A large wet snow avalanche cycle in West Greenland quantified using remote sensing and in situ observations     

Abermann  Jakob  Eckerstorfer  Markus  Malnes  Eirik  Hansen  Birger Ulf 《Natural Hazards》2019,97(2):517-534

Natural Hazards - On 11 April 2016 we observed high slushflow and wet snow avalanche activity at the environmental monitoring station Kobbefjord in W-Greenland. Snow avalanches released as a result...  相似文献   

Morphology, distribution and formation of relict marginal moraines in the Swedish mountains   总被引：2，自引：0，他引：2  

Jakob Heyman  Clas Hättestrand 《Geografiska Annaler: Series A, Physical Geography》2006,88(4):253-265

Relict marginal moraines are commonly used landforms in palaeoglaciological reconstructions. In the Swedish mountains, a large number of relict marginal moraines of variable morphology and origin occur. In this study, we have mapped 234 relict marginal moraines distributed all along the Swedish mountains and classified them into four morphological classes: cirque‐and‐valley moraines, valley‐side moraines, complex moraines and cross‐valley moraines. Of these, 46 moraines have been reclassified or are here mapped for the first time. A vast majority of the relict moraines are shown to have formed during deglaciation of an ice‐sheet, rather than by local mountain glaciers as suggested in earlier studies. The relict marginal moraines generally indicate that deglaciation throughout the mountains was characterised by a retreating ice‐sheet, successively damming glacial lakes, and downwasting around mountains. The general lack of moraines indicating valley and cirque glaciers during deglaciation suggests that climatic conditions were unfavourable for local glaciation during the last phase of the Weichselian. This interpretation contrasts with some earlier studies that have reconstructed the formation of local glaciers in the higher parts of the Swedish mountains during deglaciation.  相似文献   

Anorthositic lunar regolith breccia Dhofar 1769—Clear indications for repeated mixing of impact melt lithologies     

Addi Bischoff  Mike Komnik  Jakob Storz  Jasper Berndt 《Meteoritics & planetary science》2023,58(9):1260-1273

The lunar regolith breccia Dhofar 1769, which was found in 2012 as a single 125 g piece in the Zufar desert area of Oman, contains a relatively large, dark-colored impact melt breccia embedded in a fine-grained clastic matrix. The internal texture of the fragment indicates the repeated melt breccia formation on the lunar surface, their repeated brecciation, and mixing in second, third, and fourth generations of brecciated rock types. The chemical and mineralogical data reveal the incorporation of a feldspar-rich subophitic crystalline melt within a feldspar-rich microporphyritic crystalline melt breccia. This lithic paragenesis itself is embedded within a mafic, crystalline melt breccia. The entire breccia with the three different impact melts has been finally incorporated into the whole rock breccia. The three impact melts are mixtures of different source rocks and impact projectiles, based on the obtained minor and trace element compositions (in particular of Ni and the rare earth elements [REE]) of the impact melt lithologies. For all processes of impact melt formation, additional steps of their brecciation and re-lithification require a minimum number of seven impact processes.  相似文献   

Late Cretaceous-Paleocene extension on the Vøring Volcanic Margin     

Ren  Shicun  Skogseid  Jakob  Eldholm  Olav 《Marine Geophysical Researches》1998,20(4):343-369

High-quality seismic data document a Maastrichtian-Paleocene rift episode on the Vøring margin lasting for 20 m.y. prior to continental breakup. The rift structures are well imaged in the Fenris Graben and Gjallar Ridge region in the western Vøring Basin, and are characterized by low-angle detachment faults with variable fault geometries from south to north. The structural restoration has facilitated the division of pre- and syn-rift sediments across the extensional terrain, which is subsequently used to evaluate mode and mechanism for the lithospheric deformation. Extension estimates based on the structural restoration, subsidence analysis and crustal thickness evaluations yield stretching factors ranging between 1.5 to 2.3 across the main fault zone just landward of the early Tertiary flood basalts. The structural restoration also shows that a middle crustal dome structure, observed beneath the low-angle faults, can be explained by extensional unroofing. Thus, the dome structure may represent a possible metamorphic core complex. Calculations of the effects on vertical motion, assuming uniform and two-layer differential stretching models combined with the arrival of the Iceland mantle plume during rifting, indicate that the uniform extension model may account for both observed early rift subsidence and subsequent late rift uplift and erosion. Although the differential model can not be excluded, it implies early rift uplift which is not compatible with our seismic interpretation. The direct and indirect effects of the Iceland mantle plume may have caused as much as 1.2 km of late rift uplift. Comparison of the volcanic Vøring margin and the non-volcanic West Iberian margin shows similarities in terms of structural style as well as in mode and distribution of extension.  相似文献   

