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  • Other Sources  (205)
  • AGU (American Geophysical Union)  (132)
  • International Union of Crystallography  (36)
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  • 2020-2022  (57)
  • 1995-1999  (148)
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  • 1
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    Facilitated crystal handling using a simple device for evaporation reduction in microtiter plates (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-03-30
    Description: In the past two decades, most of the steps in a macromolecular crystallography experiment have undergone tremendous development with respect to speed, feasibility and increase of throughput. The part of the experimental workflow that is still a bottleneck, despite significant efforts, involves the manipulation and harvesting of the crystals for the diffraction experiment. Here, a novel low‐cost device is presented that functions as a cover for 96‐well crystallization plates. This device enables access to the individual experiments one at a time by its movable parts, while minimizing evaporation of all other experiments of the plate. In initial tests, drops of many typically used crystallization cocktails could be successfully protected for up to 6 h. Therefore, the manipulation and harvesting of crystals is straightforward for the experimenter, enabling significantly higher throughput. This is useful for many macromolecular crystallography experiments, especially multi‐crystal screening campaigns.
    Description: A simple and low‐cost device has been developed to minimize evaporation in microtiter plates for easy crystal handling and harvesting. image
    Keywords: 548 ; evaporation reduction ; crystal handling ; crystal harvesting ; crystallographic fragment screening
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  • 2
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    Cation distribution in Cu2ZnSnSe4, Cu2FeSnS4 and Cu2ZnSiSe4 by multiple‐edge anomalous diffraction (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-03-30
    Description: Multiple‐Edge Anomalous Diffraction (MEAD) has been applied to various quaternary sulfosalts belonging to the adamantine compound family in order to validate the distribution of copper, zinc and iron cations in the structure. Semiconductors from this group of materials are promising candidates for photovoltaic applications. Their properties strongly depend on point defects, in particular related to cation order–disorder. However, Cu+, Zn2+ and Fe2+ have very similar scattering factors and are all but indistinguishable in usual X‐ray diffraction experiments. Anomalous diffraction utilizes the dependency of the atomic scattering factors f′ and f′′ of the energy of the radiation, especially close to the element‐specific absorption edges. In the MEAD technique, individual Bragg peaks are tracked over an absorption edge. The intensity changes depending on the structure factor can be highly characteristic for Miller indices selected for a specific structural problem, but require very exact measurements. Beamline KMC‐2 at synchrotron BESSY II, Berlin, has been recently upgraded for this technique. Anomalous X‐ray powder diffraction and XAFS compliment the data. Application of this technique confirmed established cation distribution in Cu2ZnSnSe4 (CZTSe) and Cu2FeSnS4 (CFTS). In contrast to the literature, cation distribution in Cu2ZnSiSe4 (CZSiSe) is shown to adopt a highly ordered wurtz‐kesterite structure type.
    Description: Multiple‐Edge Anomalous Diffraction (MEAD) has been applied to various quaternary sulfosalts belonging to the adamantine compound family in order to validate the distribution of copper, zinc and iron cations in the structure. Application of this technique confirms established cation distribution in Cu2ZnSnSe4 (CZTSe) and Cu2FeSnS4 (CFTS), but in Cu2ZnSiSe4 (CZSiSe) the cation distribution is shown to adopt a highly ordered wurtz‐kesterite structure type in contrast to the literature. image
    Keywords: 548 ; synchrotron ; anomalous diffraction ; semiconductor ; MEAD
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  • 3
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    The copper(II)‐binding tripeptide GHK, a valuable crystallization and phasing tag for macromolecular crystallography (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-03-27
    Description: The growth of diffraction‐quality crystals and experimental phasing remain two of the main bottlenecks in protein crystallography. Here, the high‐affinity copper(II)‐binding tripeptide GHK was fused to the N‐terminus of a GFP variant and an MBP‐FG peptide fusion. The GHK tag promoted crystallization, with various residues (His, Asp, His/Pro) from symmetry molecules completing the copper(II) square‐pyramidal coordination sphere. Rapid structure determination by copper SAD phasing could be achieved, even at a very low Bijvoet ratio or after significant radiation damage. When collecting highly redundant data at a wavelength close to the copper absorption edge, residual S‐atom positions could also be located in log‐likelihood‐gradient maps and used to improve the phases. The GHK copper SAD method provides a convenient way of both crystallizing and phasing macromolecular structures, and will complement the current trend towards native sulfur SAD and MR‐SAD phasing.
    Description: A novel three‐residue tag containing the residues GHK that can be used to promote crystallization and in SAD phasing experiments using its tightly bound copper ion is described. image
    Keywords: 548 ; phasing ; crystallization ; GHK ; SAD
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  • 4
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    Determining paracrystallinity in mixed‐tacticity polyhydroxybutyrates (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-03-30
    Description: Recently, the authors reported on the development of crystallinity in mixed‐tacticity polyhydroxybutyrates. Comparable values reported in the literature vary depending on the manner of determination, the discrepancies being partially attributable to scattering from paracrystalline portions of the material. These portions can be qualified by peak profile fitting or quantified by allocation of scattered X‐ray intensities. However, the latter requires a good quality of the former, which in turn must additionally account for peak broadening inherent in the measurement setup, and due to limited crystallite sizes and the possible presence of microstrain. Since broadening due to microstrain and paracrystalline order both scale with scattering vector, they are easily confounded. In this work, a method to directionally discern these two influences on the peak shape in a Rietveld refinement is presented. Allocating intensities to amorphous, bulk and paracrystalline portions with changing tactic disturbance provided internal validations of the obtained directional numbers. In addition, the correlation between obtained thermal factors and Young's moduli, determined in earlier work, is discussed.
