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  • 1
    Online Resource
    Online Resource
    Berlin, Heidelberg :Springer Berlin / Heidelberg,
    Keywords: Biochemical engineering. ; Electronic books.
    Description / Table of Contents: This text presents key advances in calcium and magnesium isotope geochemistry including those of biological fractionation and biochemical applications. The presentation intimately links the method and applications chapters.
    Type of Medium: Online Resource
    Pages: 1 online resource (269 pages)
    Edition: 1st ed.
    ISBN: 9783540689539
    Series Statement: Advances in Isotope Geochemistry Series
    DDC: 551.9
    Language: English
    Note: Intro -- Preface -- Contents -- 1 Introduction -- Abstract -- 1 Introduction to Calcium Stable Isotope Geochemistry -- 1.1 Alkaline Earth Elements -- 1.2 Calcium and Its Isotopes -- 1.3 Notations in Ca Stable Isotope Geochemistry -- 1.4 History of Ca Stable Isotope Research -- 1.5 Applications of Ca Stable Isotope Geochemistry -- 1.6 Other Applications of Ca Isotopes: Cosmogenic 41Ca and Tracer Studies -- 2 Principles of Mass-Dependent Stable Isotope Fractionation -- 2.1 Equilibrium Isotope Partitioning -- 2.1.1 Isotope Exchange Reactions -- 2.1.2 Equilibrium Isotope Fractionation Theory -- 2.1.3 Summary of General Characteristics of Equilibrium Isotope Fractionation -- 2.2 Kinetic Stable Isotope Fractionation -- 2.3 Open System Rayleigh Fractionation and Closed System Equilibrium Fractionation -- 2.4 The Mass-Dependence of Equilibrium and Kinetic Stable Isotope Fractionations -- 2.5 Experimental Determination of Equilibrium Isotope Fractionation Factors -- References -- A selection of texts relevant for stable isotope geochemistry -- Reviews on calcium stable isotope geochemistry -- Additonal publications cited in this chapter -- 2 Analytical Methods -- Abstract -- 1 Introduction -- 2 Notations and Data Presentation -- 2.1 δ-Notation -- 2.2 Fractionation Factor (α) -- 2.3 Δ-Notation -- 2.4 εCa-Notation for Radiogenic 40Ca Ingrowth -- 2.5 ε- and µ-Notations in Cosmochemistry -- 3 Reference Materials -- 3.1 Used Reference Materials -- 3.2 Conversion of δ-Values Based on Different Reference Materials -- 4 Sample Preparation -- 4.1 Digestion and Cleaning Techniques -- 4.1.1 Carbonates -- 4.1.2 Phosphates -- 4.1.3 Sulfates -- 4.1.4 Silicate Minerals, Rocks and Soils -- 4.1.5 Organic Samples -- 4.1.6 Liquid Samples -- 4.1.7 Leachates -- 4.2 Chemical Separation -- 5 Mass Spectrometry -- 5.1 Introduction to Mass Spectrometry for Ca Isotope Analysis. , 5.2 Thermal Ionization Mass Spectrometry (TIMS) -- 5.2.1 Mass Discrimination in TIMS and the Exponential Law Correction -- 5.2.2 Analysis of Radiogenic 40Ca by TIMS -- 5.2.3 Calcium Stable Isotope Analysis by TIMS -- 5.3 Multiple Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) -- 5.3.1 Basics of MC-ICP-MS -- 5.3.2 Mass Discrimination and Matrix Effects in (MC-)ICP-MS -- 5.3.3 Interferences in (MC-)ICP-MS -- 5.3.4 Calcium Isotope Analysis by (MC-)ICP-MS -- 5.4 Double Spike Approach for Stable Isotope Analysis -- 5.4.1 Basic Principles -- 5.4.2 Double Spike Calibration -- 5.4.3 Used Double Spikes -- 5.5 Other Instrumentation -- 5.5.1 Ion Microprobe -- 5.5.2 Neutron Activation Analysis INAA -- 5.5.3 Determination of Mass Dependent Ca Isotope Fractionation by Radionuclide Tracers -- 5.6 Error