A well-preserved Cambrian impact exposed in Central Sweden     

Maurits Lindström  Johan Ekvall  Stefan E. Hagenfeldt  Brigitta Säwe  Erik F. F. Sturkell 《International Journal of Earth Sciences》1991,80(1):201-204

The Lockne impact crater south of Östersund formed in the early Middle Cambrian with a diameter of 7 km. It is identified by its rim wall of crushed Precambrian basement granite, by fragments of impact melt, and by grains of shocked quartz. The exceptional preservation, in particular of the rim wall, is due to a complicated geological history, the first stage of which consisted of burial by marine sediments. This stage lasted until the Middle Ordovician, or over 50 million years. An early Caradoc lowering of the sea-level may have induced debris flows that stripped the rim wall of much of its sedimentary cover. Because normal marine sedimentation recommenced soon after this event, the structure was not seriously damaged, as it was buried again. The Caledonian orogeny emplaced an overthrust nappe as ultimate protection, which was removed by a recent erosion episode from all but the center of the structure.Structures formed by the impact of extraterrestrial bodies are very rare throughout much of Europe, because such structures are neither well preserved nor displayed in young mountain belts or sedimentary basins. However, northern Europe has several ascertained structures of this kind (Svensson &Wickman, 1965;Svensson, 1968;Bruun &Dahlman, 1982;Kala et al., 1984;Flodén et al., 1986;Wickman, 1988). Unfortunately, the hitherto known structures are either poorly preserved or hidden by younger deposits. We are reporting the discovery of a well-preserved exposed and accessible impact structure that has escaped the notice of geologists although important features of it have been described and puzzled over by generations of researchers (Wiman, 1900;Hadding, 1927;Thorslund, 1940;Lindström et al., 1983).The structure is located in the Lockne area to the south of Östersund in central Sweden. It has a diameter of 7 km and its center is near Tramsta on the northwest shore of Lake Locknesjön (Fig. 1). Its middle is covered by folded Lower to Middle Ordovician Orthoceratite Limestone with the sheared Lower Ordovician Töyen Shale at its base. This local expanse of deformed rock is the remainder after erosion of an extensive nappe of overthrust rocks emplaced during the Caledonian orogeny. The good preservation of the impact structure is due to the nappe cover, which had to be removed before erosion could attack the underlying structures.The rim of the impact crater is outlined by a wall of strongly shattered fragments of Proterozoic crystalline rocks (Fig. 2), which formed the local bedrock (Strömberg et al., 1984) at the time of impact. The rim wall is best preserved along the western part of the structure. It was referred to as »arkose-like breccia« byThorslund (1940), who interpreted it as the result of continental weathering, but the components rather show evidence of intense crushing than of weathering (Simon, 1987a). The »arkose-like breccia« does not contain components derived from the lower Palaeozoic deposits of the area.  相似文献   

Potentiometric and Aquifer Property Changes Above Subsiding Longwall Mine Panels, Illinois Basin Coalfield     

Colin J. Booth  Erik D. Spande 《Ground water》1992,30(3):362-368

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Detecting climate adaptation with mobile network data in Bangladesh: anomalies in communication,mobility and consumption patterns during cyclone Mahasen     

Xin Lu  David J. Wrathall  Pål Roe Sundsøy  Md. Nadiruzzaman  Erik Wetter  Asif Iqbal  Taimur Qureshi  Andrew J. Tatem  Geoffrey S. Canright  Kenth Engø-Monsen  Linus Bengtsson 《Climatic change》2016,138(3-4):505-519

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A hindered settling model for the prediction of settling and consolidation of cohesive sediment     

Erik A. Toorman  Jean E. Berlamont 《Geo-Marine Letters》1991,11(3-4):179-183

The prediction of the settling and consolidation behavior of mixtures of cohesive and non-cohesive sediment is possible using a numerical model that solves the solids mass balance for each fraction. Consistency with the hindered settling theory as well as classical soil mechanics is preserved. Semi-empirical relationships for settling rate and effective stress as a function of density are required as constitutive equations. A simple procedure, using settling column experiment data, for calibration is proposed. The possibility of distinguishing different kinds of particles allows prediction of layered deposits.  相似文献   

Composition and temporal distribution of cirripede larvae in Southampton Water, England, with particular reference to the secondary production of Elminius modestus     

Muxagata  Erik; Williams  John A; Sheader  Martin 《ICES Journal of Marine Science》2004,61(4):585-595

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Food and feeding habits of juvenile flounder Platichthys flesus (L.), abd turbot Scophthalmus maximus L. in the åland archipelago, northern Baltic Sea     

Katri Aarnio  Erik Bonsdorff  Nina Rosenback 《Journal of Sea Research》1996,36(3-4)

The food choice of juvenile flounder (Platichthys flesus) and turbot (Scophthalmus maximus) was studied in the northern Baltic Sea during the years 1988, 1989, 1994 and 1995. The diet included organisms from 30 species/taxa in flounder (n = 306) and 10 species/taxa in turbot (n = 41). Flounder 45 mm mainly consumed meiofauna (dominating taxon: Harpacticoida, Copepoda) and larger fish (46–101 mm) consumed macrofauna (dominating taxa: Oligochaeta, Amphipoda and Chironomidae). In terms of biomass, macrofauna dominated for all sizes of flounders, and meiofauna was important only for the smallest fish. A strong seasonal variation could be detected in the diet. In spring, macrofauna dominated for all size classes of fish (only fish > 30 mm were caught in spring), while in summer and autumn meiofauna dominated the diets for fish 45 mm in size. Juvenile turbot (22–88 mm) consumed macrofauna and small fish. Turbot 30 mm consumed mainly amphipods, while > 30 mm turbot consumed mysid shrimps, amphipods and fish.The ontogenetic shift from meio- to macrofauna-sized prey in flounders occurs at a larger fish size in the northern Baltic Sea than reported in other areas, possibly depending on the increased relative importance of meiofauna in the northern Baltic. The seasonal variation in the diet could be due to seasonally changing abundances in the zoobenthos, or for the small fish (1-group, spring), to switching from meio- to macrofauna in order to optimize their energy gain. The 0-group flounders consumed meiofauna for a long period, possibly due to a learning-process or simply due to easy availability of meiofauna. Turbot has a much larger mouth gap than flounders, thus allowing them to consume macrofauna from the beginning of their benthic life.  相似文献   

