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  • 1
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    Publication Date: 2019-10-01
    Description: Fossil brachiopod shells are often used as valuable archives to reconstruct paleoenvironmental conditions in deep time. However, biomineralization processes can impact their fidelity as geochemical proxies. Brachiopod shells comprise an outer primary layer, a secondary fibrous layer and sometimes, a tertiary columnar layer. Therefore, it is essential to assess the potential effects of the biomineralization processes in each of the different shell microstructures of modern brachiopods. This study analyses the oxygen isotopic composition together with Li/Ca, Na/Ca Mg/Ca and Sr/Ca data at high spatial (20-50 μm) resolution in seven modern brachiopod species, focusing on differences between the primary, secondary and tertiary layers. In all studied species, δ18O values of the outer primary layer are consistently out of equilibrium with seawater. Also, this shell layer is enriched in Li, Na, Mg and Sr. Contrary to the primary layer, the innermost secondary layer is near or at oxygen isotopic and elemental equilibrium with ambient seawater. The columnar tertiary shell layer, if present, has the least variable and the heaviest oxygen isotopic composition, within the range of equilibrium values with seawater. This tertiary layer, however, is depleted in minor and trace elements relative to the other shell layers. Thus, the tertiary layer is more suitable for oxygen isotopic studies, whereas the innermost secondary layer of the most mature parts of the shell is the best target in two-layered shells. While we do not observe any clear interspecific relationships between Mg/Ca and Sr/Ca ratios, on one hand, and environmental parameters such as temperature, salinity and pH, on the other hand, there is a positive interspecific relationship between Na/Ca and salinity and a negative interspecific relationship between Li/Ca and temperature, suggesting their potential use as proxies of physicochemical parameters of seawater.
    Print ISSN: 0009-2541
    Electronic ISSN: 1872-6836
    Topics: Chemistry and Pharmacology , Geosciences
    Published by Elsevier
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  • 4
    Publication Date: 2021-02-08
    Description: Fossil carbonate skeletons of marine organisms are archives for understanding the development and evolution of palaeo-environments. However, the correct assessment of past environment dynamics is only possible when pristine skeletons and their biogenic characteristics are unequivocally distinguishable from diagenetically-altered skeletal elements and non-biogenic features. In this study, we extend our work on diagenesis of biogenic aragonite (Casella et al. 2017) to the investigation of biogenic low-Mg calcite using brachiopod shells. We examined and compared microstructural characteristics induced by laboratory-based alteration to structural features derived from diagenetic alteration in natural environments. We used four screening methods: cathodoluminescence (CL), cryogenic and conventional field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and electron backscatter diffraction (EBSD). We base our assessments of diagenetic alteration and overprint on measurements of, a) images of optical overprint signals, b) changes in calcite crystal orientation patterns, and c) crystal co-orientation statistics. According to the screening process, altered and overprinted samples define two groups. In Group 1 the entire shell is diagenetically overprinted, whereas in Group 2 the shell contains pristine as well as overprinted parts. In the case of Group 2 shells, alteration occurred either along the periphery of the shell including the primary layer or at the interior-facing surface of the fibrous/columnar layer. In addition, we observed an important mode of the overprinting process, namely the migration of diagenetic fluids through the endopunctae corroborated by mineral formation and overprinting in their immediate vicinity, while leaving shell parts between endopunctae in pristine condition. Luminescence (CL) and microstructural imaging (FE-SEM) screening give first-order observations of the degree of overprint as they cover macro-to micron scale alteration features. For a comprehensive assessment of diagenetic overprint these screening methods should be complemented by screening techniques such as EBSD and AFM. They visualise diagenetic changes at submicron and nanoscale levels depicting the replacement of pristine nanocomposite mesocrystal biocarbonate (NMB) by inorganic rhombohedral calcite (IRC). The integration of screening methods allows for the unequivocal identification of highly-detailed alteration features as well as an assessment of the degree of diagenetic alteration.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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  • 5
