Stable O and H isotopic composition of hydrous minerals as proxies for paleoclimate and -topography : application to the European Alps

The isotopie composition of clay minerals can be used to gain information on paleoaltitude, -topography and -climate during the time of their formation, as they form in isotopie equilibrium with ambient water, which has an isotopie composition that is determined by previous rain-out. In this thesis, the usability of this approach is evaluated for sample material from both the North Alpine Foreland Basin and internal parts of the European Alps. The isotopie composition of precipitation is determined by the air temperature and the extent of previous rain-out, which in turn is affected by the distance to the point of origin of the water vapor and the condensation conditions. An orographic barrier leads to uplift and adiabatic cooling of air masses and thereby enhances rainfall with a characteristic depletion in heavy 0 and H isotopes. Processes such as evaporation and mixing of water with different origin affect the isotopie composition further on the surface. Clay minerals are hydrous phyllosilicates that can form during weathering or pedogenesis in isotopie equilibrium with the present water and inherit its isotopie composition with a characteristic fractionation. If their isotopie composition was preserved after formation and the necessary parameters are known, it is possible to calculate the isotopie composition of the ambient water and thereby reconstruct climatic and topographic conditions during their formation. Due to the hygroscopic behavior of the sample material, analytical procedures needed to be modified to allow an analysis of the oxygen and hydrogen isotopie composition of the hydroxyl water built into their crystal structure and not the one of recently adsorbed water. In order to determine typical 5lsO and 8D values of meteoric water and minerals in the weathering environment in high Alpine regions, samples of surface water and soils from the valley of Visp in the Swiss Alps were studied that were collected along a transect with varying altitudes and host rock lithologies. The actual aim to attempt the reconstruction of Alpine paleoelevations was addressed by the analysis of sediments deposited under marine and terrestrial conditions during the Miocene and Oligocene, which were sampled from different locations in Switzerland, Germany and Austria. Further bentonite and fault gouge clay samples were used as reference material from low and high elevations. Results show that the isotopie composition of clay minerals can be influenced by several factors, such as the mode and site of their formation, possible isotope exchange (which is evident for 8D values), the samples' mineral composition (such as the presence of detrital material) and the deposition environment. With a consideration of these parameters and limitation to certain sample material, conclusions are drawn on the isotopie composition of the water from which the clay minerals formed. Most of the obtained values lie in the same range as has been reported in literature for foreland precipitation. The reconstruction of paleoelevation might be possible for selected sites with a geographical setting close to the Alpine chain or for intramontane basins. - La composition isotopique des argiles, qui se forment typiquement en contact avec des eaux superficielles, peut être utilisée pour obtenir des informations sur la paléoaltitude, la paléotopographie et le paléoclimat pendant leur création. La composition en isotopes stable (oxygène et hydrogène) des eaux de pluie est déterminée par l'altitude, la température, la distance depuis le lieu d'évaporation et par le taux de précipitation. Certains processus comme l'évaporation, l'interaction avec des minéraux et le mélange d'eaux d'origines variées peuvent aussi changer la composition des eaux de surface. Les argiles sont des pliyllosilicates hydratés qui sont crées par l'altération des roches au contact de ces eaux de surface et ils acquièrent une composition isotopique qui est soumise à un certain fractionnement. L'objectif de cette thèse est d'évaluer si l'effet de l'altitude des Alpes, est détectable dans la composition isotopique des sédiments du bassin d'avant-pavs pendant le Miocène. Après avoir établi la procédure analytique, des compositions isotopiques typiques de régions de hautes altitudes ont été determine sur les eaux de surfaces (rivières) et sur des échantillons de sols, le long d'une coupe dans la valée de Visp, dans les Alpes Suisses. Une reconstruction des paléoaltitudes Miocènes Alpines a ensuite été mené par l'analyse de sédiments marins et terrestres en utilisant des échantillons d'âges variés et provenant de Suisse, d'Allemagne et d'Autriche. Les résultats montrent que la composition isotopique des échantillons est impactée par de multiples facteurs, tels que le mode de et le lieu de formation. un possible échange isotopique tardif (surtout pour 5D). leur composition minéralogique et l'environnement de leur sédimentation. Après un examen prudent de ces paramètres, et en se limitant à un certain assortiment d'échatillons, on peut tirer des conclusions sur la composition isotopique des eaux originelles pour en déduire l'effet du climat et de la topographie.