Representation -- References -- 3 Calcium Isotope Fractionation During Mineral Precipitation from Aqueous Solution -- Abstract -- 1 Inorganic Precipitation Experiments -- 1.1 Carbonates -- 1.1.1 Calcite -- 1.1.2 Aragonite -- 1.1.3 Vaterite -- 1.1.4 Ikaite -- 1.1.5 Amorphous Calcium Carbonate -- 1.1.6 Magnesite, Dolomite, Dypingite and Nesquehonite -- 1.1.7 Witherite and BaMn(CO3)2 -- 1.2 Sulfates -- 1.2.1 Anhydrite -- 1.2.2 Gypsum -- 1.2.3 Barite -- 1.2.4 Epsomite -- 1.3 Other (Hydrous) Phases -- 1.4 EASI Fractionation During Mineral Precipitation from Aqueous Fluids -- 2 Calcium Isotope Fractionation Models for Calcium Carbonate Formation -- 2.1 Principles and Conceptions of Isotope Fractionation Models -- 2.2 Comparison of Ca Isotope Fractionation Models and Concluding Remarks -- 3 Inorganic Mineral Precipitation in Natural Environments -- 3.1 Carbonates -- 3.1.1 Primary and Authigenic Carbonates -- Marine Realm -- Lakes -- Soils -- 3.1.2 Carbonates Formed at Elevated Temperatures -- 3.1.3 Carbonate Recrystallization -- 3.2 Phosphates. , 3.3 Sulfates -- 3.3.1 Barite -- 3.3.2 Anhydrite and Gypsum -- Marine and Hydrothermal Ca Sulfates -- Calcium Sulfates in Soils -- 4 Diffusion, Exchange and Adsorption of Cations in Aqueous Systems -- References -- 4 Biominerals and Biomaterial -- Abstract -- 1 Procaryota-Microbial Induced Biomineralisation -- 2 Protista -- 2.1 Foraminifera -- 2.1.1 Planktic Foraminifers -- 2.1.2 Benthonic Foraminifers -- 2.2 Coccolithophores -- 2.3 Calcareous Dinoflagellates -- 2.4 Coralline Algae -- 3 Metazoa -- 3.1 Sclerosponges -- 3.2 Corals -- 3.3 Molluscs -- 3.3.1 Bivalves -- 3.3.2 Gastropods and Polyplacophora -- 3.3.3 Cephalopods -- 3.4 Brachiopods -- 3.5 Other Taxa -- 3.5.1 Echinoderms -- 3.5.2 Vertebrates -- 4 Applications, Ecosystems and Climate Change -- 4.1 Monitor of Trophic Levels -- 4.2 Archaeology -- 4.3 Paleoclimate -- References -- 5 Earth-Surface Ca Isotopic Fractionations -- Abstract -- 1 Introduction -- 2 δ44/40Ca Fractionations Related to Continental Weathering Processes -- 2.1 Range of δ44/40Ca Variations in Earth-Surface Processes -- 2.2 Forested Ecosystems -- 2.2.1 Ca Cycling Through the Vegetation -- 2.2.2 δ44/40Ca Variations Within the Vegetation -- 2.2.3 δ44/40Ca Variations Within the Soil Pool -- 2.2.4 δ44/40Ca Variations Within Atmospheric Deposits -- 2.3 Non-forested Ecosystems -- 2.3.1 Abiotic Processes -- 2.3.2 δ44/40Ca Fractionations Caused by Water-Rock Interactions -- 3 Change in δ44/40Ca Signature During Downstream Transportation into the Ocean -- 3.1 Importance of the δ44/40Ca Weathering Flux to the Oceans -- 3.2 Small Scale Catchments -- 3.3 Global Scale Catchments -- 4 Potential of Ca Isotopes Applied to Earth-Surface Processes -- 4.1 Internal Ca Cycling Processes Within the Tree -- 4.2 Ca Uptake Mechanisms Within the Rhizosphere -- 4.3 Ca Recycling by the Vegetation -- 4.4 Time Integrated Vegetal Turnover Marker. , 4.5 Hydrological Tracer -- 5 Conclusion -- References -- 6 Global Ca Cycles: Coupling of Continental and Oceanic Processes -- Abstract -- 1 Introduction -- 2 Principal Ca Reservoirs at the Earth's Surface: Estimates of {\varvec \delta}^{{\frac{{{{\bf 44}}}}{{{{\bf 40}}}}}}Ca -- 2.1 Rocks -- 2.1.1 Continental Silicates -- 2.1.2 Continental Sediments -- 2.2 Hydrosphere -- 2.2.1 Oceans -- 2.2.2 Rivers -- Partitioning Mineral Inputs of Ca to the Critical Zone -- Rivers Draining Silicate Rocks -- Rivers Draining Carbonate Rocks -- Large Rivers -- River Sediments and Colloids -- 2.2.3 Groundwaters -- 2.3 Biospheric Cycling of Ca -- 2.4 Soils -- 2.4.1 Weathering of Primary Minerals -- 2.4.2 Soil Carbonate Biomineralization -- 2.4.3 The Rhizosphere -- 2.4.4 Humic Substances -- 2.4.5 Soil Exchangeable Pool -- 2.4.6 Soil Porewaters -- 2.5 Atmospheric Ca in Dust and Rain Waters -- 2.5.1 Dry Deposition and Dust -- 2.5.2 Wet Deposition -- 2.5.3 Interaction Between the Biosphere and Atmospheric Deposition -- 3 Modern Global Budgets of Ca -- 3.1 The Continental Cycle of Ca -- 3.2 The Oceanic Cycle of Ca -- 4 Global Ca Cycling in Earth's History -- 4.1 Archives of \delta^{{\frac{44}{40}}}CaSW -- 4.1.1 Bulk Carbonate -- 4.1.2 Abiotic Records of \delta^{{\frac{44}{40}}}CaSW -- Barite Mineral Separates -- Gypsum and Anhydrite -- Apatite -- 4.1.3 Taxon-Specific Records of \delta^{{\frac{44}{40}}}CaSW -- Foraminifera -- Brachiopods -- Cephalopods -- Bivalves -- Biogenic Apatite -- Other Taxa -- 4.2 Past Changes in {\varvec \delta}^{{\frac{{{{\bf 44}}}}{{{{\bf 40}}}}}}CaSW -- 4.2.1 Neogene Seawater Records of Ca Isotopes -- 4.2.2 Phanerozoic and Deeper Time Seawater Records of Ca Isotopes -- 5 Conclusions -- References -- 7 High Temperature Geochemistry and Cosmochemistry -- Abstract. , 1 High Temperature Ca Isotope Geochemistry of Terrestrial Silicate Rocks, Minerals and Melts -- 1.1 Calcium Isotope Fractionation Between Minerals Formed at High Temperatures -- 1.2 Igneous Rocks -- 1.3 The Earth's Silicate Reservoirs and Global Tectonics -- 1.3.1 Ca Isotope Composition of the Upper Mantle and Bulk Silicate Earth -- 1.3.2 Traces of Subducted Sedimentary Carbonate in Ocean Island Basalts -- 1.3.3 Metasomatism -- 1.4 Diffusion in Silicate Melts -- 1.4.1 Chemical and Self-diffusion -- 1.4.2 Thermal Diffusion -- 2 Extraterrestrial Materials -- 2.1 Scope and Framework of Ca Isotope Cosmochemistry -- 2.2 Nucleosynthesis of Ca Isotopes -- 2.3 Nucleosynthetic Ca Isotope Signatures in Presolar Grains -- 2.4 Nucleosynthetic Anomalies in Meteorites and Calcium-Aluminum-Rich Inclusions (CAIs) -- 2.5 Mass-Dependent Variations in CAIs and Related Experiments -- 2.6 Mass-Dependent Variations in Meteorites -- 2.7 Lunar Samples -- 3 The 40K-40Ca Decay System -- 3.1 Evolution of Earth's Reservoirs -- 3.1.1 Earth's Mantle -- 3.1.2 Earth's Crust -- 3.1.3 Seawater and Exogenic Ca Cycling -- 3.2 Dating -- 3.2.1 Igneous Rocks -- 3.2.2 Authigenic Sedimentary Minerals -- 3.2.3 Evaporites -- References -- 8 Biomedical Application of Ca Stable Isotopes -- Abstract -- 1 Introduction -- 2 Ca Isotope Transport Model -- 2.1 Ca Isotopic Composition of the Diet -- 2.2 From Food to Blood -- 2.3 Fractionation Between Soft Tissue and Mineralized Tissue -- 2.4 Fractionation in the Kidneys -- 2.5 Calcium Isotope Fractionation During Milk Lactation -- 2.6 Calcium Use Index (CUI) -- 3 The Individuality of the Ca Metabolism -- 4 Current Biomedical Application of Ca Isotopes -- 4.1 Bone Loss -- 4.2 Bone Cancer -- 5 Summary and Outlook -- References.
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  • 2
    Online Resource
    Online Resource
    Berlin, Heidelberg : Springer Berlin Heidelberg