    Description: A method to robustly determine paracrystalline contents from Rietveld‐refined powder X‐ray data is presented and discussed for the example of mixed‐tacticity polyhydroxybutyrates. image
    Keywords: 548 ; polyhydroxybutyrates ; mixed tacticity ; paracrystallinity ; Rietveld refinement ; thermal factors
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  • 5
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    Concepts for nondestructive and depth‐resolved X‐ray residual stress analysis in the near‐surface region of nearly single crystalline materials with mosaic structure (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-03-27
    Description: Two evaluation concepts for nondestructive depth‐resolved X‐ray residual stress analysis in the near‐surface region of materials with cubic symmetry and nearly single crystalline structure are introduced by simulated examples. Both concepts are based on the same data acquisition strategy, which consists in the determination of lattice‐spacing depth profiles along the ⟨hkl⟩ poles by stepwise sample rotation around the scattering vector. Segmentation of these profiles parallel to the sample surface provides the lattice strain state as a function of depth. The first evaluation concept extends the crystallite group method developed for materials with pronounced crystallographic texture by the feature of depth resolution and can be applied to samples with arbitrary orientation. The second evaluation concept, which adapts the linear regression approach of the sin2ψ method for the case of single crystalline materials, is restricted to samples with (001) orientation. The influence of the strain‐free lattice parameter a0 on residual stress analysis using both evaluation concepts is discussed on the basis of explicitly derived relations.
    Description: Two data evaluation concepts are proposed for nondestructive and depth‐resolved X‐ray residual stress analysis by means of energy‐dispersive diffraction on materials featuring cubic symmetry and a nearly single crystalline structure. image
    Keywords: 548 ; residual stress ; X‐ray diffraction ; depth‐resolved analysis ; mosaic crystals
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  • 6
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    X‐ray dichroism in polyimide caused by non‐resonant scattering (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-03-30
    Description: Dichroism is one of the most important optical effects in both the visible and the X‐ray range. Besides absorption, scattering can also contribute to dichroism. This paper demonstrates that, based on the example of polyimide, materials can show tiny dichroism even far from electronic resonances due to scattering. Although the effect is small, it can lead to a measurable polarization change and might have influence on highly sensitive polarimetric experiments.
    Description: Aligned molecules, for example in polyimide foils, lead to small dichroism even far from resonances, which can be revealed by high‐precision X‐ray polarimetry. image
    Keywords: 548 ; polyimide ; polarization ; X‐ray polarimetry ; wide‐angle scattering ; X‐ray dichroism
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  • 7
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    Elastic stiffness coefficients of thiourea from thermal diffuse scattering (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-03-27
    Description: The complete elastic stiffness tensor of thiourea has been determined from thermal diffuse scattering (TDS) using high‐energy photons (100 keV). Comparison with earlier data confirms a very good agreement of the tensor coefficients. In contrast with established methods to obtain elastic stiffness coefficients (e.g. Brillouin spectroscopy, inelastic X‐ray or neutron scattering, ultrasound spectroscopy), their determination from TDS is faster, does not require large samples or intricate sample preparation, and is applicable to opaque crystals. Using high‐energy photons extends the applicability of the TDS‐based approach to organic compounds which would suffer from radiation damage at lower photon energies.
    Description: The elastic stiffness coefficients of thiourea are determined from thermal diffuse scattering. image
    Keywords: 548 ; thermal diffuse scattering ; elastic stiffness ; thiourea
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  • 8
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    Fast fitting of reflectivity data of growing thin films using neural networks (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-07-03
    Description: X‐ray reflectivity (XRR) is a powerful and popular scattering technique that can give valuable insight into the growth behavior of thin films. This study shows how a simple artificial neural network model can be used to determine the thickness, roughness and density of thin films of different organic semiconductors [diindenoperylene, copper(II) phthalocyanine and α‐sexithiophene] on silica from their XRR data with millisecond computation time and with minimal user input or a priori knowledge. For a large experimental data set of 372 XRR curves, it is shown that a simple fully connected model can provide good results with a mean absolute percentage error of 8–18% when compared with the results obtained by a genetic least mean squares fit using the classical Parratt formalism. Furthermore, current drawbacks and prospects for improvement are discussed.
    Description: Artificial neural networks trained with simulated data are shown to correctly and quickly determine film parameters from experimental X‐ray reflectivity curves.
    Keywords: 548 ; X‐ray reflectivity ; machine learning ; organic semi‐conductors ; neural networks
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  • 9
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    Numerically stable form factor of any polygon and polyhedron (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-06-05
    Description: Coordinate‐free expressions for the form factors of arbitrary polygons and polyhedra are derived using the divergence theorem and Stokes's theorem. Apparent singularities, all removable, are discussed in detail. Cancellation near the singularities causes a loss of precision that can be avoided by using series expansions. An important application domain is small‐angle scattering by nanocrystals.