    Publication Date: 2022-01-31
    Description: Highlights • Cell-reorganization; commissure: muti-cell-layered, central shell: single-cell-layered. • Individual fibres are secreted by several cells at the same time. • Tight cooperation of cells for the coordinated secretion of organic membrane and mineral. • Lack of extrapallial space between OME cells and developing fibres. • Mineral transport to sites of mineralization occurs via ion transport through cell membrane. Abstract To understand mineral transport pathways for shell secretion and to assess differences in cellular activity during mineralization, we imaged with TEM and FE-SEM ultrastructural characteristics of outer mantle epithelium (OME) cells. Imaging was carried out on Magellania venosa shells embedded/etched, chemically fixed/decalcified and high-pressure frozen/freeze-substituted samples from the commissure, central shell portions and from puncta. Imaging results are complemented with morphometric evaluations of volume fractions of membrane-bound organelles. At the commissure the OME consists of several layers of cells. These cells form oblique extensions that, in cross-section, are round below the primary layer and flat underneath fibres. At the commissure the OME is multi-cell layered, in central shell regions it is single-cell layered. When actively secreting shell carbonate extrapallial space is lacking, because OME cells are in direct contact with the calcite of the forming fibres. Upon termination of secretion, OME cells attach via apical hemidesmosomes to extracellular matrix membranes that line the proximal surface of fibres. At the commissure volume fractions for vesicles, mitochondria and lysosomes are higher relative to single-cell layered regions, whereas for endoplasmic-reticulum and Golgi apparatus there is no difference. FE-SEM, TEM imaging reveals the lack of extrapallial space between OME cells and developing fibres. In addition, there is no indication for an amorphous precursor within fibres when these are in active secretion mode. Accordingly, our results do not support transport of minerals by vesicles from cells to sites of mineralization, rather by transfer of carbonate ions via transport mechanisms associated with OME cell membranes.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: image
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  • 6
    Publication Date: 2022-01-31
    Description: Fossil brachiopod shells are often used as valuable archives to reconstruct paleoenvironmental conditions in deep time. However, biomineralization processes can impact their fidelity as geochemical proxies. Brachiopod shells comprise an outer primary layer, a secondary fibrous layer and sometimes, a tertiary columnar layer. Therefore, it is essential to assess the potential effects of the biomineralization processes in each of the different shell microstructures of modern brachiopods. This study analyses the oxygen isotopic composition together with Li/Ca, Na/Ca Mg/Ca and Sr/Ca data at high spatial (20-50 μm) resolution in seven modern brachiopod species, focusing on differences between the primary, secondary and tertiary layers. In all studied species, δ18O values of the outer primary layer are consistently out of equilibrium with seawater. Also, this shell layer is enriched in Li, Na, Mg and Sr. Contrary to the primary layer, the innermost secondary layer is near or at oxygen isotopic and elemental equilibrium with ambient seawater. The columnar tertiary shell layer, if present, has the least variable and the heaviest oxygen isotopic composition, within the range of equilibrium values with seawater. This tertiary layer, however, is depleted in minor and trace elements relative to the other shell layers. Thus, the tertiary layer is more suitable for oxygen isotopic studies, whereas the innermost secondary layer of the most mature parts of the shell is the best target in two-layered shells. While we do not observe any clear interspecific relationships between Mg/Ca and Sr/Ca ratios, on one hand, and environmental parameters such as temperature, salinity and pH, on the other hand, there is a positive interspecific relationship between Na/Ca and salinity and a negative interspecific relationship between Li/Ca and temperature, suggesting their potential use as proxies of physicochemical parameters of seawater.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 7
    Publication Date: 2022-01-31