On the geoid–quasigeoid separation in mountain areas     

Jakob Flury  Reiner Rummel 《Journal of Geodesy》2009,83(9):829-847

The separation between the reference surfaces for orthometric heights and normal heights—the geoid and the quasigeoid—is typically in the order of a few decimeters but can reach nearly 3 m in extreme cases. The knowledge of the geoid–quasigeoid separation with centimeter accuracy or better, is essential for the realization of national and international height reference frames, and for precision height determination in geodetic engineering. The largest contribution to the geoid–quasigeoid separation is due to the distribution of topographic masses. We develop a compact formulation for the rigorous treatment of topographic masses and apply it to determine the geoid–quasigeoid separation for two test areas in the Alps with very rough topography, using a very fine grid resolution of 100 m. The magnitude of the geoid–quasigeoid separation and its accuracy, its slopes, roughness, and correlation with height are analyzed. Results show that rigorous treatment of topographic masses leads to a rather small geoid–quasigeoid separation—only 30 cm at the highest summit—while results based on approximations are often larger by several decimeters. The accuracy of the topographic contribution to the geoid–quasigeoid separation is estimated to be 2–3 cm for areas with extreme topography. Analysis of roughness of the geoid–quasigeoid separation shows that a resolution of the modeling grid of 200 m or less is required to achieve these accuracies. Gravity and the vertical gravity gradient inside of topographic masses and the mean gravity along the plumbline are modeled which are important intermediate quantities for the determination of the geoid–quasigeoid separation. We conclude that a consistent determination of the geoid and quasigeoid height reference surfaces within an accuracy of few centimeters is feasible even for areas with extreme topography, and that the concepts of orthometric height and normal height can be consistently realized and used within this level of accuracy.  相似文献   

A well‐preserved Eemian incised‐valley fill in the southern North Sea Basin,Belgian Continental Shelf ‐ Coastal Plain: Implications for northwest European landscape evolution          下载免费PDF全文

M. De Clercq  T. Missiaen  J. Wallinga  O. Zurita Hurtado  A. Versendaal  M. Mathys  M. De Batist 《地球表面变化过程与地形》2018,43(9):1913-1942

This paper demonstrates that the Belgian Continental Shelf and coastal plain occupy a key position between the depositional North Sea Basin and the erosional area of the Dover Strait as it is an area where erosional landforms and fragmented sedimentary sequences provide new evidence on northwest European landscape evolution. The study area hosts 20–30 m thick penultimate to last glacial sand‐dominated sequences that are preserved within the buried palaeo‐Scheldt Valley. Here, we build on the results of previous seismo‐ and lithostratigraphical studies, and present new evidence from biostratigraphical analysis, OSL dating and depth‐converted structure maps, together revealing a complex history of deposition and landscape evolution controlled by climate change, sea‐level fluctuations and glacio‐isostasy. This study presents strong new supportive evidence on the development of the incised palaeo‐Scheldt Valley landform that became established towards the end of the penultimate glacial period (MIS 6; Saalian) as a result of glacio‐isostatic forebulge updoming, proglacial lake drainage and subsequent collapse of a forebulge between East Anglia and Belgium following ice‐sheet growth, disintegration and retreat in areas to the north. The majority of the incised‐valley fill is of estuarine to shallow marine depositional context deposited during the transgression and high‐stand of the last interglacial (MIS 5e: Eemian). A thin upper part of the valley fill consists of last glacial (MIS 5d‐2: Weichselian) fluvial sediments that show a gradual decrease and retreat of fluvial activity to inland, upstream reaches of the valley system until finally the valley ceases to exist as the combined result of climate‐driven aeolian activity and possibly also glacio‐isostatic adjustment. Thus, strong contrasts exist between the palaeo‐Scheldt Valley and estuary systems of the penultimate glacial maximum to Last Interglacial (Saalian, Eemian), the beginning of the Last Glacial (Weichselian Early Glacial and Early‐Middle Pleniglacial), and the Last Glacial Maximum to Holocene. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

