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Geowissenschaften

Die Erforschung der Erdgeschichte erfüllt den grundlegenden menschlichen Drang zu wissen, wie die Welt entstanden ist. Im Laufe ihrer 4,6 Milliarden Jahre langen Geschichte hat die Erde eine enorme Transformation durchlaufen, und die Stabilisotopenanalyse steht bei der Erforschung dieser Veränderungsprozesse an vorderster Front. Durch das Verständnis der makro- und mikrozyklischen Mechanismen, welchen die leichten stabilen Isotope in der Lithosphäre durch natürliche Prozesse unterliegen, wächst auch unser Wissen über die Erdgeschichte kontinuierlich.

Paläoklima

Eine der ersten Anwendungen der Stabilisotopenmassenspektrometrie leichter Elemente war die Rekonstruktion des Paläoklimas auf Grundlage der Sauerstoff-Isotopenzusammensetzung (δ18O) von sehr alten, konservierten marinen Carbonaten. Auch heute noch ist dies eine der weitverbreitesten Methoden zur Erforschung der Klimageschichte der Erde. Dieses Paläothermometer basiert auf dem Grundprinzip, dass die Fraktionierung von Sauerstoffisotopen in Calcit temperaturabhängig ist. Das isoprime precisION mit Dual Inlet und MultiCarb ermöglicht eine sehr präzise und zuverlässige, automatisierte Messung von Carbonaten, ideal zur Erzeugung hochauflösender Paläotemperaturaufzeichnungen.

Kosmologie

Die Schwefelisotopenanalyse von extraterrestrischen Sulfidmineralien, z.B. von Meteoriten, hat eine Vielzahl von Erkenntnissen über den Ursprung der Erde und des Sonnensystems ermöglicht. Die Isotopenzusammensetzung von meteoritischem Schwefel (Canyon Diabolo Triolite) wurde auch als der ursprüngliche Bezugspunkt für Erdkruste und -mantel verwendet, von der ausgehend die globalen Fraktionen im Schwefelkreislauf evaluiert werden. Schwefelhaltige Mineralien lassen sich leicht mit unserem vielseitigen Spektrum von EA-IRMS-Systemen analysieren, welche die Advanced Purge and Trap Technologie (APT) für eine unschlagbare Trennung und Peakfokussierung von SO2 nutzen, um eine perfekte Auflösung und eine unvergleichliche Empfindlichkeit für die Mineralschwefelanalyse zu gewährleisten.

Stratigraphie

δ13C-Fluktuationen im Laufe geologischer Zeitalter werden durch Änderungen des Gleichgewichts im Kohlenstoffkreislauf bewirkt. Da die Verweilzeit im Kohlenstoffkreislauf kurz ist (10 ka), werden Änderungen im Kohlenstofffluss im Sediment sehr genau global aufgezeichnet. Als solche sind gelegentliche Spikes im globalen δ13C als stratigraphische Marker für die Chemostratigraphie nützlich, besonders während des Paläozoikums. Der iso FLOW nutzt unsere neuartige UltiTrap-Technologie und ermöglicht so eine präzise, durchgängige Continuous Flow Analyse von Massencarbonaten, um solche Isotopenabweichungen zu identifizieren.

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248 Ergebnisse:

Sulfur isotope constraints on marine transgression in the lacustrine upper cretaceous Songliao Basin, Northeastern China
Palaeogeography, Palaeoclimatology, Palaeoecology (2016)
Hansheng Cao, Alan J. Kaufman, Xuanlong Shan, Huan Cui, Guijie Zhang

Organic-rich Cretaceous source rocks of the petroliferous Songliao Basin in northeast China are considered to be lacustrine in origin, but paleontological and organic geochemical evidence suggest episodic marine incursions. As a test of this hypothesis we applied time-series measurements of elemental and isotopic abundances on core and cutting samples to evaluate fluctuations in the sulfur and carbon cycles across the Santonian-Campanian transition preserved in the upper Yaojia and lower Nenjiang formations. The data reveal a spike in pyrite sulfur abundance and a marked negative excursion in δ34S at the base of the Nenjiang Formation when the basin expanded to its maximal extent. The elemental and isotopic data suggest that flooding was associated with rapid marine transgression that enhanced sulfate concentrations, which promoted microbial sulfate reduction in anoxic bottom waters that were episodically euxinic. Subsequent restriction of the basin and a decline in marine influence is supported by progressive upsection 34S enrichment (up to 30‰) in Nenjiang Member I, which are interpreted to reflect the distillation of sulfate through enhanced pyrite burial, followed by a gradual return to lacustrine conditions that prevailed in overlying strata.

