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171.
Fen ecosystem responses to water‐level fluctuations during the early and middle Holocene in central Europe: a case study from Wilczków,Poland
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Mateusz Płóciennik Andrzej Kruk Jacek Forysiak Dominik Pawłowski Kamila Mianowicz Scott Elias Ryszard K. Borówka Marek Kloss Milena Obremska Marek Krąpiec Piotr Kittel Sławomir Żurek 《Boreas: An International Journal of Quaternary Research》2015,44(4):721-740
This paper presents a palaeoenvironmental reconstruction of the Wilczków fen (central Poland). The fen developed in an inactive valley at the onset of the Holocene (~11 ka BP) and peat accumulation lasted until 5.7 ka BP. Multi‐proxy reconstructions were made on the basis of palaeobotanical, cladoceran, chironomid, beetle and geochemical analyses. A Kohonen self‐organizing map (SOM, unsupervised artificial neural network) of the biotic sequence distinguished four stages of fen history. Stage X1 (11.0–10.7 ka BP) was relatively wet and cool. Organic matter started to accumulate but the habitat conditions remained unstable. Moss, sedge and fern communities then developed. Sedimentary changes reveal an intensive groundwater supply at that time. Numerous and diverse chironomid and cladoceran subfossils indicate nearly permanent aquatic conditions. During stage Y1 (10.6–9.2 ka BP) conditions were dry and the upper peat layer desiccated. Cladocera nearly disappeared whereas chironomids were represented by semi‐terrestrial and predatory (Tanypodinae) species. Conditions started to be more reducing. All the remaining samples belonged to the interweaving stages X2 and Y2. Stage Y2 (mostly 9.1–7.3 and 6.0–5.7 ka BP) was also dry but humidity increased towards the top. Oxidizing conditions occurred and the pH became more alkaline, favouring Cladium mariscus. The basin received mostly allochthonous matter input at that time. Stage X2 (mostly 6.8–6.1 ka BP) was humid and warm. The groundwater supply remained low but there was an increase in precipitation, changing local conditions to ombrotrophic. Species‐rich chironomid and cladoceran communities were associated with temporary pools. Finally, conditions returned to those characteristic of stage Y2. The presented reconstruction documents long‐term abiotic and biotic changes determined by water supply, including groundwater outflow, which have rarely been detected at a multi‐proxy scale. We show that inactivated valley fens are sensitive to climate‐driven hydrological fluctuations. Kohonen neural networks appear to be a promising method for analysing variability in multi‐proxy data. 相似文献
172.
Marek Gołkowski Marek Kubicki Morris Cohen Andrzej Kułak Umran S. Inan 《Acta Geophysica》2011,59(1):183-204
Variations in the global atmospheric electric circuit are investigated using a wide range of globally spaced instruments observing
VLF (∼10 kHz) waves, ELF (∼300 Hz) waves, Schumann resonances (4–60 Hz), and the atmospheric fair weather electric field.
For the ELF/VLF observations, propagation effects are accounted for in a novel approach using established monthly averages
of lightning location provided by the Lightning Image Sensor (LIS) and applying known frequency specific attenuation parameters
for daytime/nighttime ELF/VLF propagation. Schumann resonances are analyzed using decomposition into propagating and standing
waves in the Earth-ionosphere waveguide. Derived lightning activity is compared to existing global lightning detection networks
and fair weather field observations. The results suggest that characteristics of lightning discharges vary by region and may
have diverse effects upon the ionospheric potential. 相似文献
173.
Marek Kwaśniewski 《Rock Mechanics and Rock Engineering》2011,44(1):103-111
Based on the hypothesis that shearing stresses and normal tensile stresses both play a decisive role in fracturing and brittle
failure of rock material, a novel strength criterion was developed in one of the earlier works of the author. In the criterion,
a certain parameter ν′ occurs which depends on the structure of the material. Originally, the parameter was treated as a constant, which resulted
in a linear form of the strength function F
σ
1 = f(σ
3), where σ
3 = σ
2. Although the linear strength criterion is sometimes found to be applicable to various particular rock materials, it is not,
in general, of a universal character. Analysis of the triaxial test results for 75 different sandstones revealed that parameter
ν′ usually increases in an exponential or linear manner as confining pressure increases, and only in isolated cases does it
seem to be independent of the confining pressure. For these three types of function ν′ = f(p) appropriate strength criteria F
σ
1 = f(σ
3) are given in the present paper. These criteria were used to fit all of the collected empirical data sets. In general, a
very good fit to the data was obtained. 相似文献
174.
Geochemical and statistical approach to evaluate background concentrations of Cd,Cu, Pb and Zn (case study: Eastern Poland) 总被引:2,自引:2,他引:0
Wojciech Zgłobicki Lesia Lata Andrzej Plak Marek Reszka 《Environmental Earth Sciences》2011,62(2):347-355
An attempt was made to evaluate background concentrations of Cd, Cu, Pb and Zn by means of geochemical and statistical approach.
As many as 753 samples taken from 51 profiles located in Eastern Poland were analysed. For the estimation of geochemical background
values, direct geochemical methods and a statistical analysis for the whole population of samples were applied. Average values
of heavy metal concentration in loess sediments (bedrock) as well as in profiles not affected by human activity were measured.
The iterative 2σ technique and calculated distribution function were chosen as statistical methods. The resulting values (background
concentrations range) were as follows: Cd 0.5–0.9 mg kg−1, Cu 5–16 mg kg−1, Pb 12–26 mg kg−1 and Zn 31–47 mg kg−1. All the methods applied gave similar results. The highest deviation of the background was noted for Cu and the lowest for
Zn. The lowest values of background were obtained for loess sediments and the highest in the case of the multiple 2σ method. 相似文献
175.
Dimitris Papadopoulos Michael Herty Volker Rath Marek Behr 《Computational Geosciences》2011,15(4):737-753
A shape reconstruction method for geophysical objects by temperature measurements is presented which uses adjoint equations
and a level set function approach. Temperature is measured on subdomains, e.g., representing boreholes. This information is
used to reconstruct the shape of the geophysical layers. For this purpose, shape optimization techniques are applied. The
method uses a representation of the layers by a so-called level set function. The evolution of this level set function is
then used to determine the optimal shape. The “speed” of the evolution is computed using adjoint equations. Synthetic examples
demonstrate the use of the inverse method and its behavior in different configurations. 相似文献
176.
177.
178.
179.
Is the soft excess in active galactic nuclei real? 总被引:1,自引:0,他引:1
180.
Miloslav Zejda Zdeněk Mikulášek Marek Wolf Ondřej Pejcha 《Astrophysics and Space Science》2006,304(1-4):161-163
We have re-analyzed old photometric UBV data obtained by Papoušek et al. at Skalnaté Pleso, Slovakia (1967–1980) and Brno, Czech Republic (1976–1979) observatories and analyzed new own CCD measurements (Brno, 2003–2005). In both set of photometric data the oscillations in the vicinity of the primary minimum were found. The detected periods are compatible with the previously published ones. 相似文献