Synorogenic extension in the Tethyan Himalaya documented by structural studies and the Kubler index, Lachung La area, NW India

Tectonic observations in the Tethyan Himalaya reveal an important extensional event that succeeds the emplacement of SW-verging nappes. A major thrust, called the Kum Tso Thrust, has been backfolded and reactivated by normal faulting associated with this event. Measurements of the Kubler index, coupled with characterization of clay-size paragenesis show the effect of normal faulting on the regional metamorphic zonation and indicate that important extension zones, like the Sarchu-Lachung La Normal Fault Zone (SLFZ), exist within the Tethyan Himalaya. Diagenetic limestones from within the SLFZ are characterized by the occurrence of mixed-layered clay phases, kaolinite and an illite with a 001 peak >0.4 Delta degrees2 theta. This zone is bordered by two anchizonal-to-epizonal zones, where illite peaks become narrower. Further to the NE the successive appearance of biotite, chloritoid, garnet and garnet-staurolite-kyanite assemblapes testifies to an increase in metamorphic grade. The cataclastic samples from the normal faults contain kaolinite, smectite and a `broad' illite, indicating that extension occurs under diagenetic conditions.

Formation of chlorite during thrust fault reactivation. Record of fluid origin and P-T conditions in the Monte Perdido thrust fault (southern Pyrenees)

The chemical and isotopic compositions of clay minerals such as illite and chlorite are commonly used to quantify diagenetic and low-grade metamorphic conditions, an approach that is also used in the present study of the Monte Perdido thrust fault from the South Pyrenean fold-and-thrust belt. The Monte Perdido thrust fault is a shallow thrust juxtaposing upper Cretaceous-Paleocene platform carbonates and Lower Eocene marls and turbidites from the Jaca basin. The core zone of the fault, about 6 m thick, consists of intensely deformed clay-bearing rocks bounded by major shear surfaces. Illite and chlorite are the main hydrous minerals in the fault zone. Illite is oriented along cleavage planes while chlorite formed along shear veins (< 50 mu m in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. delta O-18 and delta D values of newly formed chlorite support equilibration with sedimentary interstitial water, directly derived from the local hanging wall and footwall during deformation. Given the absence of large-scale fluid flow, the mineralization observed in the thrust faults records the P-T conditions of thrust activity. Temperatures of chlorite formation of about 240A degrees C are obtained via two independent methods: chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz. Burial depth conditions of 7 km are determined for the Monte Perdido thrust reactivation, coupling calculated temperature and fluid inclusion isochores. The present study demonstrates that both isotopic and thermodynamic methods applied to clay minerals formed in thrust fault are useful to help constrain diagenetic and low-grade metamorphic conditions.

Fluid flow through the sedimentary cover in northern Switzerland recorded by calcite-celestite veins (Oftringen borehole, Olten)

