Late Pleistocene stratigraphy and sedimentary environments of the Severnaya Dvina‐Vychegda region in northwestern Russia     

Astrid Lyså  Eiliv Larsen  Jan‐Pieter Buylaert  Ola Fredin  Maria A. Jensen  Denis Kuznetsov  Andrew S. Murray  Dmitry A. Subetto  Aurelien van Welden 《Boreas: An International Journal of Quaternary Research》2014,43(4):759-779

The Late Pleistocene stratigraphy from the Severnaya Dvina‐Vychegda region of northwestern Russia is revised based on investigations of new localities, revisiting earlier localities, introduction of about 110 new OSL dates and burial depth corrections of earlier published OSL dates, in addition to six new radiocarbon dates. Most of the OSL samples studied here are from fluvial and subaquaeous sediments, which we found to be well bleached. Six chronostratigraphical units and their sedimentary environment are described, with the oldest unit consisting of pre‐Eemian glacial beds. For the first time, Early Weichselian sediments are documented from the region and a fluvial environment with some vegetation and permafrost conditions is suggested to have persisted from the end of the Eemian until at least about 92 ka ago. The period in which a Middle Weichselian White Sea Lake could have existed is constrained to 67?62 ka, but as the lake level never reached the thresholds of the drainage basin, the lake probably existed only for a short interval within this time‐span. Blocking and reversal of fluvial drainage started again around 21?20 ka ago when the Fennoscandian Ice Sheet advanced into the area, reaching its maximum 17?15 ka ago. At that time, an ice‐dammed lake reached its maximum water level, which was around 135 m above present sea level. Drainage of the lake started shortly after 15 ka ago, and the lake was emptied within 700 years. Severe periglacial conditions, with permafrost and aeolian activity, prevailed in the area until about 10.7 ka.  相似文献   

Bifurcation of the Pacific North Equatorial Current in a wind-driven model: response to climatological winds     

Tommy G. Jensen 《Ocean Dynamics》2011,61(9):1329-1344

The sensitivity of the bifurcation of the North Equatorial Current in the Pacific to different wind products is investigated. Variations of the bifurcation latitude with season is simulated in a purely wind-driven model and is found to be in agreement with recent observations. The seasonal cycle is nearly independent of the wind climatology, but the annual average latitude depends on the wind stress curl. It is also shown that in the upper ocean, the poleward shift in bifurcation latitude with depth is realistic in our simple model. This implies that given a stratification close to the observed, it is primarily the wind forcing that determines the location of the bifurcation and its seasonal variation.  相似文献   

Why chromatic imaging matters     

Joel Sanchez-Bermudez  Florentin Millour  Fabien Baron  Roy van Boekel  Laurent Bourgès  Gilles Duvert  Paulo J. V. Garcia  Nuno Gomes  Karl-Heinz Hofmann  Thomas Henning  Jacob W. Isbell  Bruno Lopez  Alexis Matter  J-Uwe Pott  Dieter Schertl  Eric Thiébaut  Gerd Weigelt  John Young 《Experimental Astronomy》2018,46(3):457-473

During the last two decades, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the near- and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u ? v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Therefore, interferometric imaging is already a key feature of the new generation of VLTI instruments, as well as for other interferometric facilities like CHARA and JWST. It is thus imperative to account for the current image reconstruction capabilities and their expected evolutions in the coming years. Here, we present a general overview of the current situation of optical interferometric image reconstruction with a focus on new wavelength-dependent information, highlighting its main advantages and limitations. As an Appendix we include several cookbooks describing the usage and installation of several state-of-the art image reconstruction packages. To illustrate the current capabilities of the software available to the community, we recovered chromatic images, from simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we aim at showing the importance of selecting good regularization functions and their impact on the reconstruction.  相似文献   

Hypoplastic interface models for fine‐grained soils          下载免费PDF全文

Henning Stutz  David  Mašín 《国际地质力学数值与分析法杂志》2017,41(2):284-303

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Detrital zircon geochronology of Palaeozoic Novaya Zemlya – a key to understanding the basement of the Barents Shelf     