Analysis of local ionospheric time varying characteristics with singular value decomposition     

Jakob Jakobsen  Per Knudsen  Anna B. O. Jensen 《Journal of Geodesy》2010,84(7):449-456

In this paper, a time series from 1999 to 2007 of absolute total electron content (TEC) values has been computed and analyzed using singular value decomposition (SVD). The data set has been computed using a Kalman Filter and is based on dual frequency GPS data from three reference stations in Denmark located in the midlatitude region. The station separation between the three stations is 132–208 km (the time series of the TEC can be freely downloaded at ). For each year, a SVD has been performed on the TEC time series in order to identify the three time varying (daily, yearly, and 11 yearly) characteristics of the ionosphere. The applied SVD analysis provides a new method for separating the daily from the yearly components. The first singular value is very dominant (approximately six times larger than the second singular value), and this singular value corresponds clearly to the variation of the daily cycle over the year. The second singular value corresponds to variations of the width of the daily peak over the year, and the third singular value shows a clear yearly variation of the daily signal with peaks around the equinoxes. The singular values for each year show a very strong correlation with the sunspot number for all the singular values. The correlation coefficients for the first 5 sets of singular values are all above 0.96. Based on the SVD analysis yearly models of the TEC in the ionosphere can be recomposed and illustrate the three time varying characteristics of the ionosphere very clearly. By prediction of the yearly mean sunspot number, future yearly models can also be predicted. These can serve as a priori information for a real time space weather service providing information of the current status of the ionosphere. They will improve the Kalman filter processing making it more robust, but can also be used as starting values in the initialization phase in case of gaps in the data stream. Furthermore, the models can be used to detect variations from the normal local ionospheric activity.  相似文献   

Robust optical tracking of individual ejecta particles in hypervelocity impact experiments          下载免费PDF全文

Max Gulde  Lukas Kortmann  Matthias Ebert  Erkai Watson  Jakob Wilk  Frank Schäfer 《Meteoritics & planetary science》2018,53(8):1696-1704

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Numerical studies of gas hydrate systems: sensitivity to porosity-permeability models     

Jakob Hauschildt  Vikram Unnithan  Joachim Vogt 《Geo-Marine Letters》2010,30(3-4):305-312

The formation of sub-seafloor gas hydrates in marine environments can be described as a coupled transport and thermodynamic process inside a host sediment matrix undergoing structural evolution. The transport processes are driven by the sedimentary load and induced overpressure gradients, controlled by sediment permeability. In order to accurately model the resulting fluid flow profile, the decrease of sediment permeability during hydrate precipitation has to be taken into account, which affects both the transport of solutes and sediment compaction. In this paper, we investigate how total hydrate abundance is affected by regions of low permeability which deflect the flow field in their vicinity. For this purpose, a two-dimensional numerical hydrate system model was set up which permits to quantify this effect in scenarios where changes in water depth cause lateral variations of the thickness of the hydrate stability field, as well as of hydrate saturation and sediment permeability. The microscopic structure of gas hydrate crystals in the host sediment matrix defines the evolution of the permeability reduction during hydrate formation. Grain-coating precipitates have a stronger tendency to clog flow paths through pore throats than do pore-filling precipitates. Our results clearly show that these pore-scale processes affect the large-scale flow field and hydrate abundance. The sensitivity depends on the model geometry and, for a 5° slope of the seafloor, 4.1% relative difference is predicted for the hydrate saturation according to different porosity-permeability relationships.  相似文献   

Approaches to decomposing acoustic data     

Jakob B.U. Haldorsen 《Geophysical Prospecting》2023,71(1):163-167

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