Abundant veins filled by calcite, celestite and pyrite were found in the core of a 719 m deep borehole drilled in Oftringen near Olten, located in the north-western Molasse basin, close to the thrust of the Folded Jura. Host rocks are calcareous marl, argillaceous limestone and limestone of the Dogger and Malm. The delta O-18 values of vein calcite are lower than in host rock carbonate and, together with microthermometric data from fluid inclusions in vein calcite, indicate precipitation from a seawater-dominated fluid at average temperatures of 56-68A degrees C. Such temperatures were reached at the time of maximum burial of the sedimentary pile in the late Miocene. The depth profile of delta C-13 and Sr-87/Sr-86 values and Sr content of both whole-rock carbonate and vein calcite show marked trends towards negative delta C-13, high Sr-87/Sr-86, and low Sr content in the uppermost 50-150 m of the Jurassic profile (upper Oxfordian). The Sr-87/Sr-86 of vein minerals is generally higher than that of host rock carbonate, up to very high values corresponding to Burdigalian seawater (Upper Marine Molasse, Miocene), which represents the last marine incursion in the region. No evidence for internally derived radiogenic Sr (clay minerals) has been found and so an external source is required. S and O isotope composition of vein celestite and pyrite can be explained by bacterial reduction of Miocene seawater sulphate. The available data set suggests the vein mineralization precipitated from descending Burdigalian seawater and not from a fluid originating in the underlying Triassic evaporites.

Dispersion of natural arsenic in the Malcantone watershed, Southern Switzerland: field evidence for repeated sorption-desorption and oxidation-reduction processes

In recent years, elevated arsenic concentrations have been found in waters and soils of many, countries, often resulting in a health threat for the local population. Switzerland is not an exception and this paper deals with the release and subsequent fate of arsenic in a 200-km(2) mountainous watershed, characterized by crystalline silicate rocks (gneisses, schists, amphibolites) that contain abundant As-bearing sulfide ore deposits, some of which have been mined for iron and gold in the past. Using analytical methods common for mineralogical, ground water and soil studies (XRD, XRF, XAS-XANES and -EXAFS, electron microprobe, extraction, ICP, AAS with hydride generator, ion chromatography), seven different field situations and related dispersion processes of natural arsenic have been studied: (1) release by rock weathering, (2) transport and deposition by water and ice; (3) release of As to the ground and surface water due to increasing pH; (4) accumulation in humic soil horizons; (5) remobilization by reduction in water-saturated soils and stagnant ground waters; (6) remobilization by using P-rich fertilizers or dung and (7) oxidation, precipitation and dilution in surface waters. Comparison of the results with experimental adsorption studies and speciation diagrams from the literature allows us to reconstruct and identify the typical behavior of arsenic in a natural environment under temperate climatic conditions. The main parameters identified are: (a) once liberated from the primary minerals, sorption processes on Fe-oxy-hydroxides dominate over Al-phases, such as Al-hydroxides or clay minerals and limit the As concentrations in the spring and well waters between 20 and 300 mug/l. (b) Precipitation as secondary minerals is limited to the weathering domain, where the As concentrations are still high and not yet too diluted by rain and soils waters. (c) Although neutral and alkaline pH conditions clearly increase the mobility of As, the main factor to mobilize As is a low redox potential (Eh close or below 0 mV), which favors the dissolution of the Fe-oxy-hydroxides on which the As is sorbed. (d) X-ray absorption spectroscopy (XAS) of As in water-logged humic forest soils indicates that the reduction to As III only occurs at the solid-water interface and that the solid contains As as As V (e) A and Bh horizons of humic cambisols can effectively capture As when As-rich waters flow through them. Complex spatial and temporal variation of the various parameters in a watershed results in repeated mobilization and immobilization of As, which continuously transports As from the upper to the lower part of a watershed and ultimately to the ocean. (C) 2004 Elsevier B.V. All rights reserved.

Low-grade metamorphism of the Gets nappe (Western Alps)

The Gets nappe, a decollement cover nappe located at the top of the Prealps, is characterized by the occurrence of ophiolitic rocks. The metamorphic grade in the Gets nappe was determined using illite crystallinity and clay mineral assemblages. Samples from the same locality were analyzed to estimate variations in illite crystallinity values and in the parageneses of clay minerals, both in sedimentary elements of a breccia and in the embedding shaly flysch. For samples from one and the same locality, the range in illite crystallinity data between breccia elements and the shaly flysch is comparable to the variation between different shaly beds. Two S-N transects along the Gets nappe reveal the same metamorphic gradient, with the internal parts of the nappe being characterized by middle anchizonal metamorphism and the external parts showing diagenetic conditions. The metamorphic grade is higher within the Gets nappe than in its hangingwall (i.e. the Breche and Simme nappes), suggesting that the metamorphism in the Gets unit is transported. The timing and conditions of thrusting of the Gets Nappe onto the Br che and the Simme nappes is constrained by stratigraphic and metamorphic data.

