Sedimentäre Eisenerze und ihr geochemisches Bildungsmilieu in den tertiären Deckschichten der Rheinischen Braunkohle,Tagebau Frechen

Zusammenfassung Die hangenden tertiären Schichteinheiten der Rheinischen Braunkohle sind im Tagebau Frechen der Rheinische Braunkohlenwerke AG teils sideritisch, teils markasitisch-pyritisch vererzt. Beide Vererzungsarten sind postsedimentär und können daher nicht direkt als Milieuindikatoren für das Sedimentationsmilieu der Ablagerungen interpretiert werden. Die Existenz der Sideritgeoden, auf die tonigen Sedimente der Indener Schichten beschränkt, legt jedoch eine Ablagerung der Geodenmatrix in einem von Moorwässern bestimmten, limnischen Milieu nahe, da nur hier die Voraussetzungen für eine geologisch relevante Bereitstellung von Eisen gegeben erscheinen. Es wird angenommen, daß dieses zur Sideritbildung notwendige Eisen zunächst bevorzugt als Humat in die Tone eingelagert wurde. Im Rahmen der postsedimentären Oxidation organischer Substanzen bildeten sich dann CO₂-reiche Porenlösungen, in denen das Eisen, einem pH-Gefälle folgend, erneut transportiert und unter hohen pH-Werten schließlich als Siderit ausgefällt wurde. Markasit und Pyrit deuten eine ähnliche pH-Entwicklung der sie bildenden Porenlösungen an. Auch hier beobachtet man mit der Zeit eine Zunahme der Basizität der Lösungen, ausgedrückt in der Ausscheidungsfolge Markasit-Pyrit. In tonigen Sedimenten, mit geschlossenen Mikrosystemen der Porenräume, wird diese Folge nur einmal durchlaufen, in Sanden und Kiesen deuten wiederholte Ausscheidungen von Markasit und Pyrit ein offenes Porensystem an, in das im Verlauf der Diagenese auch jüngere Grundwässer Ehund pH-bestimmend eingreifen konnten.

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In the Frechen open cast mine (Rheinische Braunkohlenwerke AG) the hanging wall of the Rhenish lignites is partly mineralized by siderite and marcasite-pyrite. The mineralizations are postsedimentary and therefore, the environment existing during the deposition of the host rocks cannot be interpreted directly from the presence of siderite and marcasite-pyrite. However, since the occurrence of sideritic concretions is restricted to clayey sediments of the Inden beds (Miocene), the depositional environment may be deduced for geochemical reasons: Large-scale transport of iron is only possible in a limnetic environment influenced by bog water. It is suggested that the iron, necessary for the formation of siderite, was deposited together with the clay minerals, mainly as iron humates. Postsedimentary oxidation of organic materials formed CO₂-rich pore fluids which again facilitated mobilization of iron. In a basic environment siderite was precipitated. For the pyrite and marcasite bearing sedimentary beds a similar development of the interstitial waters is suggested. The precipitation sequence of marcasite, followed by pyrite, is indicative for an increasingly basic character of the pore fluids. In the clay sediments with closed microsystems of pore volumes, one marcasite-pyrite cycle was precipitated. The sand and gravel beds contain repeated marcasite-pyrite cycles, indicating a system open to repeated changes of its Eh-pH states by interaction with more recent groundwaters.

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Resumé Les couches tertiaires susjacentes aux lignites rhénaniennes sont minéralisées partiellement sidéritiquement et partiellement markasitiquement-pyritiquement dans les mines à ciel ouvert du Rheinische Braunkohlenwerke AG à Frechen. Ces deux types de minéralisation sont postsédimentaires et c'est ce qui explique qu'ils ne peuvent être interprétés comme des indicateurs du milieu existant à l'endroit de sédimentations. L'apparition des géodes de sidérite qui est limitée aux sédiments argileux de la formation Indènes (Miocene) rend vraisemblable que la déposition des sédiments argileux surviendra dans un milieu limnique influencé par des eaux marécageuses, étant donné que les conditions pour un transport des ions ferreux dans des quantités géologiquement intéressantes, n'existent que dans un tel milieu. Il est supposé que le fer est réparti dans les argiles sous la forme de sels humiques. Pendant l'oxidation des matériaux organiques, des solutions intersticielles riches en CO₂ se forment dans desquelles le fer est transporté de nouveau suivant l'augmentation du pH et où il est précipité sous forme de sidérite dans le milieu avec les valeurs pH plus élevées. Les formations des précipitations de markasite et de pyrite indiquent un développement semblable de leur solution intersticielle. On peut aussi observer dans ce cas une augmentation avec le temps de la basicité des solutions, marquée par la séquence de la précipitation markasite-pyrite. Cette séquence n'est suivie qu'une fois dans des sédiments argileux ayant un système intersticiel fermé; plusieurs précipitations observées dans des sables et graviers indiquent un système ouvert dans lequel les eaux souterraines peuvent intervenir déterminant le Eh-pH des processus des sédimentations plus jeunes.