    Description: CO2-induced ocean acidification and associated decrease of seawater carbonate saturation state contributed to multiple environmental crises in Earth’s history, and currently poses a major threat for marine calcifying organisms. Owing to their high abundance and good preservation in the Phanerozoic geological record, brachiopods present an advantageous taxon of marine calcifiers for palaeo-proxy applications as well as studies on biological mechanism to cope with environmental change. To investigate the geochemical and physiological responses of brachiopods to prolonged low-pH conditions we cultured Magellania venosa, Terebratella dorsata and Pajaudina atlantica under controlled experimental settings over a period of more than two years. Our experiments demonstrate that brachiopods form their calcite shells under strong biological control, which enables them to survive and grow under low-pH conditions and even in seawater strongly undersaturated with respect to calcite (pH = 7.35, Ωcal = 0.6). Using boron isotope (δ11B) systematics including MC-ICP-MS as well as SIMS analyses, validated against in vivo microelectrode measurements, we show that this resilience is achieved by strict regulation of the calcifying fluid pH between the epithelial mantle and the shell. We provide a culture-based δ11B−pH calibration, which as a result of the internal pH regulatory mechanisms deviates from the inorganic borate ion to pH relationship, but confirms a clear yet subtle pH dependency for brachiopods. At a micro-scale level, the incorporation of 11B appears to be principally driven by a physiological gradient across the shell, where the δ11B values of the innermost calcite record the internal calcifying fluid pH while the composition of the outermost layers is also influenced by seawater pH. These findings are of consequence to studies on biomineralisation processes, physiological adaptations as well as past climate reconstructions.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 8
    Publication Date: 2023-02-08
    Description: Brachiopods present a key fossil group for Phanerozoic palaeo-environmental and palaeo-oceanographical reconstructions, owing to their good preservation and abundance in the geological record. Yet to date, hardly any geochemical proxies have been calibrated in cultured brachiopods and only little is known on the mechanisms that control the incorporation of various key elements into brachiopod calcite. To evaluate the feasibility and robustness of multiple Element/Ca ratios as proxies in brachiopods, specifically Li/Ca, B/Ca, Na/Ca, Mg/Ca, Sr/Ca, Ba/Ca, as well as Li/Mg, we cultured Magellania venosa, Terebratella dorsata and Pajaudina atlantica under controlled experimental settings over a period of more than two years with closely monitored ambient conditions, carbonate system parameters and elemental composition of the culture medium. The experimental setup comprised of two control aquariums (pH0 = 8.0 and 8.15, T = 10 °C) and treatments where pCO2−pH (pH1 = 7.6 and pH2 = 7.35), temperature (T = 16 °C) and chemical composition of the culture medium were manipulated. Our results indicate that the incorporation of Li and Mg is strongly influenced by temperature, growth effects as well as carbonate chemistry, complicating the use of Li/Ca, Mg/Ca and Li/Mg ratios as straightforward reliable proxies. Boron partitioning varied greatly between the treatments, however without a clear link to carbonate system parameters or other environmental factors. The partitioning of both Ba and Na varied between individuals, but was not systematically affected by changes in the ambient conditions. We highlight Sr as a potential proxy for DIC, based on a positive trend between Sr partitioning and carbonate chemistry in the culture medium. To explain the observed dependency and provide a quantitative framework for exploring elemental variations, we devise the first biomineralisation model for brachiopods, which results in a close agreement between modelled and measured Sr distribution coefficients. We propose that in order to sustain shell growth under increased DIC, a decreased influx of Ca2+ to the calcifying fluid is necessary, driving the preferential substitution of Sr2+ for Ca2+ in the crystal lattice. Finally, we conducted micro-computed tomography analyses of the shells grown in the different experimental treatments. We present pore space – punctae – content quantification that indicates that shells built under increased environmental stress, and in particular elevated temperature, contain relatively more pore space than calcite, suggesting this parameter as a potential novel proxy for physiological stress and even environmental conditions.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 9
    Publication Date: 2024-02-07