Phosphorus and carbon burial during cretaceous oceanic anoxic events, links with climatic changes

According to Jenkyns (2010), oceanic anoxic events (OAE) record profound changes in the climatic and paleoceanographic state of the planet and represent major disturbances in the global carbon cycle. One of the most studied OAEs on a worldwide scale is the Cenomanian-Turonian OAE 2, which is characterized by a pronounced positive excursion in carbon-isotope records and the important accumulation of organic-rich sediments. The section at Gongzha (Tibet) and the sections at Barranca and Axaxacualco (Mexico) are located in remote parts of the Tethys, and show δ13C records, which are well correlated with those of classical Tethyan sections. Both sections, however, do not exhibit the presence of organic-rich sediments. Phosphorus Mass Accumulation Rates (PMAR) in Tibet show a pattern similar to that observed in the Tethys by Mort et al. (2007), which suggests enhanced Ρ regeneration during the OAE 2 time interval, though there is no evidence for anoxic conditions in Tibet. Ρ appears here to have been mainly driven by detrital influx and sea-level fluctuations. The sections at Barranca and Axaxacualco show that the Mexican carbonate platform persisted during this anoxic event, which allowed the evolution of platform fauna otherwise not present in Tethyan sections. The persistence of this carbonate platform close to the Caribbean Igneous Plateau, which is thought to have released bio-limiting metals, is explained by local uplift which delayed the drowning of the platform and a specific oceanic circulation that permitted the preservation of oligotrophic conditions in the area. The Coniacian-Santonian OAE (OAE3) appears to have been more dependent on local conditions than OAE2. The presence of black shales associated with OAE3 appear to have been restricted to shallow-water settings and epicontinental seas in areas located around the Atlantic Ocean. The sections at Olazagutia (Spain), and Ten Mile - Arbor Park (USA), two potential Global Boundary Stratotype Sections and Points (GSSP) sites, are devoid of organic-rich sediments and lack a δ13C positive excursion around the C-S boundary. The Gabal Ekma section (Sinai, Egypt) exhibits accumulations of organic-rich sediments, in addition to phosphorite bone beds layers, which may have been linked to an epicontinental upwelling zone and/or storm inputs. Our data suggest that OAE 3 is rarely expressed by truly anoxic conditions and seems to have been linked to local conditions rather than global paleoenvironmental change. The evidence for detrital-P being the likely cause of Ρ fluctuations during the OAEs studied here does not negate the idea that anoxia was the principal driver of these fluctuations in the western Tethys. However, an explanation is required as to why the Ρ accumulation signatures are mirrored in both oxic and anoxic sedimentary successions. 'Eustatic/climatic' and 'productivity/anoxic' models may have both operated simultaneously in different parts of the world depending on local conditions, both producing similar trends in Ρ accumulation. - Selon Jenkyns (2010), les événements anoxiques océaniques enregistrent de profonds changements dans le climat et la paléoceanographie de la planète et représente des perturbations majeures du cycle du carbone. L'un des plus étudiés à l'échelle mondiale est l'ΟΑΕ2 du Cénomanien-Turonien, qui est caractérisé par une très forte excursion positive des isotopes du carbone et une importante accumulation de sédiments riche en matière organique. La section de Gongzha (Tibet) et les sections de Barranca et Axaxcualco (Mexique) sont situées aux confins de la Téthys, et enregistrent une courbe isotopique en δ13C parfaitement corrélable avec les sections téthysiennes, mais ne montre pas d'accumulation de black shales. Le taux de phosphore en accumulation de masses (PMAR) au Tibet