     

Sedimentation in Lake Biel,an eutrophic,hard-water lake in northwestern Switzerland

A 2-year study of sedimentation in Lake Biel reveals that three major factors can account for the spatial and temporal patterns of tripton sedimentation. Allochthonous inputs of particulate matter mostly derived from the Aare river during spring snowmelt and other high-water periods, provide about 50% of the carbonate and 50% of the siliceous matter collected in sediment traps. Phytoplankton activity in this eutrophic, hard-water lake accounts for the rest of the carbonate and siliceous matter, the latter being mostly diatom frustules. Sedimentation rates are thus highest during periods of maximum river flooding, maximum phytoplankton activity and following the breakdown of the thermocline, at which time trapped particles settle out. Resuspension of sediment is important in at least one basin. Resolution of carbonate appears to be minor.     

Über die Vertikalverteilung und jahreszeitliche Entwicklung der Larvendichte vonDreissena polymorpha Pallas im Pelagial des unteren Zürichseebeckens im Jahre 1974

The freshwater musselDreissena polymorpha Pallas was sighted for the first time in Lake Zürich about 1969. This meant a real problem for the water treatment of lake water. By means of a pump and vertical net hauls it was tried to add more information to the scarce results on Lake Zürich. Additional dates about temperature, depth of Secchi disk visibility and beam transmittance were collected. There was a first appearance of the larvae ofD. polymorpha at the beginning of June. Following the mean temperature of the epilimnion the number of individuals/m² reached max. 210,000. The greatest concentration measured in 1974 of larvae/m³ was 54,375 at a depth of 4 m. The last appearance ofD. polymorpha was observed at the end of October.

     

Episodic Cenozoic volcanism and tectonism in the Andes of Peru

Radiometric and geologic information indicate a complex history of Cenozoic volcanism and tectonism in the central Andes. K-Ar ages on silicic pyroclastic rocks demonstrate major volcanic activity in central and southern Peru, northern Chile, and adjacent areas during the Early and Middle Miocene, and provide additional evidence for volcanism during the Late Eocene. A provisional outline of tectonic and volcanic events in the Peruvian Andes during the Cenozoic includes: one or more pulses of igneous activity and intense deformation during the Paleocene and Eocene; a period of quiescence, lasting most of Oligocene time; reinception of tectonism and volcanism at the beginning of the Miocene; and a major pulse of deformation in the Middle Miocene accompanied and followed through the Pliocene by intense volcanism and plutonism. Reinception of igneous activity and tectonism at about the Oligocene-Miocene boundary, a feature recognized in other circum-Pacific regions, may reflect an increase in the rate of rotation of the Pacific plate relative to fixed or quasifixed mantle coordinates. Middle Miocene tectonism and latest Tertiary volcanism correlates with and probably is genetically related to the beginning of very rapid spreading at the East Pacific Rise.     

Noble gases in mineral separates from three shergottites: Shergotty,Zagami, and EETA79001

Abstract— This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic‐ray exposure ages from ³He, ²¹Ne, and ³⁸Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic ⁴He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic‐ray effects. While a contribution of this low‐energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic‐ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near‐surface siting. We derive an estimate for the ⁴⁰Ar/³⁶Ar ratio of the Martian interior component by subtracting from measured Ar in the (K‐poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from ¹²⁹Xe, * excesses. Unless compromised by the presence of additional components, a high ratio of ～2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of ⁴⁰Ar/³⁶Ar in the Martian mantle.     

Ecohydrographic constraints on biodiversity and distribution of phytoplankton and zooplankton in coral reefs of the Red Sea,Saudi Arabia

An integral concept of ecological research is the constraint of biodiversity along latitudinal and environmental gradients. The Red Sea features a natural example of a latitudinal gradient of salinity, temperature and nutrient richness. Coral reefs along the Red Sea coasts are supported with allochthonous resources such as oceanic and neritic phytoplankton and zooplankton; however, relatively little is known about how the ecohydrography correlates with plankton biodiversity and abundance. In this article we present the biodiversity of phytoplankton and zooplankton in Red Sea coral reefs. Oceanographic data (temperature, salinity), water samples for nutrient analysis, particulate organic matter, phytoplankton and zooplankton, the latter with special reference to Copepoda (Crustacea), were collected at nine coral reefs over ~1500 km distance along the Red Sea coast of Saudi Arabia. The trophic state of ambient waters [as indicated by chlorophyll a (Chl a)] changed from strong oligotrophy in the north to mesotrophy in the south and was associated with increasing biomasses of Bacillariophyceae, picoeukaryotes and Synechococcus as indicated by pigment fingerprinting (CHEMTAX) and flow cytometry. Net‐phytoplankton microscopy revealed a Trichodesmium erythraeum (Cyanobacteria) bloom north of the Farasan Islands. Several potentially harmful algae, including Dinophysis miles and Gonyaulax spinifera (Dinophyceae), were encountered in larger numbers in the vicinity of the aquaculture facilities at Al Lith. Changes in zooplankton abundance were mainly correlated to the phytoplankton biomass following the latitudinal gradient. The largest zooplankton abundance was observed at the Farasan Archipelago, despite high abundances of copepodites, veligers (Gastropoda larvae) and Chaetognatha at Al Lith. Although the community composition changed over latitude, biodiversity indices of phytoplankton and zooplankton did not exhibit a systematic pattern. As this study constitutes the first current account of the plankton biodiversity in Red Sea coral reefs at a large spatial scale, the results will be informative for ecosystem‐based management along the coastline of Saudi Arabia.     