    Keywords: Earth sciences ; Earth Sciences ; Biochemical engineering ; Geochemistry ; Geotechnical engineering ; Earth sciences ; Biochemical engineering ; Geochemistry ; Geotechnical engineering ; Calciumisotop ; Isotopengeochemie ; Stabiles Isotop ; Calcium ; Biochemie ; Biogeochemie ; Biomineralisation ; Mineralchemie ; Calciumisotop ; Geochemie ; Stoffwechsel
    Description / Table of Contents: Analytical Methods -- Calcium Isotope Fractionation During Mineral Precipitation from Aqueous Solution -- Biominerals and Biomaterial -- Earth-surface Ca Isotopic Fractionations -- High Temperature Geochemistry and Cosmochemistry -- Biomedical Application of Ca Stable Isotopes.
    Type of Medium: Online Resource
    Pages: Online-Ressource (X, 260 p. 102 illus., 37 illus. in color, online resource)
    ISBN: 9783540689539
    Series Statement: Advances in Isotope Geochemistry
    RVK:
    Language: English
    Note: Includes bibliographical references
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  • 3
    Publication Date: 2021-12-13
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 4
    Publication Date: 2021-02-08
    Description: The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale. In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175°C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons. We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product. At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2020-02-06
    Description: In this study we examine the behavior of stable Sr isotopes between strontianite [SrCO3] and reactive fluid during mineral dissolution, precipitation, and at chemical equilibrium. Experiments were performed in batch reactors at 25 °C in 0.01 M NaCl solutions wherein the pH was adjusted by bubbling of a water saturated gas phase of pure CO2 or atmospheric air. The equilibrium Sr isotope fractionation between strontianite and fluid after dissolution of the solid under 1 atm CO2 atmosphere was estimated as Δ88/86SrSrCO3-fluid = δ88/86Sr SrCO3 − δ88/86Srfluid = −0.05 ± 0.01‰. On the other hand, during strontianite precipitation, an enrichment of the fluid phase in 88Sr, the heavy isotopomer, was observed. The evolution of the δ88/86Srfluid during strontianite precipitation can be modeled using a Rayleigh distillation approach and the estimated, kinetically driven, fractionation factor αSrCO3-fluid between solid and fluid is calculated to be 0.99985 ± 0.00003 corresponding to Δ88/86SrSrCO3-fluid = −0.15‰. The obtained results further support that under chemical equilibrium conditions between solid and fluid a continuous exchange of isotopes occurs until the system approaches isotopic equilibrium. This isotopic exchange is not limited to the outer surface layer of the strontianite crystal, but extends to ∼7–8 unit cells below the crystal surface. The behavior of Sr isotopes in this study is in excellent agreement with the concept of dynamic equilibrium and it suggests that the time needed for achievement of chemical equilibrium is generally shorter compared to that for isotopic equilibrium. Thus it is suggested that in natural Sr-bearing carbonates an isotopic change may still occur close to thermodynamic equilibrium, despite no observable change in aqueous elemental concentrations. As such, a secondary and ongoing change of Sr isotope signals in carbonate minerals caused by isotopic re-equilibration with fluids has to be considered in order to use Sr isotopes as environmental proxies in aquatic environments.