    Description: Coordinate‐free expressions for the form factors of arbitrary polygons and polyhedra are derived using the divergence theorem and Stokes's theorem. Series expansions are used to ensure numeric precision close to apparent singularities. image
    Keywords: 548 ; form factors ; polyhedra ; Fourier shape transform
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  • 10
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    ROBL‐II at ESRF: a synchrotron toolbox for actinide research (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-06-30
    Description: ROBL‐II provides four different experimental stations to investigate actinide and other alpha‐ and beta‐emitting radionuclides at the new EBS storage ring of ESRF within an energy range of 3 to 35 keV. The XAFS station consists of a highly automatized, high sample throughput installation in a glovebox, to measure EXAFS and conventional XANES of samples routinely at temperatures down to 10 K, and with a detection limit in the sub‐p.p.m. range. The XES station with its five bent‐crystal analyzer, Johann‐type setup with Rowland circles of 1.0 and 0.5 m radii provides high‐energy resolution fluorescence detection (HERFD) for XANES, XES, and RIXS measurements, covering both actinide L and M edges together with other elements accessible in the 3 to 20 keV energy range. The six‐circle heavy duty goniometer of XRD‐1 is equipped for both high‐resolution powder diffraction as well as surface‐sensitive CTR and RAXR techniques. Single crystal diffraction, powder diffraction with high temporal resolution, as well as X‐ray tomography experiments can be performed at a Pilatus 2M detector stage (XRD‐2). Elaborate radioprotection features enable a safe and easy exchange of samples between the four different stations to allow the combination of several methods for an unprecedented level of information on radioactive samples for both fundamental and applied actinide and environmental research.
    Description: ROBL‐II at ESRF provides four experimental stations to investigate actinides with X‐ray absorption and emission spectroscopy, and with surface, high‐resolution powder, and single‐crystal X‐ray diffractometry.
    Keywords: 549 ; actinides ; EXAFS ; XANES ; HERFD‐XANES ; XAS ; XES ; RIXS ; XRD ; CTR ; RAXR ; surface diffraction
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  • 11
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    ATSAS 3.0: expanded functionality and new tools for small‐angle scattering data analysis (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-03-30
    Description: The ATSAS software suite encompasses a number of programs for the processing, visualization, analysis and modelling of small‐angle scattering data, with a focus on the data measured from biological macromolecules. Here, new developments in the ATSAS 3.0 package are described. They include IMSIM, for simulating isotropic 2D scattering patterns; IMOP, to perform operations on 2D images and masks; DATRESAMPLE, a method for variance estimation of structural invariants through parametric resampling; DATFT, which computes the pair distance distribution function by a direct Fourier transform of the scattering data; PDDFFIT, to compute the scattering data from a pair distance distribution function, allowing comparison with the experimental data; a new module in DATMW for Bayesian consensus‐based concentration‐independent molecular weight estimation; DATMIF, an ab initio shape analysis method that optimizes the search model directly against the scattering data; DAMEMB, an application to set up the initial search volume for multiphase modelling of membrane proteins; ELLLIP, to perform quasi‐atomistic modelling of liposomes with elliptical shapes; NMATOR, which models conformational changes in nucleic acid structures through normal mode analysis in torsion angle space; DAMMIX, which reconstructs the shape of an unknown intermediate in an evolving system; and LIPMIX and BILMIX, for modelling multilamellar and asymmetric lipid vesicles, respectively. In addition, technical updates were deployed to facilitate maintainability of the package, which include porting the PRIMUS graphical interface to Qt5, updating SASpy – a PyMOL plugin to run a subset of ATSAS tools – to be both Python 2 and 3 compatible, and adding utilities to facilitate mmCIF compatibility in future ATSAS releases. All these features are implemented in ATSAS 3.0, freely available for academic users at https://www.embl‐hamburg.de/biosaxs/software.html.
    Description: ATSAS is a comprehensive software suite for the processing, visualization, analysis and modelling of small‐angle scattering data. This article describes developments in the ATSAS 3.0 release, including new programs for data simulation and for the structural modelling of lipids, nucleic acids and polydisperse systems. image
    Keywords: 548 ; small‐angle scattering ; data analysis ; biological macromolecules ; structural modelling ; ATSAS
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  • 12
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    XGANDALF – extended gradient descent algorithm for lattice finding (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-03-27
    Description: Serial crystallography records still diffraction patterns from single, randomly oriented crystals, then merges data from hundreds or thousands of them to form a complete data set. To process the data, the diffraction patterns must first be indexed, equivalent to determining the orientation of each crystal. A novel automatic indexing algorithm is presented, which in tests usually gives significantly higher indexing rates than alternative programs currently available for this task. The algorithm does not require prior knowledge of the lattice parameters but can make use of that information if provided, and also allows indexing of diffraction patterns generated by several crystals in the beam. Cases with a small number of Bragg spots per pattern appear to particularly benefit from the new approach. The algorithm has been implemented and optimized for fast execution, making it suitable for real‐time feedback during serial crystallography experiments. It is implemented in an open‐source C++ library and distributed under the LGPLv3 licence. An interface to it has been added to the CrystFEL software suite.
    Description: A description and evaluation are given of XGANDALF, extended gradient descent algorithm for lattice finding, an algorithm developed for fast and accurate indexing of snapshot diffraction patterns. image
    Keywords: 548 ; indexing ; XGANDALF ; CrystFEL ; multiple lattices ; serial crystallography
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  • 13
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    pinkIndexer – a universal indexer for pink‐beam X‐ray and electron diffraction snapshots (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-03-27
    Description: A crystallographic indexing algorithm, pinkIndexer, is presented for the analysis of snapshot diffraction patterns. It can be used in a variety of contexts including measurements made with a monochromatic radiation source, a polychromatic source or with radiation of very short wavelength. As such, the algorithm is particularly suited to automated data processing for two emerging measurement techniques for macromolecular structure determination: serial pink‐beam X‐ray crystallography and serial electron crystallography, which until now lacked reliable programs for analyzing many individual diffraction patterns from crystals of uncorrelated orientation. The algorithm requires approximate knowledge of the unit‐cell parameters of the crystal, but not the wavelengths associated with each Bragg spot. The use of pinkIndexer is demonstrated by obtaining 1005 lattices from a published pink‐beam serial crystallography data set that had previously yielded 140 indexed lattices. Additionally, in tests on experimental serial crystallography diffraction data recorded with quasi‐monochromatic X‐rays and with electrons the algorithm indexed more patterns than other programs tested.
