Reconstruction of palaeosalinity using carbon isotopes and benthic associations: a comparison

Carbon and oxygen isotopes were determined on 40 recrystallized shells of Late Jurassic bivalves from the Lusitanian Basin of Portugal. In contrast with the oxygen isotopes, which exhibited considerable diagenetic distortion, the carbon isotopes are thought to preserve a record of the salinity of the Jurassic marginal marine seas in which these bivalves lived. The reconstructed palaeosalinities range from 35%o (euhaline) to 5% (oligohaline). Comparing these values with the palaeosalinity reconstructed from a palaeoecological analysis of 17 stratigraphic levels within the basin, the independently derived values agree in most cases. Strongly differing values are explained as being due to biotic factors and to diagenetic distortion of the isotopic signal; they are less likely to be due to smallscale time-averaging or insufficient microstratigraphic sampling. On the whole, the carbon isotope analyses are thought to produce reasonable palaeosalinity values, although data from infaunal, originally aragonitic bivalves appear to be less reliable than those from epifaunal bivalves with a predominantly or exclusively calcitic shell. As diagenetic alteration of the carbon isotope signal is, however, unpredictable and biotic effects on the isotopic composition are insufficiently known, palaeosalinity reconstructions based on stable isotope data should be supported by palaeoecological data.     

Zur Sedimentologie eines arktischen Deltas (Colville Delta,Nordalaska)

Zusammenfassung Im August 1971 wurden vom Verfasser Feldarbeiten zur Sedimentologie des Colville Deltas, Alaska, durchgeführt. Aus subaquatischen Typlokalitäten wurden Probenkerne gezogen und auf Strukturen, Korngrößen und Mineralogie hin untersucht. Neben konventionellen Labormethoden kamen moderne elektronische Verfahren zum Einsatz (Röntgenradiographie, Röntgendiffraktometrie, Elektronenmikroskopie). Die Arbeiten sind Teil eines Programmes, dessen Gesamtleitung Prof. Dr. H. J. Walker, Baton Rouge, USA, hatte und das unterstützt wurde von: Arctic Program and Geography Programs, Office of Naval Research.

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In August 1971, sedimentological field work was carried out by the author in the Colville River delta, Alaska. Short cores were taken from several deltaic environments and analyzed for structure and composition. Laboratory analysis included both conventional laboratory methods and modern electronic techniques (radiography, X-ray diffractometry, electron microscopy). The research reported is part of a program headed by Prof. Dr. H. J. Walker, Baton Rouge, USA, and was supported by the Arctic Program and Geography Programs, Office of Naval Research.

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Résumé En auguste 1971 l'auteur a étudié sur le terrain la sédimentologie du delta de Colville (Alaska). On a prélevé des échantillons dans des profiles de divers environnements typiques; on en a examiné les structures et la composition par les méthodes traditionelles et électroniques (rayons X, radiographie et diffractométrie, microscope électronique). Ces investigations font partie d'un programme sous la direction du Prof. Dr. H. J. Walker, Baton Rouge, USA, subventionné par »Arctic Program and Geography Programs, Office of Naval Research«.

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1971 . , . , . , - , . , . . H. J. Walker, Baton Rouge, USA Arctic Program, and Geography Programs, Office of Naval Research.

     

Synthetic dumortierite: its PTX-dependent compositional variations in the system Al2O3-B2O3-SiO2-H2O

Dumortierite, generally simplified as Al₇BSi₃O₁₈, was synthesized in the pure system Al₂O₃–B₂O₃–SiO₂–H₂O (ABSH) using gels with variable Al/Si ratios mixed with H₃BO₃ and H₂O in known proportions as starting materials. Synthesis conditions ranged from 3 to 5 and 15 to 20 kbar fluid pressure at 650° to 880°C. On the basis of analyses, synthetic dumortierite shows relatively narrow homogeneity ranges with regard to Al/Si which, however, vary as a function of pressure: at low pressures (3–5 kbar) Al/Si is 2.77–2.94 versus 2.33–2.55 at high pressures (15–20 kbar). Outside of these homogeneity limits, dumortierite was found to coexist with quartz or corundum, depending on the starting composition. Whereas synthetic dumortierite invaribly contains 1.0 boron atom per formula unit (p.f.u.) based on 18 oxygens, the water contents vary drastically as a function of pressure and temperature (1.32–2.30 wt.% H₂O or 0.85–1.47 H p.f.u.). H₂O is an essential component in dumortierite. Structural formulae based on complete chemical analyses of the dumortierites synthesized reveal that there is invariably an Si-deficiency against the ideal number of 3.0 p.f.u. In the calculation procedure used here, this deficiency is balanced by assuming tetrahedral Al. The remaining Al, taken to occupy the octahedral sites, is always below the ideal number of 7.0 p.f.u. Charge-balancing the structure with the hydrogen found analytically leads to two different mechanisms of H incorporation: (1) 3H⁺ + octahedral vacancy for Al^[6]; (2) H⁺ + tetrahedral Al for Si^[4]. Dumortierite synthesized at high fluid pressure contains little Al^[4] and, thus, little H⁺ of type 2; its hydrogen is predominantly present as type 1. Conversely, dumortierite formed at low fluid pressures is high in Al^[4] and hydrogen type 2. The amounts of hydrogen type 1 in low-pressure dumortierites decrease with rising temperatures of synthesis. Typical structural formulae are: (Al_6.670.33)[Al_0.49Si_2.51–O_13.53(OH)_1.47](BO₃) for a low-pressure product, and (Al_6.680.32)[Al_0.09Si_2.91O_13.94(OH)_1.06](BO₃) for a high-pressure product. Independently of the synthesis conditions, dumortierite was found always to be orthorhombic, with b₀/a₀ deviating slightly, but significantly from the valid for hexagonal lattice geometry. As a function of increasing Al/Si in the synthetic crystals, their a₀, c₀, and V₀ rise, whereas b₀ decreases. Thus b₀/a₀ decreases most sensitively with rising Al/Si and also with growing Al^[4]. More experimentation is required before the compositional variations of dumortierite found here can be applied successfully to geothermobarometry of natural rocks.     