    Description: Lithium has proven a powerful tracer of weathering processes and chemical seawater evolution. Skeletal components of marine calcifying organisms, and in particular brachiopods, present promising archives of Li signatures. However, Li incorporation mechanisms and potential influence from biological processes or environmental conditions require a careful assessment. In order to constrain Li systematics in brachiopod shells, we present Li concentrations and isotope compositions for 11 calcitic brachiopod species collected from six different geographic regions, paralleled with data from culturing experiments where brachiopods were grown under varying environmental conditions and seawater chemistry (pH–pCO2, temperature, Mg/Ca ratio). The recent brachiopod specimens collected across different temperate and polar environments showed broadly consistent δ7Li values ranging from 25.2 to 28.1‰ (with mean δ7Li of 26.9 ± 1.5‰), irrespective of taxonomic rank, indicating that incorporation of Li isotopes into brachiopod shells is not strongly affected by vital effects related to differences among species. This results in Δ7Licalcite–seawater values (per mil difference in 7Li/6Li between brachiopod calcite shell and seawater) from −2.9‰ to −5.8‰ (with mean Δ7Licalcite–seawater value of −3.6‰), which is larger than the Δ7Licalcite–seawater values calculated based on data from planktonic foraminifera (~0‰ to ~−4‰). This range of values is further supported by results from brachiopods cultured experimentally. Under controlled culturing conditions simulating the natural marine environment, the Δ7Licalcite–seawater for Magellania venosa was −2.5‰ and not affected by an increase in temperature from 10 to 16 °C. In contrast, a decrease in Mg/Ca (or Li/Ca) ratio of seawater by addition of CaCl2 as well as elevated pCO2, and hence low-pH conditions, resulted in an increased Δ7Licalcite-seawater up to −4.6‰. Collectively, our results indicate that brachiopods represent valuable archives and provide an envelope for robust Li-based reconstruction of seawater evolution over the Phanerozoic.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 10
    Publication Date: 2018-06-05
    Description: Fossil carbonate skeletons of marine organisms are archives for understanding the development and evolution of palaeo-environments. However, the correct assessment of past environment dynamics is only possible when pristine skeletons and their biogenic characteristics are unequivocally distinguishable from diagenetically-alteredskeletal elements and non-biogenic features. In this study, we extend our work on diagenesis of biogenic aragonite (Casella et al. 2017) to the investigation of biogenic low-Mg calcite using brachiopod shells. We examined and compared microstructural characteristics inducedby laboratory-based alteration to structural features derived from diagenetic alteration in natural environments. We used four screening methods: cathodoluminescence (CL), cryogenic and conventional field emission-scanning electronmicroscopy (FE-SEM), atomic force microscopy (AFM) and electron backscatter diffraction (EBSD).We base our assessments of diagenetic alteration and overprint on measurements of, a) images of optical overprint signals, b) changes in calcite crystal orientation patterns, and c) crystal co-orientation statistics. According to the screening process, altered and overprinted samples define two groups. In Group 1 the entire shell is diagenetically overprinted, whereas in Group 2 the shell contains pristine as well as overprinted parts. In the case of Group 2 shells, alteration occurred either along the periphery of the shell including the primary layer or at the interior-facing surface of the fibrous/columnar layer. In addition, we observed an important mode of the overprinting process, namely the migration of diagenetic fluids through the endopunctae corroborated by mineral formation and overprinting in their immediate vicinity, while leaving shell parts between endopunctae in pristine condition. Luminescence (CL) and microstructural imaging (FE-SEM) screening give first-order observations of the degree of overprint as they cover macro-to micron scale alteration features. For a comprehensive assessment of diagenetic overprint these screening methods should be complemented by screening techniques such as EBSD and AFM. They visualise diagenetic changes at submicron and nanoscale levels depicting the replacement of pristine nanocomposite mesocrystal biocarbonate (NMB) by inorganic rhombohedral calcite (IRC). The integration of screening methods allows for the unequivocal identification of highly-detailed alteration features as well as an assessment of the degree of diagenetic alteration.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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