et al.
(författare)
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Oxygen isotopes and implications for the cavity-grown quartz crystals in the Lockne impact structure, Sweden
- 2011
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Ingår i: GFF. - : The Geological Society of Sweden. - 1103-5897 .- 2000-0863. ; 133:02-jan, s. 101-107
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Tidskriftsartikel (refereegranskat)abstract
- Well-developed quartz crystals are found in open cavities in the intensely fractured crystalline basement of the marine-target impact structure at Lockne in central Sweden. The 458 Ma impact structure has a well-preserved crater in Precambrian granitic basement rock that is covered by resurge deposits composed of breccias and fine-grained sedimentary units of mixed Ordovician limestone, Cambrian black bituminous shales and the basement granite. Directly after the impact, the resurge deposits formed when the seawater rushed back into the crater. The residual heat from the impact facilitated a short-lived hydrothermal system accompanied by the inflowing seawater, which resulted in mineral growth in fractures and open cavities of the granite basement. The oxygen isotope values of the first-precipitated minerals, the cavity-grown quartz crystals, range from +15.2 to +16.2 parts per thousand (Vienna Standard Mean Ocean Water) and differ from those of the hosting granite basement rock with delta(18)O quartz between +10.1 and +11.7 parts per thousand. The delta(18)O values of the quartz are more consistent with derivation from a fluid of relatively high delta(18)O probably attributable to oxygen isotope exchange during seawater-rock interactions in the resurge deposits. The occurrence of organic matter in association with the cavity-grown quartz strongly indicates a relationship to the black bituminous shale in the matrix of the breccia that rests on the crater floor. Comparing the results with previously obtained oxygen data on fracture-grown calcite from Lockne shows that oxygen isotope composition of the cavity-grown quartz crystals is less variable and probably more accurately reflects the original fluid source.
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| 10. |
- de Zeeuw-van Dalfsen, E., et al.
(författare)
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Geodetic data shed light on ongoing caldera subsidence at Askja, Iceland
- 2013
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 75:5
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Tidskriftsartikel (refereegranskat)abstract
- Subsidence within the main caldera of Askja volcano in the North of Iceland has been in progress since 1983. Here, we present new ground- and satellite-based deformation data, which we interpret together with new and existing micro-gravity data, to help understand which processes may be responsible for the unrest. From 2003 to 2007, we observe a net micro-gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro-gravity increase while the subsidence continues. We infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary. Mass movements at active volcanoes can be caused by several processes, including water table/lake level movements, hydrothermal activity and magma movements. We suggest that, here, magma movement and/or a steam cap in the geothermal system of Askja at depth are responsible for the observed micro-gravity variations. In this respect, we rule out the possibility of a shallow intrusion as an explanation for the observed micro-gravity increase but suggest magma may have flowed into the residing shallow magma chamber at Askja despite continued subsidence. In particular, variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation.
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