The effect of specific background electrolytes on water structure and solute hydration: Consequences for crystal dissolution and growth

Barium sulfate is used as a model system to illustrate how solution composition can affect processes of crystal dissolution and growth. Rates and modes of reactions as well as morphological features can be modified by the introduction of simple ionic salts (KCl, NaCl, LiCl, CsCl, NaF, NaNO₃), due to the effects of these electrolytes on water structure dynamics and solute hydration. Based on the results of AFM in situ experiments performed at supersaturation (Ω) = 10.6 ± 0.1 and ionic strength (IS) in the range of 0.005-0.1 M we show that growth and dissolution behavior of barite changes under conditions of constant thermodynamic driving force (Ω) and constant IS in a systematic way depending on the specific background electrolyte used to adjust IS. The results are interpreted in terms of the relationships between solution composition, ion properties and the consequent growth and dissolution behavior. Island spreading rate is affected by salt-specific effects on the activation energy barrier of expelling water molecules from solvation shells of barite building units. Dissolution kinetics depends on the balance between the energy expended on breaking solvent structure and the energy gain on hydrating Ba²⁺ and ions, which are specific for different electrolyte solutions. Nucleation rates are determined by the frequency of water exchange around a barium cation which also depends on solution composition. Relating the structure of the solution to its composition can help to understand phenomena such as growth and dissolution in the presence of organic additives or impurity incorporation.     

Paleomagnetism of Carboniferous sediments from the West Sudetes (SW Poland)

The studied Carboniferous flysch and molasse sediments from the Intra-Sudetic Basin correspond to the period from Middle Visean to Early Autunian. Main magnetic minerals carrying the natural remanent magnetization (NRM) are goethite, magnetite, maghemite and hematite, all usually secondarily formed and/or remagnetized due to several tectonometamorphic events. In most samples several NRM components were isolated. One of them is usually a Jurassic-Triassic overprint. Some others define the Westphalian-Early Permian segment of the declination and inclination trajectory for the Sudetes calculated according to the reference apparent polar wander path for the Baltica plate. The Sudetic path is slightly shifted to the east compared to the reference path, suggesting the possibility of independent movements of the Sudetes during this time. The majority of isolated NRM components are secondary and related to the Sudetic orogenic phase and later tectonometamorphic activity.     

Predictability of stemflow in a species‐rich tropical forest

Numerous studies investigated the influence of abiotic (meteorological conditions) and biotic factors (tree characteristics) on stemflow generation. Although these studies identified the variables that influence stemflow volumes in simply structured forests, the combination of tree characteristics that allows a robust prediction of stemflow volumes in species‐rich forests is not well known. Many hydrological applications, however, require at least a rough estimate of stemflow volumes based on the characteristics of a forest stand. The need for robust predictions of stemflow motivated us to investigate the relationships between tree characteristics and stemflow volumes in a species‐rich tropical forest located in central Panama. Based on a sampling setup consisting of ten rainfall collectors, 300 throughfall samplers and 60 stemflow collectors and cumulated data comprising 26 rain events, we derive three main findings. Firstly, stemflow represents a minor hydrological component in the studied 1‐ha forest patch (1.0% of cumulated rainfall). Secondly, in the studied species‐rich forest, single tree characteristics are only weakly related to stemflow volumes. The influence of multiple tree parameters (e.g. crown diameter, presence of large epiphytes and inclination of branches) and the dependencies among these parameters require a multivariate approach to understand the generation of stemflow. Thirdly, predicting stemflow in species‐rich forests based on tree parameters is a difficult task. Although our best model can capture the variation in stemflow to some degree, a critical validation reveals that the model cannot provide robust predictions of stemflow. A reanalysis of data from previous studies in species‐rich forests corroborates this finding. Based on these results and considering that for most hydrological applications, stemflow is only one parameter among others to estimate, we advocate using the base model, i.e. the mean of the stemflow data, to quantify stemflow volumes for a given study area. Studies in species‐rich forests that wish to obtain predictions of stemflow based on tree parameters probably need to conduct a much more extensive sampling than currently implemented by most studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Valence state mapping and quantitative electron spectroscopic imaging of exsolution in titanohematite by energy-filtered TEM

