Application of Gibbs Energy Minimization to ModelEarly-Diagenetic Solid-Solution Aqueous-Solution EquilibriaInvolving Authigenic Rhodochrosites in Anoxic BalticSea Sediments

The natural early-diagenetic environment ``anoxicporewater – authigenic mineral phases' has beencharacterized in sediment of the Gotland Deep,Baltic Sea, by a closed-system model. Occurrence ofcarbonate precipitates as thin almost pure whitelaminae was considered as a natural experiment forlong-term equilibration between these phases andporewater. Plots of distribution coefficientsindicate that metastable equilibrium exists betweenporewater and the authigenic Ca-rich rhodochrositephases below 7 cm depth. A thermodynamic model ofporewater geochemistry at in situ P = 25 barand T = 5 °C was developed using the Gibbsenergy minimization (GEM) approach. The values of isobaric-isothermal potentials of Mn, Ca, Fe, Mg,Sr, Ba, C, and O, calculated from the porewatercomposition, were used in a new ``dual thermodynamic'calculation approach to estimate solid activitycoefficients of the end-members in the non-idealsolid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO₃,i.e., at full major and minor multi-componentcomplexity. The regular Margules interactionparameters for the composing binaries estimated bythis model were _Mn-Ca = 1.9 ± 0.5,_Mn-Mg = 0.6,_Ca-Mg = 3.7,_Mn-Fe = 0.2,_Ca-Fe = 2.8,_Mn-Sr = 9.7,_Ca-Sr = 2.15,_Mn-Ba = 4.0,_Ca-Ba = 1.4,validating the theoretical predictions given byLippmann in his pioneering 1980's paper. Thestrictly thermodynamic equilibrium model is not onlyable to match both the measured porewater andcarbonate solid-solution composition, but also topredict that the porewater pH, pe, alkalinity, anddissolved Mn, Fe, and S concentrations arecontrolled by the authigenic mineral bufferingassemblage mackinawite-greigite-rhodochrosite. Ourmodel is only compatible with the idea of ACRformation with typical composition (X_Mnbetween 70–75%) in the topmost sediment layerwhich, however, needs a major source ofMn_aq ^II. This is provided by reduction ofparticulate Mn oxides precipitated in significantamounts in the water column upon major inflow eventsin the Baltic Sea. The model enables also to set upscenarios of changing environmental conditions, e.g.,to predict the non-linear response of the carbonatesolid-solution composition to changes in Mn loading,alkalinity and salinity of the sediment-watersystem. The results suggest that the major andespecially minor element contents (Sr, Mg, Ba) inauthigenic carbonates can be applied as anenvironmental paleoproxy.     

Cathodoluminescence, electron microscopy, and Raman spectroscopy of experimentally shock-metamorphosed zircon

Thorough understanding of the shock metamorphic signatures of zircon could be the basis for the use of this mineral as a powerful tool for the study of old, deeply eroded, and metamorphically overprinted impact structures and formations. This study of the cathodoluminescence (CL) and Raman spectroscopic signatures of experimentally (20-60 GPa) shock-metamorphosed zircon single crystals contributes to the understanding of high-pressure microdeformation in zircon. For all samples, an inverse relationship between the brightness of the backscattered electron (BSE) signal and the corresponding cathodoluminescence intensity was observed. The unshocked sample shows crosscutting, irregular fractures. The 20 GPa sample displays some kind of mosaic texture of CL brighter and darker domains, but does not exhibit any shock metamorphic features in BSE or CL images. The 40 GPa sample shows a high density of lamellar features, which might be explained by the phase transformation between zircon- and scheelite-structure phases of zircon and resulting differences in the energy levels of the activator elements. The CL spectra of unshocked and shocked (20, 40, and 60 GPa) zircon samples are dominated by narrow emission lines and broad bands in the region of visible light and in the near-UV range. The emission lines result from rare earth element activators and the broad bands might be associated with lattice defects. Raman spectra revealed that the unshocked and 20 GPa samples represent zircon-structure material, whereas the 40 GPa sample yielded additional peaks with relatively high peak intensities, which are indicative of the presence of the scheelite-type high-pressure phase. The 60 GPa sample has a Raman signature that is similar to that of an amorphous phase, in contrast to the observations of an earlier TEM study that the crystalline scheelite-structure phase is stable at this shock pressure. The 60 GPa Raman signature cannot be explained at this stage. The results show a clear dependence of the CL and Raman properties of zircon on shock pressure, which confirm the possible usage of these methods as shock indicators.     