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2020-06-26
    Description: In order to apply Sr/Ca and 44Ca/40Ca fractionation during calcium carbonate (CaCO3) formation as a proxy to reconstruct paleo-environments, it is essential to evaluate the impact of various environmental factors. In this study, a CO2 diffusion technique was used to crystallize inorganic calcite from aqueous solutions at different ionic strength/salinity by the addition of NaCl at 25 °C. Results show that the discrimination of Sr2+ versus Ca2+ during calcite formation is mainly controlled by precipitation rate (R in μmol/m2/h) and is weakly influenced by ionic strength/salinity. In analogy to Sr incorporation, 44Ca/40Ca fractionation during precipitation of calcite is weakly influenced by ionic strength/salinity too. At 25 °C the calcium isotope fractionation between calcite and aqueous calcium ions (Δ44/40Cacalcite-aq = δ44/40Cacalcite − δ44/40Caaq) correlates inversely to log R values for all experiments. In addition, an inverse relationship between Δ44/40Cacalcite-aq and log DSr, which is independent of temperature, precipitation rate, and aqueous (Sr/Ca)aq ratio, is not affected by ionic strength/salinity either. Considering the log DSr and Δ44/40Cacalcite-aq relationship, Sr/Ca and δ44/40Cacalcite values of precipitated calcite can be used as an excellent multi-proxy approach to reconstruct environmental conditions (e.g., temperature, precipitation rate) of calcite growth and diagenetic alteration.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2019-09-23
    Description: Conflicting results have been reported for the stable Sr isotope fractionation, specifically with respect to the influence of temperature. In an experimental study we have investigated the stable Sr isotope systematics for inorganically precipitated and biogenic (coral) aragonite (natural and laboratory-cultured). Inorganic aragonite precipitation experiments were performed from natural seawater using the CO2 diffusion technique. The experiments were performed at different temperatures and different carbonate ion concentrations. 88Sr/86Sr of the inorganic aragonite precipitated in the experiments are 0.2‰ lighter than seawater, but showed no correlation to the water temperature or to CO32− concentration. Similar observations are made in different coral species (Cladocora caespitosa, Porites sp. and Acropora sp.), with identical fractionation from the bulk solution and no correlation to temperature or CO32− concentration. The lack of 88Sr/86Sr variability in corals at different environmental parameters and the similarity to the 88Sr/86Sr fractionation in inorganic aragonite may indicate a similar Sr incorporation mechanism in corals skeleton and inorganic aragonite, and therefore the previously proposed Rayleigh-based multi element model (Gaetani et al., 2011) cannot explain the process of Sr incorporation in the coral skeletal material. It is proposed that the relatively constant 88Sr/86Sr fractionation in aragonite can be used for paleo reconstruction of seawater 88Sr/86Sr composition. The seawater 88Sr/86Sr ratio reconstruction can be further used in calcite samples to reconstruct paleo precipitation rates.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 8
    Publication Date: 2019-02-01
    Description: Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the Phanerozoic in marine waters. Consequently, traditional and non-traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties. This paper tests the sensitivity of brachiopod shell magnesium isotope (δ26Mg) data to diagenetic alteration. We apply a dual approach by: (i) performing hydrothermal alteration experiments using meteoric, marine, and burial reactive fluids; and (ii) comparing these data to naturally altered, ancient brachiopod shells. The degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy. The absence of luminescence might indicate both well-preserved shell material, but also the secondary enrichment of quenching elements such as iron along diagenetic pathways. Complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells. Brachiopod shell magnesium isotope values respond to differential fluid temperature, chemistry, and experiment durations. The patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals. Generally, the range in bulk δ26Mg from experimentally altered (1.52‰) and that of bulk samples from ancient, diagenetically altered brachiopod valves (1.53‰) exceed the geochemical variability of δ26Mgbrachiopod bulk values of most recent specimens (1.26‰) in the lower and upper range. More 26Mg enriched (0.8‰) and more 26Mg depleted (0.7‰) values, respectively, are found in altered shells in comparison to unaltered ones. The data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives. Consequently, we propose tentative guidelines for magnesium isotope research applied to ancient carbonates.