 The microstructure of hematite-ilmenite exsolution intergrowth of a natural titanohematite crystal from a granulite has been investigated in a transmission electron microscope equipped with an energy filter. Special emphasis is on quantitative compositional mapping at the nanometre scale using electron spectroscopic imaging, as well as mapping the Fe³⁺ and Fe²⁺ valence distribution in the intergrowth. Quantitative point analyses by energy-dispersive X-ray analysis have been compared with results from electron energy-loss spectroscopy and element-distribution mapping. The results indicate that the coexisting compositions of the two phases (Ilm₈₈Hem₁₂ and Ilm₁₆Hem₈₄) are independent of the size of the exsolution. The application of quantitative mapping to determining diffusion profiles around precipitates is demonstrated. Received: 30 March 2000 / Accepted: 7 September 2000     

Book reviews

Pure and Applied Geophysics -     

A mechanism of mineral replacement: isotope tracing in the model system KCl-KBr-H2O

The solid solution-aqueous solution system KCl-KBr-H₂O is used as a model system to determine the mechanism of the replacement process of one crystalline phase by another. A single crystal of KBr was allowed to react with a saturated KCl solution enriched in ⁴⁰K and the resulting new product phases were analyzed for changes in both anion and K isotope composition, using SEM, EDX, microprobe analysis, mass spectrometry and X-ray diffraction. The results show that the new product, K(Cl,Br), contains elements derived from both the original fluid and solid phases, indicating that both anions and K isotopes are exchanged during the replacement reaction. The interface between the advancing reaction front and the original parent crystal is sharp on a micron scale, showing no reaction profile that could indicate a solid state diffusion mechanism. Macroscopically the new phase is turbid due to the development of porosity that is consistent with a net volume deficit replacement reaction and this porosity may act as an indication that a replacement process has taken place. Single crystal X-ray diffraction patterns show the preservation of the crystallographic orientation during the replacement process. The replacement mechanism is interpreted as a result of a coupled process of dissolution and recrystallization occurring at the fluid-crystal interface.     

The Glueckstadt Graben, a sedimentary record between the North and Baltic Sea in north Central Europe

The Glueckstadt Graben is one of the deepest post-Permian structures within the Central European Basin system and is located right at its “heart” at the transition from the North Sea to the Baltic Sea and from the Lower Saxony Basin to the Rynkoebing–Fyn High.The Mesozoic to recent evolution is investigated by use of selected seismic lines, seismic flattening and a 3D structural model. A major tectonic event in the latest Middle–Late Triassic (Keuper) was accompanied by strong salt tectonics within the Glueckstadt Graben. At that time, a rapid subsidence took place within the central part, which provides the “core” of the Glueckstadt Graben. The post-Triassic tectonic evolution of the area does not follow the typical scheme of thermal subsidence. In contrast, it seems that there is a slow progressive activation of salt movements triggered by the initial Triassic event. Starting with the Jurassic, the subsidence centre partitioned into two parts located adjacent to the Triassic “core.” In comparison with other areas of the Central European Basin system, the Glueckstadt Graben was not strongly affected by additional Jurassic and Cretaceous events. During the late Jurassic to Early Cretaceous, the area around the Glueckstadt Graben was affected by relative uplift with regional erosion of the elevated relief. However, subsidence was reactivated and accelerated during the Cenozoic when a strong subsidence centre developed in the North Sea. During Paleogene and Quaternary–Neogene, the two centres of sedimentation moved gradually towards the flanks of the basin.The data indeed point toward a control of post-Permian evolution by gradual withdrawal of salt triggered by the initial exhaustion along the Triassic subsidence centre. In this sense, the Glueckstadt Graben was formed at least partially as “basin scale rim syncline” during post-Permian times. The present day Hamburger, East and Westholstein Troughs are the actual final state of this long-term process which still may continue and may play a role in terms of young processes and, e.g., for coastal protection.     

Structure and evolution of the Glueckstadt Graben due to salt movements

The salt tectonics of the Glueckstadt Graben has been investigated in relation to major tectonic events within the basin. The lithologic features of salt sections from Rotliegend, Zechstein and Keuper show that almost pure salt is prominent in the Zechstein, dominating diapiric movements that have influenced the regional evolution of the Glueckstadt Graben. Three main phases of growth of the salt structures have been identified from the analysis of the seismic pattern. The strongest salt movements occurred at the beginning of the Keuper when the area was affected by extension. This activation of salt tectonics was followed by a Jurassic extensional event in the Pompeckj Block and Lower Saxony Basin and possibly in the Glueckstadt Graben. The Paleogene–Neogene tectonic event caused significant growth and amplification of the salt structures mainly at the margins of the basin. This event was extensional with a possible horizontal component of the tectonic movements. 3D modelling shows that the distribution of the initial thickness of the Permian salt controls the structural style of the basin, regionally. Where salt was thick, salt diapirs and walls formed and where salt was relatively thin, simple salt pillows and shallow anticlines developed.