Henning Lorenz  David G. Gee  Evgeny Korago  Galina Kovaleva  William C. McClelland  Jane A. Gilotti  Dirk Frei 《地学学报》2013,25(6):496-503

The Novaya Zemlya fold‐and‐thrust‐belt is the northern continuation of the late Palaeozoic Uralide Orogen. Little is known about its deeper structure and the basement history of the adjacent Barents and Kara shelves. Based on geological evidence and detrital zircon analysis of 28 samples from the northeastern and stratigraphically deepest part of the archipelago, we demonstrate that Cambro‐Ordovician turbidite‐dominated deposition was almost exclusively sourced from rocks consolidated during the Timanian orogeny (Timanian basement). A profound change in provenance occurred near the end of the Ordovician. Over 90% of the zircons from Silurian and about 80% from Devonian strata have ages characteristic of the Sveconorwegian Orogen, implying uplift of these rocks in the vicinity of Novaya Zemlya. The presence of Sveconorwegian and Grenvillian rocks in the high Arctic suggests revision of recent reconstructions of the Rodinia supercontinent, its break‐up and subsequent Caledonian orogeny.  相似文献   

A nonlinear PDE formulation for offshore vessel pipeline installation     

Gullik A. Jensen  Niklas Säfström  Thor I. Fossen 《Ocean Engineering》2010,37(4):365-2819

In this paper a nonlinear dynamic PDE formulation for a pipe string suspended from a pipelay vessel to the seabed in a pipelay operation is developed. This model extends a three-dimensional beam model capable of undergoing finite extension, shearing, twist and bending, to apply for marine applications by adding the effects of restoring forces, hydrodynamic drag and seabed interaction. The model is validated against the natural catenary equation and the FEM code RIFLEX. The model is extended to include the pipelay vessel dynamics by applying a potential theory formulation of a surface vessel, suited for dynamic positioning and low speed maneuvering, as a boundary condition for the PDE. This system is found to be input-output passive and stable. Pipeline installation applications where the presented model is suited are e.g., analysis and simulation of the installation operation, operability analysis, hardware-in-the-loop (HIL) testing for vessel control systems, and automation of the pipelay operation.  相似文献   

“Nonlinear energy stability in a compressible atmosphere”     

M. S. Chance  J. M. Greene  T. H. Jensen 《地球物理与天体物理流体动力学》2013,107(1-4):203-204

Abstract

A magnetic field line topology with nulls, generated by superimposing a uniform magnetic field onto the field from a distributed ring current, is analyzed. This simple model, which is reminiscent of the structures found in laboratory field reversed configurations and detached plasmoids, is amenable to substantial analytical progress and also facilitates the visualization of the three dimensional field geometry. Four nulls are seen to exist and representative field lines and tubes of flux found by numerical integration are presented. An infinite number of topologically distinct flux bundles is found. These are distinguished by the number of times they encircle a circular magnetic field line. A convenient mapping is described which proves very useful in distinguishing between and following the paths of the different tubes of flux as they traverse through the null system. The separatrices that divide these flux bundles are described. The complexities already present in this simple but nontrivial configuration serve to emphasize the difficulties in analyzing more complicated geometries, but the intuition gained from this study proves beneficial in those cases. One such example is the comparison of the generic features of our model with those found in a topologically different model of plasmoid formations in the earth's magnetotail.  相似文献   

Thermophysical properties of Almahata Sitta meteorites (asteroid 2008 TC3) for high‐fidelity entry modeling          下载免费PDF全文

Stefan Loehle  Peter Jenniskens  Hannah Böhrk  Thomas Bauer  Henning Elsäβer  Derek W. Sears  Michael E. Zolensky  Muawia H. Shaddad 《Meteoritics & planetary science》2017,52(2):197-205