Examples of structural control of hydrothermal mineralization: Fault zones in epicontinental sedimentary basins — A review

By compiling data available from progressive geoscientific research and by complementary studies it seems possible for an increasing number of mineral districts to find indications of the regional structural control of hydrothermal ore deposits. The relationship between hydrothermal mineralization, faulting or fault reactivation and related processes can, of course, best be examined at the less deformed sedimentary record of ancient epicontinental sedimentary basins. There is increasing evidence that the sediment-hosted submarine-hydrothermal Zn-Pb-Ba mineralizations such as Meggen, Rammelsberg, Howards Pass and McArthur River are spatially and genetically related to deep synsedimentarily active fault systems of intracontinental basins or stretched passive continental margins. For long periods of basin development the faults provided favorable conduits for episodically released basin-dewatering brines, which precipitated the leached metals into third-order basins situated next to these faults. There are many similarities to epigenetic mineralizations, which formed either pre-, syn- or postorogenetically from brines leaching the (meta)sediments and associated magmatites and migrating along fault zones to their near-surface trap structures.

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Zusammenfassung Für viele Erzprovinzen bietet sich durch Kompilation der in den letzten Jahrzehnten rasch angewachsenen Zahl geowissenschaftlicher Daten und durch gezielte ergänzende Untersuchungen die Möglichkeit, Anhaltspunkte oder Belege für die regionale Strukturbindung von hydrothermalen Vererzungen zu finden. Naturgemäß lassen sich die Zusammenhänge zwischen hydrothermaler Mineralisation, Störungstektonik und begleitenden Prozessen am besten an nur gering orogen überprägten ehemaligen Sedimentbecken untersuchen. So gibt es vermehrt Belege dafür, daß die sediment-gebundenen, submarin-hydrothermalen Zn-Pb-Ba Vererzungen wie Meggen, Rammeisberg, Howards Pass, McArthur River etc. räumlich und genetisch an tiefreichende Störungssysteme gebunden sind, die während der Sedimentation in intrakontinentalen Riftbecken oder auf ausgedünnten passiven Kontinentalrändern aktiv waren. Diese Störungssysteme bündelten die salinaren Beckenentwässerungsströme und ermöglichten über lange Zeiträume ihren episodischen Aufstieg zum Meeresboden, wo die aus den Sedimenten aufgenommenen Metalle als Sulfide in Spezialbecken abgesetzt wurden. Es bestehen prinzipielle Ähnlichkeiten mit vielen epigenetischen prä-, syn- oder postorogenen Mineralisationen; auch hier migrierten die salinaren Lösungen, die ihren Metallgehalt aus den (Meta)Sedimenten und eingelagerten Magmatiten bezogen, entlang von Störungssystemen bis zu den oberflächennahen Fallenstrukturen.

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Résumé La compilation des données géologiques qui se sont largement multipliées au cours des dernières décennies, complétée par des rechercùhes complémentaires particulières, nous offre pour beaucoup de régions la possibilité de trouver des indices ou même des preuves d'un contrôle structural des minéralisations hydrothermales. Il va de soi que les relations entre la minéralisation hydrothermale, la formation et la réactivation des failles ainsi que les processus associés sont les plus faciles à étudier dans les couches sédimentaires les moins touchées par une orogenèse postérieure. Ainsi, il existe des preuves que les gisements stratiformes et sédimentaires de Zn-Pb-Ba du type Howards Pass, McArthur River et Meggen sont liés à des systèmes de failles synsédimentaires profondes, en relation avec des bassins intracontinentaux du genre rift et avec des marges continentales passives en extension. Ces systèmes de failles synsédimentaires ont drainé des eaux salines intersticielles et ont rendu possible leur mouvement ascendant vers les fonds marins où leur contenu métallifère s'est déposé dans des bassins locaux. Des nombreuses recherches récentes mettent en évidence la ressemblance avec les minéralisations épigénétiques formées avant, pendant et après l'orogenèse; là aussi, les eaux interstitielles, après avoir reçu leur contenu métallifère des (meta)sédiments et des magmatites associées ont migré le long de systèmes de failles jusqu'aux pièges structuraux proches de la surface.