Hi-Fix—ein neues Hilfsmittel zur Ortsbestimmung in küstennahen Seegebieten

Zusammenfassung Das Hi-Fix-Verfahren ist ein funktechnisches Hilfsmittel zur Ortsbestimmung auf der Grundlage der Hyperbelnavigation, das in küstennahen Seegebieten eingesetzt wird. Es werden das Prinzip, die Technik und die Anwendung dieses Verfahrens behandelt. Über die im praktischen Einsatz gesammelten Erfahrungen, sowie über die Aufgabenstellung für die Auswertung zahlreichen Registriermaterials, das während des Großversuches einiger Bundesbehörden in den Jahren 1964 und 1965 anfiel, wird berichtet. Es soll dabei untersucht werden, welchen Einfluß geophysikalische Größen, wie z. B. die Bodenleitfähigkeit und atmosphärische Störungen (Gewitter) sowie Sekundärstrahlungen von metallischen Gebilden in Empfängernähe auf die Anzeigegenauigkeit der Hi-Fix-Geräte ausüben.

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Hi-Fix — a new tool of position fixing in near-coastal sea areas

Summary Hi-Fix is a new tool of position fixing on the basis of hyperbolic navigation that is used in near-coastal sea areas. Principle, technique, and application of this system are treated. A report is given of the experience gained from field work as well as of the objects of the evaluation of the numerous recordings obtained from a large-scale experiment conducted by several federal agencies in the years 1964 and 1965. The effect of geophysical quantities, such as soil conductivity and atmospherics (thunderstorm) as well as secondary radiation from metallic bodies in the vicinity of the receiver on the indication accuracy of the Hi-Fix devices shall be studied.

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Hi-Fix — un nouveau procédé de localisation en mer à proximité de la côle

Résumé Le procédé Hi-Fix apporte une nouvelle contribution au problème de la localisation; il est basé sur le système hyperbolique et utilisé dans les eaux côtières. Il est rendu compte des expériences réalisées pratiquement ainsi que de la tâche qu'imposait, à quelques autorités fédérales, l'exploitation des nombreuses informations recueillies au cours des essais à grande échelle des années 1964 et 1965. A cette occasion on cherche à mesurer l'influence qu'exercent sur la précision fournie par les appareils Hi-Fix, des grandeurs atmosphériques (orages) ainsi que les rayonnements secondaires de formations métalliques au voisinage du récepteur.

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Nach einem Vortrag, gehalten auf der Meteorologen-Geophysiker-Tagung am 4. 4. 1968 in Hamburg.

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Hi-Fix ist eine Abkürzung für High Accuracy Position Fixing, es wird ebensowenig aufgelöst wie z. B. Radar. Die Redaktion.     

Determination of the submerged macrophyte coverage of greifswalder bodden

The extent to which a water body can support submerged macrophytic vegetation provides an indicator of its degree of eutrophication. Regular, quantitative assessment of macrophyte populations enables changes in the trophic state of the system to be observed. A combination of aerial photography and underwater videography has proved to be an efficient method for calculating macrophyte coverage of large coastal areas.     

A new equation of state for high compression

A new equation of state for metals is developed. The starting point for this development is a realistic expression for the free energy, allowance being made not only for the lattice energy but also for the Fermi energy, the exchange and correlation energies of the electrons. This expression is extended by means of three free parameters. The equation of state is successfully transformed in such a way that only macrophysical material parameters occur in it. In this way, the equation can be applied both to experimental high-pressure physics and to geophysics, its advantage resting in the fact that the bonding structure has been taken into account better than in other equations of state.The new equation, along with two other well-proven equations of state, was tried out on isothermal and Hugoniot data of 40 materials. Good agreement with the measured data was found not only for metallic elements, but also for halides and some oxides of geophysical importance. This agreement even proved to be better for most of the materials than that obtained with the other two equations of state.     

High Energetic Electrons Accelerated by a Coronal Shock Wave

Combined SOHO (Solar and Helisopheric Observatory) and ground based radio observations show evidently signatures of electrons accelerated by a shock wave during the event on July 9, 1996. A solar type II radio burst has been received as a signature of a coronal shock wave at 300 MHz on 9:10:54 UT. It was accompanied with electron beams appearing as type III radio bursts below 80 MHz. Simultaneously, the COSTEP (Comprehensive Suprathermal and Energetic Particle Analyzer) instrument aboard SOHO has measured enhanced electron fluxes in the range 30 keV – 3 MeV. This indicates that a coronal shock wave was able to produce high energetic electrons. A mechanism of electron acceleration up to relativistic velocities is presented and compared with the observations. The electron acceleration takes place at substructures of quasi-parallel collisionless shocks. This revised version was published online in July 2006 with corrections to the Cover Date.     