Asteroid 2008 TC₃ was characterized in a unique manner prior to impacting Earth's atmosphere, making its October 7, 2008, impact a suitable field test for or validating the application of high‐fidelity re‐entry modeling to asteroid entry. The accurate modeling of the behavior of 2008 TC₃ during its entry in Earth's atmosphere requires detailed information about the thermophysical properties of the asteroid's meteoritic materials at temperatures ranging from room temperature up to the point of ablation (T ~ 1400 K). Here, we present measurements of the thermophysical properties up to these temperatures (in a 1 atm. pressure of argon) for two samples of the Almahata Sitta meteorites from asteroid 2008 TC₃: a thick flat‐faced ureilite suitably shaped for emissivity measurements and a thin flat‐faced EL6 enstatite chondrite suitable for diffusivity measurements. Heat capacity was determined from the elemental composition and density from a 3‐D laser scan of the sample. We find that the thermal conductivity of the enstatite chondrite material decreases more gradually as a function of temperature than expected, while the emissivity of the ureilitic material decreases at a rate of 9.5 × 10^?5 K^?1 above 770 K. The entry scenario is the result of the actual flight path being the boundary to the load the meteorite will be affected with when entering. An accurate heat load prediction depends on the thermophysical properties. Finally, based on these data, the breakup can be calculated accurately leading to a risk assessment for ground damage.  相似文献   

Fossilisation of a silica diagenesis reaction front on the mid-Norwegian margin     

Raluca Cristina Neagu  Joe Cartwright  Richard Davies  Lars Jensen 《Marine and Petroleum Geology》2010

The diagenetic transformation of biogenic silica from opal-A to opal-CT was recognised on seismic reflection data over an area of 78 × 10³ km² on the mid-Norwegian margin. The opal-A/CT diagenetic boundary appears as a positive, high amplitude reflection that generally cross-cuts the hosting stratigraphy. We demonstrate that it is not a sea bottom simulating reflection (BSR) and also that is not in thermal equilibrium with the present day isotherms. We present arguments that three styles of deformation associated with the opal-A/CT reflection – polygonal faulting, regional anticlines and synclines and differential compaction folding – indicate that the silica diagenesis reaction front is fossilised at a regional scale. Isochore maps demonstrate the degree of conformity between the opal-A/CT reflection and three seismic horizons of Late Miocene to Early Pliocene age that potentially represent the paleo-seabed when ‘fossilisation’ of the reaction front took place. The seismic interpretational criteria for recognition of a fossilised diagenetic front are evaluated and the results of our study are integrated with previous studies from other basins of the NE Atlantic in order to determine if the arrest of silica diagenesis was diachronous along this continental margin.  相似文献   

Evidence for delayed poleward expansion of North Atlantic surface waters during the last interglacial (MIS 5e)     

Nicolas Van Nieuwenhove  Henning A. Bauch  Frédérique Eynaud  Evguenia Kandiano  Elsa Cortijo  Jean-Louis Turon 《Quaternary Science Reviews》2011,30(7-8):934-946

The Last Interglacial (Marine Isotopic Stage or MIS 5e) surface ocean heat flux from the Rockall Basin (NE Atlantic) towards the Arctic Ocean was reconstructed by analysing dinoflagellate cyst (dinocyst) assemblages in four sediment cores. Together with records of stable isotopes and ice-rafted detritus, the assemblage data reflect the northward retreat of ice(berg)-laden waters and the gradual development towards interglacial conditions at the transition from the Saalian deglaciation (Termination II) into MIS 5e. At the Rockall Basin, this onset of the Last Interglacial is soon followed by the appearance of the thermophilic dinocyst species Spiniferites mirabilis, with relative abundances higher than those observed at present in the area. North of the Iceland-Scotland Ridge, however, S. mirabilis only appears in significant numbers during late MIS 5e, between ～118 and 116.5 ka. Hence, fully marine Last Interglacial conditions with most intense Atlantic surface water influence occurred during late MIS 5e in the Nordic seas, and consequently also farther north in the Arctic Ocean, and at times when northern hemisphere summer insolation was already significantly decreased. The stratigraphic position of this Late Interglacial optimum is supported by planktic foraminifers and contrasts with the timing of the early Holocene climatic optimum in this area. We interpret the delayed northward expansion of Atlantic waters towards the polar latitudes as a result of the Saalian ice sheet deglaciation and its specific impact on the subsequent water mass evolution in this region.  相似文献