Deciphering processes controlling mid-Jurassic coccolith turnover
Marine Micropaleontology (2016)
Fabienne Giraud, Emanuela Mattioli, Gatsby Emperatriz López-Otálvaro, Christophe Lécuyer, Baptiste Suchéras-Marx, Yves Alméras, François Martineau, Florent Arnaud-Godet, Eric de Kænel

The Middle Jurassic is characterized by major changes within the fossil coccolithophorid community, with a transition from Lotharingius-dominated to Watznaueria-dominated assemblages, concomitant with a significant increase in the pelagic carbonate production. The mechanisms that triggered this overturn remain poorly understood. Here, we present a compilation (new and previously published data) of Lotharingius and Watznaueria abundances through the Early–Middle Jurassic transition. Alongside this, trends in newly-acquired and literature-derived carbon and oxygen isotope data were used to represent paleoceanographic indicators, such as nutrient and temperature changes. The nannofossil data show a rapid (around 1.5Myr) turnover around the Aalenian–Bajocian transition. Across the Aalenian/Bajocian boundary, assemblages dominated by Lotharingius spp. give way to assemblages dominated by Watznaueria spp., coinciding with a peak in a particular morphological group of Watznaueria (species with a cross in the central area). The proliferation of this morphogroup occurred during a time of oceanic opening and rearrangement of ocean circulation. This led on to the evolution of pioneering coccolithophorid taxa, but also to extinctions in several marine groups. In the Early Bajocian, the proliferation of two other morphogroups (Watznaueria without a central-area structure and Watznaueria with a bar) corresponds to the major diversification of Watznaueria, and the beginning of its Mesozoic dominance. The Watznaueria diversification and dominance are associated with radiation in other marine groups, and these biotic changes occurred during a time of putative enhanced oceanic fertility and relatively low temperatures. This study suggests that restructuring of fossil coccolithophorid communities may be favored during short turnover intervals related to major paleoceanographic change.
Schlagworte: carbon , oxygen , geol , mulitcarb

Water concentrations and hydrogen isotope compositions of alkaline basalt hosted clinopyroxene megacrysts and amphibole clinopyroxenites : the role of structural hydroxyl groups and molecular water
Contributions to Mineralogy and Petrology (2016)
Istvan Kovacs, Francois Fourel, Qun-ke Xia, Liu Jia, Etienne Deloule, Istvan Kovacs

The aim of this study was to determine both ‘water’ contents (as OH- and H2O) and δD values of several clinopyroxene samples from alkaline basalts. These parameters were first obtained from five clinopyroxene samples using both the classical ‘off-line’ vacuum extraction technique and the ‘on-line’ high-temperature pyrolysis technique. Blanks measured with the ‘on-line’ gas extraction techniques were low enough to prevent any contamination by atmospheric water vapour. The comparison of data has revealed that our ‘on-line’ procedure is more effective for the extraction of ‘water’ from clinopyroxenes and, consequently, this ‘on-line’ technique was applied to ten additional clinopyroxene samples. Sample δD values cover a similar range from -95‰ to -45‰ (VSMOW) regardless the studied locations, whereas the total ‘water’ content varies from ~115 to ~2570 ppm. The structural hydroxyl content of clinopyroxene samples measured by micro-FTIR spectrometry varies from ~0 to 476 ppm expressed in molecular water equivalent. The total ‘water’ concentrations determined by mass spectrometry differs considerably from structural hydroxyl contents constrained by micro-FTIR, thus indicating that considerable proportion of the ‘water’ may be present in (nano)-inclusions. The structural hydroxyl concentration - apart from clinopyroxenes separated from amphibole clinopyroxenite xenoliths - correlates positively with the δD values of clinopyroxene megacrysts for each locality, indicating that structurally bond hydrogen in clinopyroxenes may have δD values higher than molecular water in inclusions. This implies that that there may be a significant hydrogen isotope fractionation for structural hydroxyl during crystallization of clinopyroxene, while for molecular water there may be no or only negligible isotope fractionation

I-n-Atei palaeolake documents past environmental changes in central Sahara at the time of the “Green Sahara”: Charcoal, carbon isotope and diatom records
Palaeogeography, Palaeoclimatology, Palaeoecology (2016)
Christophe Lécuyer, Anne-Marie Lézine, François Fourel, Françoise Gasse, Florence Sylvestre, Christine Pailles, Christophe Grenier, Yves Travi, Abel Barral