    Description: pinkIndexer, an algorithm developed for indexing of snapshot diffraction patterns recorded with pink‐beam X‐rays, monochromatic X‐rays and electrons, is described and its use evaluated. image
    Keywords: 548 ; indexing ; pinkIndexer ; CrystFEL ; pink X‐ray beam ; serial electron diffraction
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  • 14
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    Crystal structure of human CRM1, covalently modified by 2‐mercaptoethanol on Cys528, in complex with RanGTP (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-06-17
    Description: CRM1 is a nuclear export receptor that has been intensively targeted over the last decade for the development of antitumor and antiviral drugs. Structural analysis of several inhibitor compounds bound to CRM1 revealed that their mechanism of action relies on the covalent modification of a critical cysteine residue (Cys528 in the human receptor) located in the nuclear export signal‐binding cleft. This study presents the crystal structure of human CRM1, covalently modified by 2‐mercaptoethanol on Cys528, in complex with RanGTP at 2.58 Å resolution. The results demonstrate that buffer components can interfere with the characterization of cysteine‐dependent inhibitor compounds.
    Description: The covalent modification of human CRM1 by 2‐mercaptoethanol interferes with the characterization of cysteine‐dependent inhibitor compounds. image
    Keywords: 548 ; nuclear export ; cancer ; exportin 1 ; cysteine modification
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  • 15
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    Staircase array of inclined refractive multi‐lenses for large field of view pixel super‐resolution scanning transmission hard X‐ray microscopy (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-06-26
    Description: Owing to the development of X‐ray focusing optics during the past decades, synchrotron‐based X‐ray microscopy techniques allow the study of specimens with unprecedented spatial resolution, down to 10 nm, using soft and medium X‐ray photon energies, though at the expense of the field of view (FOV). One of the approaches to increase the FOV to square millimetres is raster‐scanning of the specimen using a single nanoprobe; however, this results in a long data acquisition time. This work employs an array of inclined biconcave parabolic refractive multi‐lenses (RMLs), fabricated by deep X‐ray lithography and electroplating to generate a large number of long X‐ray foci. Since the FOV is limited by the pattern height if a single RML is used by impinging X‐rays parallel to the substrate, many RMLs at regular intervals in the orthogonal direction were fabricated by tilted exposure. By inclining the substrate correspondingly to the tilted exposure, 378000 X‐ray line foci were generated with a length in the centimetre range and constant intervals in the sub‐micrometre range. The capability of this new X‐ray focusing device was first confirmed using ray‐tracing simulations and then using synchrotron radiation at BL20B2 of SPring‐8, Japan. Taking account of the fact that the refractive lens is effective for focusing high‐energy X‐rays, the experiment was performed with 35 keV X‐rays. Next, by scanning a specimen through the line foci, this device was used to perform large FOV pixel super‐resolution scanning transmission hard X‐ray microscopy (PSR‐STHXM) with a 780 ± 40 nm spatial resolution within an FOV of 1.64 cm × 1.64 cm (limited by the detector area) and a total scanning time of 4 min. Biomedical implant abutments fabricated via selective laser melting using Ti–6Al–4V medical alloy were measured by PSR‐STHXM, suggesting its unique potential for studying extended and thick specimens. Although the super‐resolution function was realized in one dimension in this study, it can be expanded to two dimensions by aligning a pair of presented devices orthogonally.
    Description: A new X‐ray focusing device generates hundreds of thousands of line foci, periodically spaced in the sub‐micrometre range, with centimetre length. It enables to achieve large FOV pixel super‐resolution scanning transmission hard X‐ray microscopy. image
    Keywords: 502.82 ; inclined refractive X‐ray multi‐lens array ; pixel super‐resolution ; scanning transmission hard X‐ray microscopy ; deep X‐ray lithography and electroplating
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  • 16
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    A new concept for temporal gating of synchrotron X‐ray pulses (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-07-02
    Description: A new concept for temporal gating of synchrotron X‐ray pulses based on laser‐induced thermal transient gratings is presented. First experimental tests of the concept yield a diffraction efficiency of 0.18%; however, the calculations indicate a theoretical efficiency and contrast of 〉30% and 10−5, respectively. The full efficiency of the pulse picker has not been reached yet due to a long‐range thermal deformation of the sample after absorption of the excitation laser. This method can be implemented in a broad spectral range (100 eV to 20 keV) and is only minimally invasive to an existing setup.
    Description: A new concept for temporal gating of synchrotron X‐ray pulses based on laser‐induced thermal transient gratings is presented.
