Lead Bioavailability to Freshwater Microalgae in the Presence of Dissolved Organic Matter: Contrasting Effect of Model Humic Substances and Marsh Water Fractions Obtained by Ultrafiltration     

Isabelle A. M. Worms  Vera I. Slaveykova  Kevin J. Wilkinson 《Aquatic Geochemistry》2015,21(2-4):217-230

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Tephra from andesitic Shiveluch volcano,Kamchatka, NW Pacific: chronology of explosive eruptions and geochemical fingerprinting of volcanic glass     

Vera Ponomareva  Maxim Portnyagin  Maria Pevzner  Maarten Blaauw  Philip Kyle  Alexander Derkachev 《International Journal of Earth Sciences》2015,104(5):1459-1482

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Observed spatiotemporal characteristics of drought on various time scales over the Czech Republic   总被引：4，自引：2，他引：2  

Vera Potop  Constanţa Boroneanţ  Martin Možný  Petr Štěpánek  Petr Skalák 《Theoretical and Applied Climatology》2014,115(3-4):563-581

This paper analyses the observed spatiotemporal characteristics of drought in the Czech Republic during the growing season (April to September) as quantified using the Standardised Precipitation Evapotranspiration Index (SPEI) on various time scales. The SPEI was calculated for various lags (1, 3, 6, 12, and 24 months) from monthly records of mean temperature and precipitation totals using a dense network of 184 climatological stations for the period 1961–2010. The characteristics of drought were analysed in terms of the temporal evolution of the SPEI, the frequency distribution and duration of drought at the country level, and for three regions delimited by station altitude. The driest and the wettest years during the growing season were identified. The frequency distribution of the SPEI values for seven drought category classes (in per cent) indicates that normal moisture conditions represent approximately 65 % of the total SPEI values for all time scales in all three regions, whereas moderate drought and moderate wet conditions are almost equally distributed around 10.5 %. Differences in extremely dry conditions (5 %) compared with extremely wet conditions (1.5 %) were observed with increasing SPEI time scales. The results of the non-parametric Mann–Kendall trend test applied to the SPEI series indicate prevailing negative trends (drought) at the majority of the stations. The percentage of stations displaying a significant negative trend for the 90, 95, 99, and 99.9 % confidence levels is approximately 40 %. An Empirical Orthogonal Functions (EOF) analysis was used to identify the principal patterns of variability of the SPEI during the growing season that accounted for the highest amount of statistical variance. The variance explained by the leading EOF range 66 to 56 %, whereas for EOF2 and EOF3, the value is between 7 and 11 % and between 4 and 7 %, respectively, for the SPEI is calculated for 1- to 24-month lags.  相似文献   

Comparative Study Of The Dust Polarimetric Properties In Split And Normal Comets     

Kiselev  Nikolai  Jockers  Klaus  Rosenbush  Vera 《Earth, Moon, and Planets》2002,90(1-4):167-176

Our polarimetric database contains six comets, C/1975 V1 (West), 16P/Brooks 2,C/1988 A1 (Liller), D/1996 Q1 (Tabur), C/1999 S4 (LINEAR), and C/2001 A2(LINEAR), which can be related to the group of split comets. Comets West, S4(LINEAR) and A2 (LINEAR) were observed during splitting. We compare thepolarimetric measurements of the dust particles in these comets, sometimes togetherwith available photometric and colorimetric data, with those in normal comets. Weconclude that there is no significant evidence for differences of polarization betweentidally split comets (e.g., Brooks 2), dissipating comets (e.g., Tabur), non-tidally splitcomets (e.g., West) and normal comets. The total disintegration of Comet S4 (LINEAR), however, did produce significant changes in the observed properties of dust.  相似文献   

A heat flow and physical properties package for the surface of Mercury     

Tilman Spohn  Andrew J. Ball  Karsten Seiferlin  Vera Conzelmann  Axel Hagermann  Norbert I. mle  Günter Kargl 《Planetary and Space Science》2001,49(14-15)

European Space Agencies fifth cornerstone mission BepiColombo includes a ‘Surface Element’ to land a scientific payload on the surface of Mercury. The current strawman payload includes a heat flow and physical properties package (HP³), focussing on key thermal and mechanical properties of the near-surface material (down to a depth of 2–5 m) and the measurement of heat flow from Mercury's interior, an important constraining parameter for models of the planet's interior and evolution. We present here an overview of the HP³ experiment package and its possible accommodation in a self-inserting ‘mole’ device. A mole is considered to be the most appropriate deployment method for HP³, at least in the currently-assumed case of an airbag-assisted soft landing architecture for the Mercury Surface Element.  相似文献   

Hamiltonian Dynamics of a Gyrostat in the N-Body Problem: Relative Equilibria     

J. A. Vera  A. Vigueras 《Celestial Mechanics and Dynamical Astronomy》2006,94(3):289-315

We consider the non-canonical Hamiltonian dynamics of a gyrostat in Newtonian interaction with n spherical rigid bodies. Using the symmetries of the system we carry out two reductions. Then, working in the reduced problem, we obtain the equations of motion, a Casimir function of the system and the equations that determine the relative equilibria. Global conditions for existence of relative equilibria are given. Besides, we give the variational characterization of these equilibria and three invariant manifolds of the problem; being calculated the equations of motion in these manifolds, which are described by means of a canonical Hamiltonian system. We give some Eulerian and Lagrangian equilibria for the four body problem with a gyrostat. Finally, certain classical problems of Celestial Mechanics are generalized.  相似文献   