montre un pattern similaire observé également par Mort et al. (2007) dans la Téthys, suggérant un model de régénération du Ρ durant l'anoxie, cependant aucune conditions anoxiques régnent dans la région du Tibet. Ρ apparaît donc principalement guidé par le détritisme et les fluctuations du niveau marin. Les sections de Barranca et d'Axaxacualco montrent que la plateforme carbonatée mexicaine persiste durant cet événement anoxique, et permet le développement d'une faune de plateforme qui n'est pas présente dans les sections téthysiennes. La persistance de cette plateforme carbonatée si proche du plateau Caribéen, qui est connu pour le relâchement de métaux bio-limitant, peut être expliqué par un soulèvement tectonique local qui inhibe l'ennoiement de la plateforme et une circulation océanique spécifique qui permet la préservation de conditions oligotrophiques dans cette région. L'événement anoxique océanique du Coniacien-Santonien apparaît plus dépendant des conditions locales que pour l'ΟΑΕ2. Les black shales associés à POAE3 sont restreints aux zones situées autour de l'océan Atlantique et plus particulièrement aux eaux peu profondes et épicontinentales. Les sections d'Olazagutia (Espagne), Ten Mile Creek et Arbor Park (USA), qui sont deux potentielles sections GSSP (Sections de stratotype de limite globaux et de points), ne montre pas d'accumulation de black shales et pas de forte excursion positive en δ13C autour de la limite C-S. La section de Gabal Ekma (Sinai, Egypte) montre des accumulations de black shales, en plus des couches de phosphorites et d'accumulation d'os (« bone beds »), vraisemblablement lié à des zones active d'upwelling épicontinentale et/ou d'apport de tempêtes. Nos données suggèrent que l'OAE 3 est rarement exprimé par de vraies conditions anoxiques et semble être plus lié à des conditions plus locales que des changements paléo-environnementaux globaux, comme observés pour le Cénomanien- Turonien. Les arguments pour un modèle lié au phosphore détritique qui serait la cause des fluctuations du phosphore total durant les OAEs, n'écartent pas l'idée que l'anoxie est la principale cause de ces fluctuations dans les sections riches en matière organique de l'Ouest téthysien. Cependant une explication est nécessaire pour comprendre pourquoi la signature de l'accumulation du phosphore est semblable dans les successions sédimentaires déposées dans des conditions oxygénées et anoxiques. Les modèles « Eustatisme/Climat » et « Productivité/anoxie » ont simultanément opéré dans les différentes parties du monde dépendant de conditions locales, et ont produit des tendances similaires en accumulation de phosphore.

Illite "crystallinity'' revisited

The Kubler Index (KI) is defined as the full width at half-maximum height (FWHM) of the 10-Angstrom X-ray diffraction peak of illite-smectite interstratified (I-S) clay minerals, The only parameters controlling the Kubler index are assumed to be the mean number of layers (N) in the coherent scattering domains (CSD). the variance of the distribution of the number of layers of the CSB, the mean percentage of smectite layers in I-S (%S), and the probability of layer stocking (Reichweite). The Kubler-Index measurements on ah-dried (KIAD) and ethylene-glycolated (KIEG) samples were compared to N and %S using the NEWMOD computer program to simulate X-ray diffraction patterns. Charts of KIAD versus KIEG corrected for instrumental broadening were made and isolines were mapped for constant N and CLS. Isolines allow a direct and rapid determination of N and RS from KI measurements. The method allows quantification of the metamorphic anchizone limits by considering mean thickness of fundamental particles in MacEwan crystallites. The transition from diagenesis to the anchizone and from the anchizone to the epizone of low-grade metamorphism corresponds to thicknesses of 20- and 70-layer fundamental particles, respectively.