    Keywords: 548 ; synchrotron ; time‐resolved ; thermal deformation ; transient grating ; pulse picking
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  • 17
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    The time‐resolved hard X‐ray diffraction endstation KMC‐3 XPP at BESSY II (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-06-06
    Description: The time‐resolved hard X‐ray diffraction endstation KMC‐3 XPP for optical pump/X‐ray probe experiments at the electron storage ring BESSY II is dedicated to investigating the structural response of thin film samples and heterostructures after their excitation with ultrashort laser pulses and/or electric field pulses. It enables experiments with access to symmetric and asymmetric Bragg reflections via a four‐circle diffractometer and it is possible to keep the sample in high vacuum and vary the sample temperature between ∼15 K and 350 K. The femtosecond laser system permanently installed at the beamline allows for optical excitation of the sample at 1028 nm. A non‐linear optical setup enables the sample excitation also at 514 nm and 343 nm. A time‐resolution of 17 ps is achieved with the `low‐α' operation mode of the storage ring and an electronic variation of the delay between optical pump and hard X‐ray probe pulse conveniently accesses picosecond to microsecond timescales. Direct time‐resolved detection of the diffracted hard X‐ray synchrotron pulses use a gated area pixel detector or a fast point detector in single photon counting mode. The range of experiments that are reliably conducted at the endstation and that detect structural dynamics of samples excited by laser pulses or electric fields are presented.
    Description: The KMC‐3 XPP endstation of the synchrotron BESSY II is dedicated to time‐resolved studies of structural dynamics of matter upon optical and/or electrical excitation using hard X‐ray diffraction with an accessible time range from 17 ps to several microseconds. image
    Keywords: 548 ; beamline instrumentation ; time‐resolved X‐ray diffraction ; optical excitation ; thermal transport ; ferroelectric switching
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  • 18
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    Disordered sodium alkoxides from powder data: crystal structures of sodium ethoxide, propoxide, butoxide and pentoxide, and some of their solvates (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
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    Publication Date: 2021-03-27
    Description: The crystal structures of sodium ethoxide (sodium ethanolate, NaOEt), sodium n‐propoxide (sodium n‐propanolate, NaOnPr), sodium n‐butoxide (sodium n‐butanolate, NaOnBu) and sodium n‐pentoxide (sodium n‐amylate, NaOnAm) were determined from powder X‐ray diffraction data. NaOEt crystallizes in space group P421m, with Z = 2, and the other alkoxides crystallize in P4/nmm, with Z = 2. To resolve space‐group ambiguities, a Bärnighausen tree was set up, and Rietveld refinements were performed with different models. In all structures, the Na and O atoms form a quadratic net, with the alkyl groups pointing outwards on both sides (anti‐PbO type). The alkyl groups are disordered. The disorder becomes even more pronounced with increasing chain length. Recrystallization from the corresponding alcohols yielded four sodium alkoxide solvates: sodium ethoxide ethanol disolvate (NaOEt·2EtOH), sodium n‐propoxide n‐propanol disolvate (NaOnPr·2nPrOH), sodium isopropoxide isopropanol pentasolvate (NaOiPr·5iPrOH) and sodium tert‐amylate tert‐amyl alcohol monosolvate (NaOtAm·tAmOH, tAm = 2‐methyl‐2‐butyl). Their crystal structures were determined by single‐crystal X‐ray diffraction. All these solvates form chain structures consisting of Na+, –O− and –OH groups, encased by alkyl groups. The hydrogen‐bond networks diverge widely among the solvate structures. The hydrogen‐bond topology of the iPrOH network in NaOiPr·5iPrOH shows branched hydrogen bonds and differs considerably from the networks in pure crystalline iPrOH.
    Description: The crystal structures of NaOEt, NaOPr, NaOBu and NaOAm (Am = amyl = pentyl) were determined from powder data. These compounds crystallize in an anti‐PbO structure in the space groups P21m and P4/nmm. Additionally, solvates with the composition NaOEt·2EtOH, NaOPr·2PrOH, NaOiPr·5iPrOH and NaOtAm·tAmOH were synthesized, and their structures were determined from single crystals. They form interesting chain structures of different compositions and topologies. image
    Keywords: 548 ; sodium alkoxide ; powder data ; solvate ; isopropanol ; Bärnighausen tree ; PXRD
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  • 19
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    Small‐angle X‐ray scattering from GaN nanowires on Si(111): facet truncation rods, facet roughness and Porod's law (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-03-27
    Description: Small‐angle X‐ray scattering from GaN nanowires grown on Si(111) is measured in the grazing‐incidence geometry and modelled by means of a Monte Carlo simulation that takes into account the orientational distribution of the faceted nanowires and the roughness of their side facets. It is found that the scattering intensity at large wavevectors does not follow Porod's law I(q) ∝ q−4. The intensity depends on the orientation of the side facets with respect to the incident X‐ray beam. It is maximum when the scattering vector is directed along a facet normal, reminiscent of surface truncation rod scattering. At large wavevectors q, the scattering intensity is reduced by surface roughness. A root‐mean‐square roughness of 0.9 nm, which is the height of just 3–4 atomic steps per micrometre‐long facet, already gives rise to a strong intensity reduction.
    Description: The intensity of small‐angle X‐ray scattering from GaN nanowires on Si(111) depends on the orientation of the side facets with respect to the incident beam. This reminiscence of truncation rod scattering gives rise to a deviation from Porod's law. A roughness of just 3–4 atomic steps per micrometre‐long side facet notably changes the intensity curves. image
    Keywords: 548 ; nanowires ; Porod's law ; facet truncation rods ; small‐angle X‐ray scattering ; SAXS ; grazing‐incidence small‐angle X‐ray scattering ; GISAXS
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  • 20
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    The accuracy of protein models automatically built into cryo‐EM maps with ARP/wARP (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-28
    Description: Recent developments in cryogenic electron microscopy (cryo‐EM) have enabled structural studies of large macromolecular complexes at resolutions previously only attainable using macromolecular crystallography. Although a number of methods can already assist in de novo building of models into high‐resolution cryo‐EM maps, automated and reliable map interpretation remains a challenge. Presented here is a systematic study of the accuracy of models built into cryo‐EM maps using ARP/wARP. It is demonstrated that the local resolution is a good indicator of map interpretability, and for the majority of the test cases ARP/wARP correctly builds 90% of main‐chain fragments in regions where the local resolution is 4.0 Å or better. It is also demonstrated that the coordinate accuracy for models built into cryo‐EM maps is comparable to that of X‐ray crystallographic models at similar local cryo‐EM and crystallographic resolutions. The model accuracy also correlates with the refined atomic displacement parameters.