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Studies of dust grain properties in infrared reflection nebulae

We have developed a model for reflection nebulae around luminous infrared sources embedded in dense dust clouds. The aim of this study is to determine the sizes of the scattering grains. In our analysis, we have adopted an MRN-like power-law size distribution (Mathis, Rumpl, and Nordsieck) of graphite and silicate grains, but other current dust models would give results which were substantially the same. In the optically thin limit, the intensity of the scattered light is proportional to the dust column density, while in the optically thick limit, it reflects the grain albedo. The results show that the shape of the infrared spectrum is the result of a combination of the scattering properties of the dust, the spectrum of the illuminating source, and foreground extinction, while geometry plays a minor role. Comparison of our model results with infrared observations of the reflection nebula surrounding OMC-2/IRS 1 shows that either a grain size distribution like that found in the diffuse interstellar medium, or one consisting of larger grains, can explain the observed shape of the spectrum. However, the absolute intensity level of the scattered light, as well as the observed polarization, requires large grains (approximately 5000 angstroms). By adding water ice mantles to the silicate and graphite cores, we have modeled the 3.08 micrometers ice band feature, which has been observed in the spectra of several infrared reflection nebulae. We show that this ice band arises naturally in optically thick reflection nebulae containing ice-coated grains. We show that the shape of the ice band is diagnostic of the presence of large grains, as previously suggested by Knacke and McCorkle. Comparison with observations of the BN/KL reflection nebula in the OMC-1 cloud shows that large ice grains (approximately 5000 angstroms) contribute substantially to the scattered light.     

The equation of state of the solar interior: A comparison of results from two competing formalisms

A recently developed treatment of partition functions in the equation of state (Mihalas, Hummer, Däppen, MH&D) has led to a substantial improvement in the agreement between observed and theoretically predicted solar p-mode oscillation frequencies. The MH&D equation of state is a realization of the free-energyminimization method, based on the so-called chemical picture, in which ionization and dissociation reactions are assumed to be those that maximize entropy, or equivalently, minimize the free energy. An alternative equation of state has recently been developed at Livermore. It realizes a virial expansion of pressure, and is based on the physical picture, in which explicitly only fundamental species (i.e., electrons and nuclei) appear. Results of a first comparison between thermodynamic quantities of the MH&D and Livermore equations of state are presented. For simplicity, a mixture with only hydrogen and helium (90% H and 10% He by number) is chosen. The comparison is made for a low-density and a high-density case. In the first case, the conditions are those of the hydrogen and helium ionization zones of the Sun, in the second case those of the solar centre. In both cases, the MH&D and Livermore results agree strikingly, despite the very different formalisms they are based on.     

Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon(ISM),perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air.Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau.However,given the large biases in simulating the monsoon in CMIP5 models,such results may be model dependent.This study investigates the impact of orographic forcing from the Tibetan Plateau,Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon(EASM) in the UK Met Office's Had GEM3-GA6 and China's Institute of Atmospheric Physics FGOALS-FAMIL global climate models.The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies.However,considerable differences exist in the rainfall responses over India and China,and in the detailed aspects such as onset and retreat dates.In particular,the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed.Our results show that a multi-model approach,as suggested in the forthcoming Global Monsoon Model Intercomparison Project(GMMIP) associated with CMIP6,is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.     

Regional geoid determination in Antarctica utilizing airborne gravity and topography data

Antarctica is the only continent that suffers major gaps in terrestrial gravity data coverage. To overcome this problem and to close these gaps as well as to densify the global satellite gravity field solutions, the International Association of Geodesy (IAG) Commission Project 2.4 “Antarctic Geoid” was set into action. This paper reviews the current situation concerning the gravity field in Antarctica. It is shown that airborne geophysical surveys are the most promising tools to gain new gravity data in Antarctica. In this context, a number of projects to be carried out during the International Polar Year 2007/2008 will contribute to this goal. To demonstrate the feasibility of the regional geoid improvement in Antarctica, we present a case study using gravity and topography data of the southern Prince Charles Mountains, East Antarctica. During the processing, the remove–compute– restore (RCR) technique and least-squares collocation (LSC) were applied. Adding signal parts of up to 6 m to the global gravity field model that was used as a basis, the calculated regional quasigeoid reveals the dominant features of bedrock topography in that region, namely the graben structure of the Lambert glacier system. The accuracy of the improved regional quasigeoid is estimated to be at the level of 15 cm.