During the ‘Green Sahara event’, water bodies developed throughout the Sahara and Sahel, reflecting the enhanced influence of the Atlantic monsoon rainfall. Major lakes then dried out between 6.5 and 3.5ka. This study investigates land cover change and lacustrine environment during the Holocene at I-n-Atei, Southern Algeria, a desert region lying in the hyperarid core of the Sahara. This site is remarkable by its extent (up to 80km2) and by the exceptional preservation and thickness of the lacustrine deposits (7.2m). I-n-Atei was a lake from 11 to 7.4ka, then it dried out and left place to a swampy environment. Charcoal concentrations show that the surroundings of the lake were vegetated throughout the wet period with two short phases of possible vegetation deterioration associated with a lowering of the lake level at 9.3 and 8.2ka, coeval with well-known dry events in the tropics. The stable carbon isotope record reflects the penetration of C4 herbaceous populations in replacement of the original C3, typical of the regional vegetation at the time of the maximum lake expansion. The δ13C of charcoals increase non-linearly with the 14C-based ages from −24.5‰ to −13.0‰ (V-PDB). Assuming that these extreme values sample both C3 and C4 plant end-members, mass balance calculations suggest that C3 were replaced by C4 plants according to an exponential decay law with a half-life (t1/2) of 850±110years. The replacement of C3 by C4 plants occurred in two main steps: a mixed C3–C4 vegetation of “wooded grassland” type was present from 10ka to 8.4ka while a C4 exclusive vegetation developed after 8.4ka. After the end of the lacustrine phase a catastrophic event (flooding?) provoked the lifting of most of the lacustrine deposits and their re-deposition above the lacustrine sequence.

Interaction of coal and oil in confined pyrolysis experiments: Insight from the yields and carbon isotopes of gas and liquid hydrocarbons
Marine and Petroleum Geology (2016)
Erting Li, Changchun Pan, Shuang Yu, Xiaodong Jin, Jinzhong Liu

Isothermal confined (gold capsule) pyrolysis experiments were performed for coal alone, oil alone and coal plus oil with oil/coal ratios ranging from 0.006 to 0.171 at 315 °C, 345 °C and 375 °C, respectively and 50 MPa for 72 h. In the experiment for coal plus oil, the amounts and compositions of hydrocarbon gases are substantially different from those predicted from the results in the experiments for oil alone and coal alone. The results of these experiments demonstrate that kerogen and oil do not crack separately in the experiments of coal plus oil. The interaction between kerogen and oil components leads to the generation of hydrocarbon gases. With oil/coal ratio increasing, the amounts of individual and total hydrocarbon gases decrease at first, and then increase rapidly up to several times those calculated from the yields of these components in the experiments for oil alone and coal alone. The C1/ΣC1–5 ratios of hydrocarbon gases decrease and are increasingly lower than those calculated from the yields of hydrocarbon gases in the experiments for oil alone and coal alone. The amount and carbon isotopes of individual n-alkane demonstrate that the free liquid n-alkanes were incorporated into kerogen and replaced the bound liquid n-alkanes (covalently bonded alkyl groups in kerogen). Carbon isotopes of hydrocarbon gases further suggest that the bound liquid n-alkanes in kerogen preferentially crack into hydrocarbon gases.
Schlagworte: carbon , geol , oilg , gaschrom

Methane-derived authigenic carbonates of mid-Cretaceous age in southern Tibet: Types of carbonate concretions, carbon sources, and formation processes
Journal of Asian Earth Sciences (2016)
Huimin Liang, Xi Chen, Chengshan Wang, Dekun Zhao, Helmut Weissert

