Geochemistry and mineralogy of shallow alluvial aquifers in Daudkandi upazila in the Meghna flood plain, Bangladesh

The shallow alluvial aquifers of the delta plains and flood plains of Bangladesh, comprises about 70% of total land area are mostly affected by elevated concentrations of arsenic (As) in groundwater exposing a population of more than 35 million to As toxicity. Geochemical studies of shallow alluvial aquifer in the Meghna flood plain show that the uppermost yellowish grey sediment is low in As (1.03 mg/kg) compared to the lower dark grey to black sediment (5.24 mg/kg) rich in mica and organic matter. Sequential extraction data show that solid phase As bound to poorly crystalline and amorphous metal (Fe, Mn, Al)-oxyhydroxides is dominant in the grey to dark grey sediment and reaches its maximum level (3.05 mg/kg) in the mica rich layers. Amount of As bound to sulphides and organic matter also peaks in the dark grey to black sediment. Vertical distributions of major elements determined by X-ray fluorescence (XRF) show that iron (Fe₂O₃), aluminum (Al₂O₃) and manganese (MnO) follow the general trend of distribution of As in the sediments. Concentrations of As, Mn, Fe, HCO₃ ⁻, SO₄ ²⁻ and NO₃ ⁻ in groundwater reflect the redox status of the aquifer and are consistent with solid phase geochemistry. Mineralogical analysis by X-ray diffraction (XRD) and scanning electron microscopy (SEM) fitted with energy dispersive X-ray spectrometer (EDS) revealed dominance of crystalline iron oxides and hydroxides like magnetite, hematite and goethite in the oxidised yellowish grey sediment. Amorphous Fe-oxyhydroxides identified as grain coating in the mica and organic matter rich sediment suggests weathering of biotite is playing a critical role as the source of Fe(III)-oxyhydroxides which in turn act as sink for As. Presence of authigenic pyrite in the dark grey sediment indicates active reduction in the aquifer.     

Major shoreline retreat and sediment starvation following Snowball Earth

While cap dolostones are integral to the Snowball Earth hypothesis, the current depositional model does not account for multiple geological observations. Here we propose a model that rationalises palaeomagnetic, sequence‐stratigraphic and sedimentological data and supports rapid deglaciation with protracted cap dolostone precipitation. The Snowball Earth hypothesis posits that a runaway ice‐albedo can explain the climate paradox of Neoproterozoic glacial deposits occurring at low palaeolatitudes. This scenario invokes volcanic degassing to increase atmospheric greenhouse gases to a critical threshold that overcomes the albedo effect and brings the planet back from the ice‐covered state. Once this occurs, Earth should shift rapidly from a snowball to an extreme greenhouse. However, cap dolostone units overlying glacial sediments, typically interpreted as transgressive deposits, exhibit multiple magnetic reversals indicating they accumulated in >10⁵ years. By reviewing modern post‐glacial systems, sequence stratigraphic concepts and principles of sedimentology, we suggest that cap dolostones are not restricted to the transgression but rather represent sediment starvation following a major landward shoreline migration associated with the demise of Snowball Earth. Thus, the duration in which cap dolostone accumulated is not directly coupled to the timescale of the Snowball Earth deglaciation.     

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.     

Spectral analyses of [WCL]-type central stars

The optical spectra of the seven late-type Wolf-Rayet central stars NGC-40 (PNG 120.0+09.8, subtype [WC8]), He 2-99 (PNG 309.0–04.2, [WC9]), BD+30°3639 (PN G 064.7+05.0, [WC9]), CPD-56°8032 (PNG 332.9–09.9, [WC11]), He2-113 (PNG 321.0+03.9, [WC11]), M4-18 (PNG 146.7+07.6, [WC11]) and K2-16 (PNG 352.9+11.4, [WC11]) are analyzed by means of spherically expanding model atmospheres. The NLTE simulations account for the elements hydrogen, helium, carbon and oxygen. As main results effective temperature, element abundances and final velocity of the wind are determined for each star. Assuming distances or luminosities for the objects, also the stellar radii and the mass-loss rates can be fixed. The results of these analyses establish empirical constraints for the evolutionary status of WC-type central stars as post-AGB objects and provide input for modeling their planetary nebulae.     

Climatic fluctuations in Bohemia from the 16th century until the present

Summary This is paper appraises the possibilities of using written sources of weather information and indirect indicators of the climate of Bohemia. The data are related mainly to the socalled Bohemian vine and hop growing region (the lower reaches of the Ohe River). On the basis of these data climatic fluctuations from the 16th century up to the present are described. Characteristics of climatic types are presented: the continental type (1501–1560, 1681–1750), the oceanic type (1561–1630), and the ambiguous type (1631–1680). On the basis of instrumental observations of the station Prague Klementinum it is possible to describe fluctuations of air temperature since 1771 and fluctuations of precipitation since 1804. These series are the basis material for the analysis of the cyclicity of climatic fluctuations and for the study of possible causes of this cyclicity. The obtained information of the climate of Bohemia since the 16th century for forecasting the climate fluctuations by means of the so-called diagnostic methods, which are based on the method of analogies.With 7 Figures     

Biogeographical drivers of ragweed pollen concentrations in Europe

Theoretical and Applied Climatology - The drivers of spatial variation in ragweed pollen concentrations, contributing to severe allergic rhinitis and asthma, are poorly quantified. We analysed the...     

Stratigraphy of nutrients and metals in sediment profiles of two dimictic lakes in North-Eastern Germany

A sedimentological study of two dimitic lakes in North-Eastern Germany provided a record of anthropogenic impacts and historical changes of water quality. The upper 50 cm sediment profiles were compared for major nutrients and selected major trace elements. The sediments were dated by ²¹⁰Pb and ¹³⁷Cs measurement. The upper 50 cm sediment profiles represent approximately the last 100 years of history in both lakes. Element analyses show different characteristic stratigraphic patterns in both lakes. Based on the nutrient and metal stratification, three characteristic time periods can be documented for both lakes. In addition to agricultural use of the catchment area, atmospheric pollution greatly influenced the metal concentration in the sediment layers. Variation in the external loading and redox conditions in the hypolimnion explain the variation in the composition and accumulation of metals in the sediment stratigraphy. No increases or changes in the trophic level of either lake could be documented based on the accumulation of the nutrients C, N and P. The ratio of Fe/Mn and Fe/Ca characterized the changing redox conditions. The stratigraphy of Pb and Zn agrees with the historical variation in atmospheric pollution and confirms literature values for Central and North Europe. The drop in Pb and Zn over the last 10–15 years is a regional effect in North-Eastern Germany.     

Windstorm of the eighteenth century in the Czech Lands: course,extent, impacts

Theoretical and Applied Climatology - This paper addresses the course, extent, and impacts of a windstorm that occurred on 20–21 December 1740, in the Czech Lands. The analysis is based on...