    Keywords: 548 ; ARP/wARP ; model building ; cryo‐EM ; model accuracy ; sequence assignment
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  • 21
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    Synthesis, crystal structure and structure–property relations of strontium orthocarbonate, Sr2CO4 (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-05
    Description: Carbonates containing CO4 groups as building blocks have recently been discovered. A new orthocarbonate, Sr2CO4 is synthesized at 92 GPa and at a temperature of 2500 K. Its crystal structure was determined by in situ synchrotron single‐crystal X‐ray diffraction, selecting a grain from a polycrystalline sample. Strontium orthocarbonate crystallizes in the orthorhombic crystal system (space group Pnma) with CO4, SrO9 and SrO11 polyhedra as the main building blocks. It is isostructural to Ca2CO4. DFT calculations reproduce the experimental findings very well and have, therefore, been used to predict the equation of state, Raman and IR spectra, and to assist in the discussion of bonding in this compound.
    Description: A new orthocarbonate, Sr2CO4, was synthesized under extreme pressure and temperature conditions of 92 GPa and 2500 K, respectively. The crystal structure of the compound s fully characterized in situ by synchrotron single‐crystal X‐ray diffraction and DFT calculations were employed to provide insight into its equation of state, Raman and IR spectra, and bonding. image
    Keywords: 548 ; orthocarbonates ; crystal structure ; single‐crystal X‐ray diffraction ; high pressure ; Sr2CO4
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  • 22
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    Anomalous SAXS at P12 beamline EMBL Hamburg: instrumentation and applications (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-28
    Description: Small‐angle X‐ray scattering (SAXS) is an established method for studying nanostructured systems and in particular biological macromolecules in solution. To obtain element‐specific information about the sample, anomalous SAXS (ASAXS) exploits changes of the scattering properties of selected atoms when the energy of the incident X‐rays is close to the binding energy of their electrons. While ASAXS is widely applied to condensed matter and inorganic systems, its use for biological macromolecules is challenging because of the weak anomalous effect. Biological objects are often only available in small quantities and are prone to radiation damage, which makes biological ASAXS measurements very challenging. The BioSAXS beamline P12 operated by the European Molecular Biology Laboratory (EMBL) at the PETRA III storage ring (DESY, Hamburg) is dedicated to studies of weakly scattering objects. Here, recent developments at P12 allowing for ASAXS measurements are presented. The beamline control, data acquisition and data reduction pipeline of the beamline were adapted to conduct ASAXS experiments. Modelling tools were developed to compute ASAXS patterns from atomic models, which can be used to analyze the data and to help designing appropriate data collection strategies. These developments are illustrated with ASAXS experiments on different model systems performed at the P12 beamline.
    Keywords: 548 ; ASAXS ; biological SAXS ; metalloproteins ; gold nanoparticles ; anomalous scattering ; beamline development
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  • 23
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    Symmetry relations in wurtzite nitrides and oxide nitrides and the curious case of Pmc21 (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-27
    Description: Binary III–V nitrides such as AlN, GaN and InN in the wurtzite‐type structure have long been considered as potent semiconducting materials because of their optoelectronic properties, amongst others. With rising concerns over the utilization of scarce elements, a replacement of the trivalent cations by others in ternary and multinary nitrides has led to the development of different variants of nitrides and oxide nitrides crystallizing in lower‐symmetry variants of wurtzite. This work presents the symmetry relationships between these structural types specific to nitrides and oxide nitrides and updates some prior work on this matter. The non‐existence of compounds crystallizing in Pmc21, formally the highest subgroup of the wurtzite type fulfilling Pauling's rules for 1:1:2 stoichiometries, has been puzzling scientists for a while; a rationalization is given, from a crystallographic basis, of why this space group is unlikely to be adopted.
    Keywords: 548 ; group–subgroup relationships ; nitride materials ; wurtzite type
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  • 24
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    Comparison and evaluation of pair distribution functions, using a similarity measure based on cross‐correlation functions (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-20
    Description: An approach for the comparison of pair distribution functions (PDFs) has been developed using a similarity measure based on cross‐correlation functions. The PDF is very sensitive to changes in the local structure, i.e. small deviations in the structure can cause large signal shifts and significant discrepancies between the PDFs. Therefore, a comparison based on pointwise differences (e.g. R values and difference curves) may lead to the assumption that the investigated PDFs as well as the corresponding structural models are not in agreement at all, whereas a careful visual inspection of the investigated structural models and corresponding PDFs may reveal a relatively good match. To quantify the agreement of different PDFs for those cases an alternative approach is introduced: the similarity measure based on cross‐correlation functions. In this paper, the power of this application of the similarity measure to the analysis of PDFs is highlighted. The similarity measure is compared with the classical Rwp values as representative of the comparison based on pointwise differences as well as with the Pearson product‐moment correlation coefficient, using polymorph IV of barbituric acid as an example.