Methane-derived authigenic carbonates with distinctive structures and morphologies have been documented worldwide, but they are rarely found from ancient strata in the Eastern Tethys Ocean. The methane-derived authigenic carbonates found in southern Tibet are developed in calcareous or silty shales of mid-Cretaceous age in the Xigaze forearc basin and in the Tethyan Himalaya tectonic zone. The morphology, mineralogy, elemental geochemistry and composition of carbon and oxygen isotopes of these carbonates are studied in detail. The carbonates have nodular, tubular, and tabular morphologies. They are primarily composed of carbonate cement that binds and partly replaces host sediment grains; host siliciclastic sediments are composed mainly of quartz and plagioclase feldspar; a few foraminifers; and framboidal or subhedral to euhedral pyrite. Carbonate cements dominantly are micritic calcite, with minor contribution of dolomite. Nodular concretions are characterized by depleted δ13C values, commonly ranging from −30‰ to −5‰. The δ13C values show a gradual decrease from the periphery to the center, and the CaO, SiO2, Fe2O3, Al2O3, K2O, and TiO2 contents generally show a gradual change. These features indicate that the nodular concretions grew from an early-formed center toward the periphery, and that the carbon source of the nodular concretions was derived from a mixture of methane, methanogenic CO2, and seawater-dissolved inorganic carbon. The tubular concretions are characterized by δ13C values of −8.85‰ to −3.47‰ in the Shangba Section, and −27.37‰ to −23.85‰ in the upper Gamba Section. Unlike the nodular concretions, the tubular concretions show central conduits, which are possible pathways of methane-rich fluids, suggesting that the cementation of tubular concretions begins at the periphery and proceeds inward. Moreover, the tubular concretions show morphological similarity with the methane-derived carbonate chimneys, pipes and slabs reported in present-day cold seep settings. We suggest that the carbon source of the tubular concretions was derived from a mixture of seawater-dissolved inorganic carbon and oxidized methane formed by released hydrate. The tabular concretions are characterized by δ13C values of −21.87‰ to −6.67‰ in the Xiege Sections. These depleted δ13C values suggest that the carbon of the tabular concretions was derived at least in part, from AOM. The tabular concretions are characterized by δ13C values of −28.81‰ to −12.99‰ in the Gamba Section. According to the δ13C values and field observation, we infer that their carbon source was more likely to be a mixture of the oxidized methane formed by released hydrate and seawater-dissolved inorganic carbon.
Schlagworte: carbon , oxygen , geol , gashead

Palaeotemperature reconstruction during the Last Glacial from δ18O of earthworm calcite granules from Nussloch loess sequence, Germany
Earth and Planetary Science Letters (2016)
Charlotte Prud'homme, Christophe Lécuyer, Pierre Antoine, Olivier Moine, Christine Hatté, François Fourel, François Martineau, Denis-Didier Rousseau

The Nussloch loess–palaeosol sequence (Rhine Valley, Germany) is considered to be one of the most complete records of the last glacial period in Western Europe due to its very high sedimentation rate and its good chronological control. This sequence is therefore a good framework in which to develop new proxies for palaeoenvironmental reconstructions. In this study, we explore, for the first time, the potential of earthworm calcite granules as a new bio-indicator and climatic proxy of absolute air and soil temperature in the context of Last Glacial loess. These granules are composed of rhomboedric calcite crystals, organized in a radial crystalline structure. As these granules are individually generated by earthworms at a relative fast rate, they are expected to record intra-annual variations in the available sources of oxygen: percolating waters of meteoric origin. We extracted thirty earthworm calcite granules from 11 of 5 cm layers thick from tundra gley and brown soil horizons previously, dated at 45 to 23 ka. Oxygen isotope ratios were measured on each individual granule. The δ18O of calcite granules and interlinked transfer functions between water cycle, air and soil temperatures allowed us to estimate air temperatures ranging from 10 to 12 ±4 ◦C, which most likely reflect the warm periods of the year when earthworms were the most active.
Schlagworte: oxygen , geol , clim , mulitcarb

Hydrogeochemical and isotopic tracers for identification of seasonal and long-term over-exploitation of the Pleistocene thermal waters
Environmental Monitoring and Assessment (2016)
Nina Rman

The aim of the study was to develop and test an optimal and cost-effective regional quality monitor- ing system in depleted transboundary low-temperature Neogene geothermal aquifers in the west Pannonian basin. Potential tracers for identification of seasonal and long-term quality changes of the Pleistocene ther- mal waters were investigated at four multiple-screened wells some 720 to 1570 m deep in Slovenia. These thermal waters are of great balneological value owing to their curative effects and were sampled monthly between February 2014 and January 2015. Linear cor- relation and regression analyses, ANOVA and Kolmogorov–Smirnov two-sample test for two indepen- dent samples were used to determine their seasonal and long-term differences. Temperature, pH, electrical con- ductivity, redox potential and dissolved oxygen did not identify varying inflow conditions; however, they pro- vided sufficient information to distinguish between the four end-members. Characteristic (sodium) and conser- vative (chloride) tracers outlined long-term trends in changes in quality but could not differentiate between the seasons. Stable isotopes of δ18Oand δ2Hwere used to identify sequential monthly and long-termtrends, and origin and mixing of waters, but failed to distinguish the difference between the seasons. A new local paleo- meteoric water line (δ2H= 9.2*δ18O+ 26.3) was outlined for the active regional groundwater flow system in the Pannonian to Pliocene loose sandstone and gravel. A new regression line (δ2H=2.3*δ18O– 45.2) was calculated for thermomineral water from the more isolated Badenian to Lower Pannonian turbiditic sandstone, indicating dilution of formation water.Water composition was generally stable over the 1-year period, but long-term trends indicate that changes in quality occur, implying deterioration of the aquifers status.
Schlagworte: hydrogen , oxygen , geol , gashead

