Internal structure and sources of selected ice jams on the lower Vistula River

The lower stretch of the Vistula is the most ice‐jammed river section on the North European Plain. Since 1982, the structure of hanging dams has been studied by means of a mechanical non‐core sampler. In this article, a selected of field research results of the hanging dams' structure and the degree of filling of the cross section with ice obtained during surface ice‐jam events in the years 1995–2014 are presented, along with an explanation of their causes. Surface ice jams occurred during spring snowmelt surges and ice breakups and also during freeze‐up and ice‐covered periods. Their main cause was changes in the river flow and was also those affected by anthropogenic sources. A characteristic feature of the analysed cross sections was the considerable share of the underhanging ice dam's firm accumulation with ice floes, when the cross section would be filled with ice in excess of 70%. In most cases, due to low river discharge, there was no substantial flooding damage. Copyright © 2016 John Wiley & Sons, Ltd.     

Pinched between the plates: Armenia's voluminous record of volcanic activity

Located in the heart of the Lesser Caucasus mountains, where the Arabian and Eurasian tectonic plates collide, Armenia occupies an exceptional geological position shaped through millions of years of subduction and collision. It is a unique place on the Earth recording extensive intrusive and volcanic activity related to the long-standing continental convergence. The volcanoes of Armenia provide a rare opportunity to study the sources and processes involved in this unusual type of magmatism. More than 500 Quaternary volcanoes have been mapped in Armenia, most of them formed from single eruptive episodes. Among several large composite volcanoes, the mighty Aragats stands out as the largest volcano in Armenia and the region altogether. Volcanic deposits testify to the range of eruptive styles—from the ignimbrites formed in eruptions as explosive and voluminous as any seen globally in the modern era to the enormous fissure-fed lava flows that form the Southern Caucasus flood basalt province, the smallest and youngest Large Igneous Province in the world. Several pre-historical and historical eruptions have been documented, highlighting the potential for future volcanic activity in the region. In recent years, research has focused on the volcanic hazards associated with the Armenian Nuclear Power Plant, located in the foothills of Aragats volcano. This article highlights some of the extraordinary volcanic and intrusive features observed in Armenia and summarizes aspects of recent volcanological and petrological research.     

Seasonal stability of snow cover in Poland in relation to the atmospheric circulation

The seasonal stability of snow cover (ISS) was defined as a percentage ratio of the real and the potential snow cover duration in a winter season. Main results of the study are as follows: (1) alternately occurring periods of high and low values of the index of snow cover stability did not appeared simultaneously in mountainous and non-mountainous areas; (2) in the majority of Poland area both zonal and meridional components of the atmospheric circulation influence the ISS; however, in south the meridional air flow reveals the stronger impact, mostly due to the intensification of the southern advection by the foehn effect; and (3) changes of two or three indices describing atmospheric circulation explain up to 50 % of the ISS in Poland. The diminishing stability of snow cover in Poland corresponds with an increasing intensity of the advection from the western sector in winter in the second half of the twentieth century in Europe.     

<Emphasis Type="Italic">Salvinia natans</Emphasis> in medieval wetland deposits in Gdańsk,northern Poland: evidence for the early medieval climate warming

Large numbers of sub-fossil remains of the aquatic fern Salvinia natans (L.) All. have been found in several early medieval sites in Gdańsk, N. Poland. This record indicates a population expansion of this species around 7th–8th century A.D., similar to the recently observed rapid spread and high population dynamics of S. natans in northern Poland, which recent studies have attributed to climate warming. Our results suggest that in the Vistula deltaic area the S. natans expansion in the Early Middle Ages (7th–8th century A.D.) was similarly stimulated by climate warming, while its subsequent decline was mainly due to climate cooling, especially during the Little Ice Age. Warmer winters and springs and a longer growing season seem to be the most important factors forcing the expansion of S. natans in medieval times. According to our data, the co-occurrence of S. natans with other aquatic plant species was similar in both the medieval and present-day vegetation. Also, the high density of S. natans in the medieval population caused impoverishment of the local ecosystems in a way that has been observed in recent water bodies affected by invasive pleustophytes (free-floating plants).     