Boron abundances and isotopic ratios of olivine grains on Itokawa returned by the Hayabusa spacecraft

We report the B abundances and isotopic ratios of two olivine grains from the S‐type asteroid Itokawa sampled by the Hayabusa spacecraft. Olivine grains from the Dar al Gani (DaG) 989 LL6 chondrite were used as a reference. Since we analyzed polished thin sections in both cases, we expect the contribution from the solar wind B (rich in ¹⁰B) to be minimal because the solar wind was implanted only within very thin layers of the grain surface. The Itokawa and DaG 989 olivine grains have homogeneous B abundances (~400 ppb) and ¹¹B/¹⁰B ratios compatible with the terrestrial standard and bulk chondrites. The observed homogeneous B abundances and isotopic ratios of the Itokawa olivine grains are likely the result of thermal metamorphism which occurred in the parent asteroid of Itokawa, which had a similar composition as LL chondrites. The chondritic B isotopic ratios of the Itokawa samples suggest that they contain little cosmogenic B (from cosmic‐ray spallation reactions) rich in ¹⁰B. This observation is consistent with the short cosmic‐ray exposure ages of Itokawa samples inferred from the small concentrations of cosmogenic ²¹Ne. If other Itokawa samples have little cosmogenic B as well, the enrichment in ¹⁰B found previously on the surface of another Itokawa particle (as opposed to the bulk grain study here) may be attributed to implanted solar wind B.     

Shock experiments on anhydrite and new constraints on the impact‐induced SOx release at the K‐Pg boundary

Abstract– We have performed six shock experiments at nominal peak‐shock pressures of 12.5, 20, 33, 46.5, 64, and 85 GPa using polycrystalline anhydrite discs embedded in ARMCO‐Fe sample containers and the shock reverberation technique. The recovered samples were analyzed using X‐ray powder diffraction and transmission electron microscopy (TEM). The X‐ray diffraction patterns recorded on all samples are compatible with the anhydrite structure; extra‐peaks have not been observed. Peak intensities decrease and peak broadening increases progressively in the pressure range from 0 to 46.5 GPa. At higher pressures, peak broadening diminishes and the X‐ray diffraction pattern of the 85 GPa sample resembles essentially that of unshocked, well‐crystallized anhydrite. Related structural changes at the nanoscale include in the pressure regime up to 20 GPa “cold” deformation phenomena such as cracks and deformation twins. Dislocation density increases up to 33 GPa and the strain increases up to 46.5 GPa. In the pressure range from 46.5 to 85 GPa, high postshock temperatures caused annealing of the deformation features. Increasing density and size of voids in the anhydrite samples shocked at 64 and 85 GPa indicate partial decomposition of anhydrite. Recalculation of the peak‐shock pressure in the experiments to a more realistic natural loading path indicates the onset of degassing of anhydrite in the pressure range of 30–41 GPa.     

Search for extinct aluminum‐26 and titanium‐44 in nanodiamonds from the Allende CV3 and Murchison CM2 meteorites

Abstract– We report Mg‐Al and Ca‐Ti isotopic data for meteoritic nanodiamonds separated from the Allende CV3 and Murchison CM2 meteorites. The goal of this study was to search for excesses in ²⁶Mg and ⁴⁴Ca, which can be attributed to the in situ decay of radioactive and now extinct ²⁶Al and ⁴⁴Ti, respectively. Previous work on presolar SiC and graphite had shown that ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios in presolar grains can be used to discriminate between different types of stellar sources. Aluminum and Ti concentrations are low in the meteoritic nanodiamonds of this study. Murchison nanodiamonds have higher Al and Ti concentrations than the Allende nanodiamonds. This can be attributed to contamination and the presence of presolar SiC in the Murchison nanodiamond samples. ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca ratios are close to normal in Allende nanodiamonds with upper limits on the initial ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios of approximately 1 × 10^?3. These ratios are factors of 10–1000 and, respectively, 1–1000 lower than those of presolar SiC and graphite grains from supernovae. The ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti data for nanodiamonds are compatible with an asymptotic giant branch star or solar system origin, but not with a supernova origin of a major fraction of meteoritic nanodiamonds. The latter possibility cannot be excluded, though, as the diamond separates may contain significant amounts of contaminating Al and Ti, which would lower the inferred ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios considerably.