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2019-01-21
    Description: Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth 〈 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l−1 and in concentrations of Ca2+, Mg2+, Na+ and Si(OH)4 compared to the shallow type. The δ2H and δ18O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ13CDIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO2 from local soil horizons, whereas δ13CDIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO2, δ13CDIC = +1‰ of the deep groundwater with highest PCO2 = 10−0.9 atm yields δ13CCO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO2. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO2 from soil and magmatic origin. The uptake of magmatic CO2 results in elevated cations and Si(OH)4 concentrations. Weathering of local basalts is documented by 87Sr/86Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO2 from the solution leading to secondary calcite formation.
    Type: Article , PeerReviewed
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  • 10
    Publication Date: 2020-10-16
    Description: We have investigated the strontium isotope fractionation (Δ88/86Srcarb–aq) between inorganic calcite and aqueous Sr2+ ions by precipitation experiments at a constant temperature of 25 °C and precipitation rates (R) ranging from 102.3 to 104.2 μmol/m2/h. Strontium isotope ratios were measured using the 87Sr–84Sr double spike technique. It was found that strontium isotope fractionation in these calcites is strongly dependent on the precipitation rate: View the MathML source The measured δ88/86Sr values are significantly correlated with previously measured δ44/40Ca and Sr/Ca values of the same calcite samples: Δ88/86Srcarb–aq=+0.18∗Δ44/40Cacarb–aq-0.01 View the MathML source The slope of 88Sr/86Sr versus 44Ca/40Ca fractionation is 0.18 ± 0.04 and compatible with a kinetic fractionation during dehydration of the strontium and calcium ions, but not with isotope fractionation in a diffusive boundary layer. Using published equilibrium Δ44/40Cacarb–aq and View the MathML source values we estimate the equilibrium isotope fractionation of strontium to be very close to zero (Δ88/86Sreq(carb–aq) = −0.01 ± 0.06‰). This estimate is confirmed by strontium isotope values of natural inorganic calcites that precipitated very slowly in basalts of the ocean crust. The results from the inorganic calcites are used to explain strontium isotope fractionation of planktic foraminifera. Specimens of two warm water species (Globigerinoides ruber and Globigerinoides sacculifer) were picked from the Holocene section of a Caribbean sediment core. We found no significant difference in δ88/86Sr between the two species. In addition, G. ruber specimens from Marine Isotope Stage 2 in the same core show δ88/86Sr values identical to the Holocene specimens. The strontium isotopes of both foraminifera species are strongly fractionated (Δ88/86Srcarb–aq = −0.248 ± 0.005‰) when compared to published data of other major marine calcifiers. Applying the results from the inorganic precipitation experiments we find that the strong foraminiferal strontium isotope fractionation can be explained by calcification in a largely open system at high precipitation rates, comparable in magnitude to rates known from scleractinian reef corals. This interpretation is in good agreement with the kinetic calcification model for planktic foraminifera by Kisakürek et al. (2011), which was based on calcium isotopes and elemental Sr/Ca ratios.
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