    Description: A novel approach to the quantification of the agreement between pair distribution functions by a similarity measure based on cross‐correlation functions is introduced and evaluated. image
    Keywords: 548 ; pair distribution functions ; similarity measures ; total scattering techniques ; cross‐correlation functions ; R values
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  • 25
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    Novel experimental setup for megahertz X‐ray diffraction in a diamond anvil cell at the High Energy Density (HED) instrument of the European X‐ray Free‐Electron Laser (EuXFEL) (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-26
    Description: The high‐precision X‐ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X‐ray Free‐Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump–probe X‐ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X‐ray heating and diffraction of Bi under pressure, obtained using 20 fs X‐ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC.
    Description: The high‐precision X‐ray diffraction (XRD) setup for work with diamond anvil cells (DACs) in Interaction Chamber 2 of the High Energy Density (HED) instrument of the European X‐ray Free‐Electron Laser is described. image
    Keywords: 548 ; diamond anvil cells ; X‐ray free‐electron lasers ; high‐precision X‐ray diffraction ; finite element modeling
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  • 26
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    The anisotropy in the optical constants of quartz crystals for soft X‐rays (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-26
    Description: The refractive index of a y‐cut SiO2 crystal surface is reconstructed from orientation‐dependent soft X‐ray reflectometry measurements in the energy range from 45 to 620 eV. Owing to the anisotropy of the crystal structure in the (100) and (001) directions, a significant deviation of the measured reflectance at the Si L2,3 and O K absorption edges is observed. The anisotropy in the optical constants reconstructed from these data is also confirmed by ab initio Bethe–Salpeter equation calculations for the O K edge. This new experimental data set expands the existing literature data for quartz crystal optical constants significantly, particularly in the near‐edge regions.
    Description: The refractive index of a y‐cut SiO2 crystal surface is reconstructed from polarization‐dependent soft X‐ray reflectometry measurements in the energy range from 45 to 620 eV. The reconstructed anisotropy in the optical constants is also confirmed by ab initio Bethe–Salpeter equation calculations of the O K edge. image
    Keywords: 548 ; optical constants ; quartz ; anisotropy ; soft X‐ray reflectometry
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  • 27
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    Unravelling the crystal structure of Nd5.8WO12−δ and Nd5.7W0.75Mo0.25O12−δ mixed ionic electronic conductors (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-07-05
    Description: Mixed ionic electronic conducting ceramics Nd6−yWO12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2O or H2S. Due to this chemical stability, they are promising and cost‐efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La6−yWO12−δ, the ionic/electronic transport mechanism in Nd6−yWO12−δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd5.8WO12−δ and molybdenum‐substituted Nd5.7W0.75Mo0.25O12−δ prepared by the citrate complexation route. High‐resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd5.8WO12−δ and Nd5.7W0.75Mo0.25O12−δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm3m and doubled unit‐cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La6−yWO12−δ. X‐ray absorption spectroscopy as a function of partial pressure pO2 in the near‐edge regions excludes oxidation state changes of Nd (Nd3+) and W (W6+) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo‐substituted Nd6−yWO12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.
    Description: The crystal structures of non‐substituted and Mo‐substituted neodymium tungstates are described in detail through neutron diffraction and high‐resolution X‐ray diffraction. Combined X‐ray and neutron diffraction refinements and electron probe micro‐analysis were employed to locate Mo atoms in the crystal structure of Nd6−yW1−zMozO12−δ (z = 0, 0.25), while X‐ray absorption spectroscopy in the near‐edge regions confirmed no changes in the oxidation states of Nd and W.
    Keywords: 548 ; powder diffraction ; mixed conductors ; X‐ray absorption spectroscopy (XAS) ; Nd6−yWO12−δ
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  • 28
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    High‐temperature treatments of niobium under high vacuum, dilute air‐ and nitrogen‐atmospheres as investigated by in situ X‐ray absorption spectroscopy (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-30
    Description: Niobium metal foils were heat‐treated at 900°C under different conditions and in situ investigated with time‐resolved X‐ray absorption fine‐structure (EXAFS and XANES) measurements. The present study aims to mimic the conditions usually applied for heat treatments of Nb materials used for superconducting radiofrequency cavities, in order to better understand the evolving processes during vacuum annealing as well as for heat treatments in controlled dilute gases. Annealing in vacuum in a commercially available cell showed a substantial amount of oxidation, so that a designated new cell was designed and realized, allowing treatments under clean high‐vacuum conditions as well as under well controllable gas atmospheres. The experiments performed under vacuum demonstrated that the original structure of the Nb foils is preserved, while a detailed evaluation of the X‐ray absorption fine‐structure data acquired during treatments in dilute air atmospheres (10−5 mbar to 10−3 mbar) revealed a linear oxidation with the time of the treatment, and an oxidation rate proportional to the oxygen (air) pressure. The structure of the oxide appears to be very similar to that of polycrystalline NbO. The cell also permits controlled exposures to other reactive gases at elevated temperatures; here the Nb foils were exposed to dilute nitrogen atmospheres after a pre‐conditioning of the studied Nb material for one hour under high‐vacuum conditions, in order to imitate typical conditions used for nitrogen doping of cavity materials. Clear structural changes induced by the N2 exposure were found; however, no evidence for the formation of niobium nitride could be derived from the EXAFS and XANES experiments. The presented results establish the feasibility to study the structural changes of the Nb materials in situ during heat treatments in reactive gases with temporal resolution, which are important to better understand the underlaying mechanisms and the dynamics of phase formation during those heat treatments in more detail.