Jurassic guyots on the Southern Iberian Continental Margin: a model of isolated carbonate platforms on volcanic submarine edifices     

J. A. Vera  J. M. Molina  P. Montero  & F. Bea 《地学学报》1997,9(4):163-166

During the middle Jurassic on the Southern Iberian Continental Margin (at the westernmost end of the northern Tethys) isolated carbonate platforms developed over volcanic edifices, forming guyots. The volcanic edifices were composed of K-rich pillow-lavas and pyroclastic rocks with a radiometric age ≈ 170 Myr. Such phenomena have not been described until now in this continental margin nor in other passive continental margins of Alpine domains. The presence of a shallowing-upward megasequence (Bajocian–Bathonian) with hummocky cross- stratification strata below oolitic shoals, shows that very shallow isolated carbonate platforms developed above the volcanic edifices, with similar facies to those recognized in other guyots, but with a different age and geodynamic context.  相似文献   

Analysis of development and depth of backward erosion pipes in the presence of a coarse sand barrier     

Akrami  Sepideh  Bezuijen  Adam  van Beek  Vera  Rosenbrand  Esther  Terwindt  Jarno  Förster  Ulrich 《Acta Geotechnica》2021,16(2):381-397

Backward erosion piping (BEP) is a failure mechanism that can affect the safety of water-retaining structures. It can occur when a local anomaly on the downstream side of an embankment causes a concentration of seepage flow at that location. Shallow pipes may then form, progressing in the upstream direction and leading to a collapse of the water-retaining structure. A novel and economically appealing measure against BEP is the coarse sand barrier (CSB), which is now being developed in a multiscale experimental programme in the Netherlands. The method involves placing a trench filled with coarse sand below the blanket layer on the downstream side of the embankment. The CSB prevents the upstream progression of the pipe and significantly enhances resistance to BEP. This paper presents medium-scale laboratory tests involving a range of sands, barrier depths and relative densities. The piping process and the observations of pipe progression in the presence of a CSB are presented, followed by a conceptual model. The presence of a CSB changed the erosion pattern. It resulted in pipe formation perpendicular to the flow direction over the entire width of the barrier before the barrier was damaged. The findings also demonstrate the effect of material properties on pipe initiation, progression and pipe depth. Measurements of the pipe depth are presented and analysed, revealing the significance of pipe depth for understanding the piping process. This analysis shows considerable erosion in the downstream background sand and demonstrates that erosion profiles and measured pipe depths are significantly larger than in BEP tests without a CSB.

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Embedded Behavior: Human Activities and the Construction of the Mesolithic Shellmound of Cabeço da Amoreira,Muge, Portugal     

Vera Aldeias  Nuno Bicho 《Geoarchaeology》2016,31(6):530-549

Shellmound formation processes are greatly influenced by human inputs associated with an intensive exploitation of marine and estuarine resources. The complex stratigraphy of shellmounds has been difficult to decipher and few studies have focused on the microstratigraphic record of midden formation, especially in European Mesolithic contexts. Cabeço da Amoreira is a long‐known Mesolithic shellmound located on the shores of the Muge River, a tributary of the Tagus River in central Portugal. The abundance of shellfish refuse favors an intricate and laterally variable stratigraphic succession of layers and lenses, which results in an extensive artificial mound. Here, we use micromorphological and microfacies analysis to investigate aspects pertaining to site formation, and the nature of individual anthropogenic activities at the site. This methodology allowed for a distinction between primary activities (e.g., discrete shell tossing events, trampled occupational surfaces, and activity areas), and secondary position of the sediments in intentionally raked‐up and dumped deposits. Nondepositional hiatuses and discrete geogenic sedimentation are associated with periods of abandonment. The complex superposition of events seen at Cabeço da Amoreira underlines the role of micromorphology for the identification of behavioral signatures and its relevance in deciphering shellmound adaptations.  相似文献   

Influence of fluid flow on the regional thermal field: results from 3D numerical modelling for the area of Brandenburg (North German Basin)   总被引：1，自引：1，他引：0  

Vera Noack  Magdalena Scheck-Wenderoth  Mauro Cacace  Michael Schneider 《Environmental Earth Sciences》2013,70(8):3523-3544

We analyse the effect of fluid flow on the recent thermal field for the Brandenburg region (North German Basin) which is strongly affected by salt structures. The basin fill is modified by a thick layer of mobilized salt (Zechstein, Upper Permian) that decouples the overburden from deeper parts of the lithosphere and is responsible for thermal anomalies since salt has a distinctly higher thermal conductivity than the surrounding sediments and is impermeable to fluid flow. Numerical simulations of coupled fluid flow and heat transfer are carried out to investigate the influence of fluid flow on the shallow temperature field above the Zechstein salt, based on the finite element method. A comparison of results from conductive and coupled modelling reveals that the temperature field down to the low-permeable Triassic Muschelkalk is influenced by fluids, where the shallow low-permeable Tertiary Rupelian-clay is absent. Overall cooling is induced by forced convective forces, the depth range of which is controlled by the communication pathways between the different aquifers. Moreover, buoyancy-induced effects are found in response to temperature-dependent differences in the fluid density where forced convective forces are weak. The range of influence is controlled by the thickness and the permeability of the permeable strata above the Triassic Muschelkalk. With increasing depth, thermal conduction mainly controls the short-wavelength pattern of the temperature distribution, whereas the long-wavelength pattern results from interaction between the highly conductive crust and low-conductive sediments. Our results provide generic implications for basins affected by salt tectonics.  相似文献