Sulfur and carbon geochemistry of the Santa Elena peridotites: Comparing oceanic and continental processes during peridotite alteration
Lithos (2016)
Esther M. Schwarzenbach, Benjamin C. Gill, Esteban Gazel, Pilar Madrigal

Ultramafic rocks exposed on the continent serve as a window into oceanic and continental processes of water–peridotite interaction, so called serpentinization. In both environments there are active carbon and sulfur cycles that contain abiogenic and biogenic processes, which are eventually imprinted in the geochemical signatures of the basement rocks and the calcite and magnesite deposits associated with fluids that issue from these systems. Here, we present the carbon and sulfur geochemistry of ultramafic rocks and carbonate deposits from the Santa Elena ophiolite in Costa Rica. The aim of this study is to leverage the geochemistry of the ultramafic sequence and associated deposits to distinguish between processes that were dominant during ocean floor alteration and those dominant during low-temperature, continental water–peridotite interaction. The peridotites are variably serpentinized with total sulfur concentrations up to 877ppm that is typically dominated by sulfide over sulfate. With the exception of one sample the ultramafic rocks are characterized by positive δ34Ssulfide (up to +23.1‰) and δ34Ssulfate values (up to +35.0‰). Carbon contents in the peridotites are low and are isotopically distinct from typical oceanic serpentinites. In particular, δ13C of the inorganic carbon suggests that the carbon is not derived from seawater, but rather the product of the interaction of meteoric water with the ultramafic rocks. In contrast, the sulfur isotope data from sulfide minerals in the peridotites preserve evidence for interaction with a hydrothermal fluid. Specifically, they indicate closed system abiogenic sulfate reduction suggesting that oceanic serpentinization occurred with limited input of seawater. Overall, the geochemical signatures preserve evidence for both oceanic and continental water–rock interaction with the majority of carbon (and possibly sulfate) being incorporated during continental water–rock interaction. Furthermore, there is evidence for microbial activity that was possibly stimulated by carbon sourced from water–rock interaction with adjacent sediments or fluid inclusions. This study provides detailed insight into the complex hydrothermal history of continental serpentinization systems and adds to our understanding of the carbon and sulfur cycling within peridotite-hosted hydrothermal systems.
Schlagworte: carbon , sulfur , geol , elem

Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition
Geological Society of America Bulletin (2016)
Nathan D. Sheldon, Stephen T. Grimes, Jerry J. Hooker, Margaret E. Collinson, Melanie J. Bugler, Michael T. Hren, Gregory D. Price, Paul A. Sutton

While marine records of the Eocene-Oligocene transition indicate a generally coherent response to global cooling and the growth of continental ice on Antarctica, continental records indicate substantial spatial variability. Marine Eocene-Oligocene transition records are marked by an ~+1.1{per thousand} foraminiferal {delta}18O shift, but continental records rarely record the same geochemical signature, making both correlation and linking of causal mechanisms between marine and continental records challenging. Here, a new high-resolution continental {delta}18O record, derived from the freshwater gill-breathing gastropod Viviparus lentus, is presented from the Hampshire Basin, UK. The Solent Group records marine incursions and has an established magnetostratigraphy, making it possible to correlate the succession directly with marine records. The V. lentus {delta}18O record indicates a penecontemporaneous, higher-magnitude shift (>+1.4{per thousand}) than marine records, which reflects both cooling and a source moisture compositional shift consistent with the growth of Antarctic ice. When combined with "clumped" isotope measurements from the same succession, about half of the isotopic shift can be attributed to cooling and about half to source moisture change, proportions similar to marine foraminiferal records. Thus, the new record indicates strong hydrological cycle connections between marine and marginal continental environments during the Eocene-Oligocene transition not observed in continental interior records.
Schlagworte: oxygen , geol , ocea , clim , gashead