Carbon and oxygen isotopic composition and foraminifers of condensed basal Zechstein (Upper Permian) strata in western Poland: environmental and stratigraphic implications

The basinal facies of the Lopingian Zechstein Limestone in SW Poland consists of thin (often less than 1 m thick) limestones and/or dolomites, often containing the Kupferschiefer (few tens of centimetres thick) at their base, and local thick (up to 90 m) reefal carbonates. The δ¹³C curve of these basal Zechstein deposits strongly suggests that even when the Kupferschiefer is lacking, the thin (condensed) sequences record the entire interval of the Zechstein prior to the onset of evaporite deposition, in contrast to the thick reef sequences which lack the characteristic δ¹³C curve for the lowermost part of the Zechstein. The calcite samples show considerable ranges of δ¹⁸O values. If the maximum δ¹⁸O values are considered to be the closest to the pristine original ones and if δ¹⁸O_water value = 0 is assumed, then the calculated range of palaeotemperatures for the Kupferschiefer and Zechstein Limestone calcite ranges from 19 to 34 °C. The faunal restriction, common dwarf foraminifers and the predominance of lagenids in the foraminiferal assemblage indicate continual dysaerobic conditions and possibly elevated salinity of seawater during deposition of thin basinal Zechstein Limestone deposits. The mixing of shallow and deeper waters in the stratified Zechstein Basin caused by upwelling could result in prolific carbonate precipitation in reefs located at the slope of the marginal carbonate platform of the Zechstein Limestone and in isolated reefs related to palaeohighs within the basin; however, there is no isotopic record of eventual upwelling. Copyright © 2014 John Wiley & Sons, Ltd.     

Evaluation of electric parameters of anisotropic sandy-shaly Miocene formations on the basis of resistivity logs

In this paper, electrical properties of an anisotropic rock formation are discussed. Electrical anisotropy parameters, i.e., horizontal resistivity, R _H , and vertical resistivity, R _V , and anisotropy coefficient in the sandyshaly Miocene formation, are determined on the basis of induction logs and laterologs. Graphs of R _V = f(R _H ) calculated for different resistivities of sandstones and shales and variable volumes of those two components allow us to conclude about the correctness of the applied method for determining the resistivity parameters of autochthonous Miocene formations.     

First-order theory of weakly eccentric orbital motion

Starting from the analytical theory of perturbed circular motions presented in Celestial Mechanics (Bois, 1994), this paper presents an extended resolution valid also for small eccentricity orbits. The solution is of the first order of a small parameter characterizing the magnitude of disturbing forces. The solution has the form of Fourier series with the coefficients given by iterative formation laws. The solution is free from singularities due to small eccentricity or inclination. As an example of numerical application the equatorial artificial satellite orbits are analyzed. For some high satellite orbits with small eccentricity the difference between the numerical integration and the analytical model does not exceed few centimeters per one revolution.On leave from Astronomical Observatory of A. Mickiewicz University, Soneczna 36, PL60-286 Pozna, Poland.     

Unusual reaction of diatom assemblage on climate changes during the last millennium: a record from Spitsbergen lake

A sediment core from the Arctic Revvatnet (Hornsund area, SW Spitsbergen, Svalbard) provided data on environmental changes over the last 3100 years. Diatom analysis showing the domination of planktonic Cyclotella forms suggested good edaphic conditions until the middle of the nineteenth century, even during the Little Ice Age. A thermally stratified and relatively stable water column with good mixing allowed small, less heavily silicified Cyclotella sensu lato to develop during this time. The climate warming at the beginning of the twentieth century induced intensification of erosion processes in the catchment of the lake and caused an increase in the sedimentation rate. These processes have caused a lack of thermal stratification by disturbances in the water column and an increase of nutrients, consequently driving changes in the diatom species composition, which became dominated by benthic forms. In this period, almost all planktonic taxa disappeared or abruptly decreased in frequency. Higher temperatures accelerated the melting of nearby glaciers, which caused an increase in the activity of diatoms typical of running waters. Also a few Cladocera species appeared at the first time in the youngest samples.