Effects of Dredging Activities on Growth of Laminaria saccharina     

Jens Erik  Lyngby Sverre Mohr  Mortensen 《Marine Ecology》1996,17(1-3):345-354

Abstract. The possible effects of fine-grained deposits on the photosynthesis, growth, and nutrient uptake of Laminaria have been studied in field and laboratory experiments. Some of the plants were treated with fine-grained material for two hours and then transplanted in rows together with untreated plants. At two-week intervals the treatment was repeated and samples collected. The macroalgae were analyzed for growth as well as chlorophyll a , nitrogen, and phosphorus content. In the laboratory experiments, plants of a uniform size were incubated in Plexiglas containers after leaf-marking and treatment in a sediment suspension. At weekly intervals, photosynthesis, growth, chlorophyll a , and nutrients were analyzed.
The field study showed that Laminaria growth was significantly affected by the fine-grained material. The growth in the control experiment was 20% higher than in the treated plants. The chlorophyll a concentrations in the treated plants were significantly lower than in the control experiment, and the results indicated a reduced nutrient uptake in the treated plants. In the laboratory experiments the growth of the treated plants was only 30% of the controls, but no differences in photosynthetic capacities were detected. Nitrogen uptake was reduced in the treated plants, but no effects were observed on phosphorus uptake. Thick layers of fine-grained material caused direct harmful effects and rotting.
The results are discussed in relation to dredging activities; it is concluded that in heavy sedimentation areas, Laminaria growth may be significantly reduced beyond the indirect effect of shading from suspended material in the water.  相似文献   

Constancy of the refraction constant     

Erik Høg 《Astrophysics and Space Science》1991,177(1-2):203-208

The flexure term and a correction to the atmospheric refraction constant calculated from meteorological data may be determined from the zenith distances of FK5 stars observed each night on a meridian circle. All such determinations on meridian circles and vertical circles have been corrupted-and in fact dominated-byinternal refraction which is often larger than 1" and quite variable. After removing internal refraction by a simple equipment at the CAMC on La Palma, it became apparent that the true flexure and the true correction to the calculated refraction constant vary by less than 0."09 (rms) from night to night. This is a reduction of the estimated uncertainty of these quantities by a factor of about four. This improvement is expected at any meridian circle which is equipped with sufficiently precise (photoelectric) micrometers if internal refraction were removed.Alternative forms of the observation equation for declinations are discussed. Observations over a period of two months may be solved in oneglobal least-squares solution keeping the refraction and the flexure terms constant. Furthermore, one per star is obtained, instead of a different each night for each star.  相似文献   

Geophysical signature of Målingen,the minor crater of the Lockne–Målingen doublet impact structure          下载免费PDF全文

Irene Melero-Asensio  Jens Ormö  Erik Sturkell  Gabrielle Stockmann  Joakim Mansfeld 《Meteoritics & planetary science》2018,53(7):1456-1475

Målingen is the 0.7 km wide minor crater associated to the 10 times larger Lockne crater in the unique Lockne–Målingen doublet. The craters formed at 458 Ma by the impact of a binary asteroid related to the well-known 470 Ma Main Belt breakup event responsible for a large number of Ordovician craters and fossil meteorites. The binary asteroid struck a target sequence including ~500 m of sea water, ~80 m of limestone, ~30 m of dark mud, and a peneplainized Precambrian crystalline basement. Although the Lockne crater has been extensively studied by core drillings and geophysics, little is known about the subsurface morphology of Målingen. We performed magnetic susceptibility and remanence, as well as density, measurements combined with gravity, and magnetic field surveys over the crater and its close vicinity as a base for forward magnetic and gravity modeling. The interior of the crater shows a general magnetic low of 90–100 nT broken by a clustered set of high-amplitude, short wavelength anomalies caused by bodies of mafic rock in the target below the crater and as allogenic blocks in the crater infill. The gravity shows a general −1.4 mgal anomaly over the crater caused by low-density breccia infill and fractured crystalline rocks below the crater floor. The modeling also revealed a slightly asymmetrical shape of the crater that together with the irregular ejecta distribution supports an oblique impact from the east, which is consistent with the direction of impact suggested for the Lockne crater.  相似文献   

Wave run-up on slender circular cylindrical foundations for offshore wind turbines in nonlinear random waves     

Dag Myrhaug  Lars Erik Holmedal 《Coastal Engineering》2010

A practical method for estimating the wave run-up height on a slender circular cylindrical foundation for wind turbines in nonlinear random waves is provided. The approach is based on the velocity stagnation head theory and Stokes second order wave theory by assuming the basic harmonic wave motion to be a stationary Gaussian narrow-band random process. Comparisons are made with measurements by De Vos et al. (2007), and some of the highest wave run-up events that were predicted agree with those measured.  相似文献