Early diagenetic processes during Mn-carbonate formation: evidence from the isotopic composition of authigenic Ca-rhodochrosites of the Baltic Sea

The formation of authigenic Ca-rich rhodochrosite (ACR) in sapropelic sediments of the Gotland Basin, Baltic Sea, is governed by deepwater renewal processes whereby saline water from the North Atlantic flushes the brackish anoxic Baltic Deeps. The carbon and oxygen isotopic compositions of these Mn-carbonates suggest that ACR formation takes place just below the sediment surface and that dissolved compounds from the deepwater column, such as water and bicarbonate molecules, were incorporated in ACR during authigenesis. Porewaters near the sediment surface display δ¹⁸O values of −5.4‰ (VSMOW) and are generally depleted in ¹⁸O, compared to the oxygen isotopic composition of water in equilibrium with Mn-carbonate solid solutions (ACR δ¹⁸O values are −4.6‰). This suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during Mn-carbonate formation. The reduction of sulfate having δ¹⁸O values of +8.4‰ accounts for a permanent enrichment of porewater ¹⁸O and observed δ¹⁸O values at depth equal to −4.6‰. However, this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and ACR close to the sediment surface where Mn-carbonate formation takes place. Based on isotopic mass balance calculations, we suggest that MnO₂ with δ¹⁸O values of +8.9‰ released oxygen enriched in ¹⁸O into the anoxic porewaters close below the sediment surface. This process should occur after oxygenation events during deepwater renewal when MnO₂ accumulates at the surface of anoxic sediments. Manganese carbonates formed in these waters display δ¹⁸O values of ∼1.0‰ heavier than values expected solely from the initial deepwater composition. This quantitatively explains the discrepancy between paleosalinities calculated from ACR δ¹⁸O based on Mn-carbonate/water isotopic equilibrium fractionation and direct observations for the same period. Our results emphasize the important role of microbial MnO₂ reduction during rhodochrosite authigenesis and suggest that Mn(II) activity, rather than alkalinity, is the limiting component for sedimentary Mn-carbonate formation.     

Linking shock textures revealed by BSE,CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure,Brazil

A silicious impact melt rock from polymict impact breccia of the northern part of the alkali granite core of the Araguainha impact structure, central Brazil, has been investigated. The melt rock is thought to represent a large mass of impact‐generated melt in suevite. In particular, a diverse population of zircon grains, with different impact‐induced microstructures, has been analyzed for U‐Pb isotopic systematics. Backscattered electron and cathodoluminescence images reveal heterogeneous intragrain domains with vesicular, granular, vesicular plus granular, and vesicular plus (presumably) baddeleyite textures, among others. The small likely baddeleyite inclusions are not only preferentially located along grain margins but also occur locally within grain interiors. LA‐ICP‐MS U‐Pb data from different domains yield lower intercept ages of 220, 240, and 260 Ma, a result difficult to reconcile with the previous “best age” estimate for the impact event at 254.7 ± 2.7 Ma. SIMS U‐Pb data, too, show a relatively large range of ages from 245 to 262 Ma. A subset of granular grains that yielded concordant SIMS ages were analyzed for crystallographic orientation by EBSD. Orientation mapping shows that this population consists of approximately micrometer‐sized neoblasts that preserve systematic orientation evidence for the former presence of the high‐pressure polymorph reidite. In one partially granular grain (#36), the neoblasts occur in linear arrays that likely represent former reidite lamellae. Such grains are referred to as FRIGN zircon. The best estimate for the age of the Araguainha impact event from our data set from a previously not analyzed type of impact melt rock is based on concordant SIMS data from FRIGN zircon grains. This age is 251.5 ± 2.9 Ma (2σ, MSWD = 0.45, p = 0.50, n = 4 analyses on three grains), indistinguishable from previous estimates based on zircon and monazite from other impact melt lithologies at Araguainha. Our work provides a new example of how FRIGN zircon can be combined with in situ U‐Pb geochronology to extract an accurate age for an impact event.