    Keywords: 548 ; in situ EXAFS ; high temperature ; time‐resolved EXAFS ; niobium
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  • 29
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    Pressure‐induced Pb–Pb bonding and phase transition in Pb2SnO4 (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-03-27
    Description: High‐pressure single‐crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of Pb2SnO4 strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa. This insulator‐to‐semiconductor transition is accompanied by a reversible appearance change from transparent to opaque. Density functional theory‐based calculations show that at ambient conditions the channels in the structure host the stereochemically‐active Pb 6s2 lone electron pairs. On compression the lone electron pairs form bonds between Pb2+ ions. Also provided is an assignment of irreducible representations to the experimentally observed Raman bands.
    Description: The structure of Pb2SnO4 is found to strongly distort on compression and a structural phase transition with a change of space group from Pbam to Pnam occurs at ∼11 GPa. Our complementary DFT‐based calculations show that at ambient conditions, the channels in the structure host the stereochemically active Pb 6s2 lone electron pairs which form bonds between the Pb2+ ions with increasing pressure. image
    Keywords: 548 ; lead stannate (Pb2SnO4) ; density functional theory ; high‐pressure X‐ray diffraction ; pressure‐induced phase transition ; insulator–semiconductor transition
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  • 30
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    A discussion on `Domain formation and phase transitions in the wurtzite‐based heterovalent ternaries: a Landau theory analysis' (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-05
    Description: Heterovalent ternary nitrides are considered one of the promising classes of materials for photovoltaics, combining attractive physical properties with low toxicity and element abundance. One of the front‐runner systems under consideration is ZnSnN2. Although it is nominally a ternary compound, no clear crystallographic evidence for cation ordering has been observed so far. An attempt to elucidate this discrepancy [Quayle (2020). Acta Cryst. A76, 410–420] was the trigger for an intensive discussion between the authors, and an agreement was reached to elaborate on some points in order to set things in perspective. Rather than using a conventional comment–answer scheme, this is published in the form of a joint discussion to celebrate constructive criticism and collegiality.
    Description: A scientific exchange on an earlier paper [Quayle (2020). Acta Cryst. A76, 410–420] has led to the clarification of some of the points. image
    Keywords: 548 ; group–subgroup relationships ; nitride materials ; wurtzite type
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    X‐SPEC: a 70 eV to 15 keV undulator beamline for X‐ray and electron spectroscopies (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-26
    Description: X‐SPEC is a high‐flux spectroscopy beamline at the KIT (Karlsruhe Institute of Technology) Synchrotron for electron and X‐ray spectroscopy featuring a wide photon energy range. The beamline is equipped with a permanent magnet undulator with two magnetic structures of different period lengths, a focusing variable‐line‐space plane‐grating monochromator, a double‐crystal monochromator and three Kirkpatrick–Baez mirror pairs. By selectively moving these elements in or out of the beam, X‐SPEC is capable of covering an energy range from 70 eV up to 15 keV. The flux of the beamline is maximized by optimizing the magnetic design of the undulator, minimizing the number of optical elements and optimizing their parameters. The beam can be focused into two experimental stations while maintaining the same spot position throughout the entire energy range. The first experimental station is optimized for measuring solid samples under ultra‐high‐vacuum conditions, while the second experimental station allows in situ and operando studies under ambient conditions. Measurement techniques include X‐ray absorption spectroscopy (XAS), extended X‐ray absorption fine structure (EXAFS), photoelectron spectroscopy (PES) and hard X‐ray PES (HAXPES), as well as X‐ray emission spectroscopy (XES) and resonant inelastic X‐ray scattering (RIXS).
    Description: X‐SPEC is a high‐flux undulator beamline for electron and X‐ray spectroscopy with an energy range from 70 eV to 15 keV. It offers X‐ray absorption spectroscopy (XAS), extended X‐ray absorption fine structure (EXAFS), photoelectron spectroscopy (PES) and hard X‐ray PES (HAXPES), as well as X‐ray emission spectroscopy (XES) and resonant inelastic X‐ray scattering (RIXS) for in vacuo, in situ and operando sample environments. image
    Keywords: 548 ; undulator beamline ; soft X‐ray ; tender X‐ray ; hard X‐ray ; in situ ; operando ; HAXPES ; RIXS ; XAS ; XES
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    Structure determination of organic compounds by a fit to the pair distribution function from scratch without prior indexing (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-06-26
    Description: A method for the ab initio crystal structure determination of organic compounds by a fit to the pair distribution function (PDF), without prior knowledge of lattice parameters and space group, has been developed. The method is called `PDF‐Global‐Fit' and is implemented by extension of the program FIDEL (fit with deviating lattice parameters). The structure solution is based on a global optimization approach starting from random structural models in selected space groups. No prior indexing of the powder data is needed. The new method requires only the molecular geometry and a carefully determined PDF. The generated random structures are compared with the experimental PDF and ranked by a similarity measure based on cross‐correlation functions. The most promising structure candidates are fitted to the experimental PDF data using a restricted simulated annealing structure solution approach within the program TOPAS, followed by a structure refinement against the PDF to identify the correct crystal structure. With the PDF‐Global‐Fit it is possible to determine the local structure of crystalline and disordered organic materials, as well as to determine the local structure of unindexable powder patterns, such as nanocrystalline samples, by a fit to the PDF. The success of the method is demonstrated using barbituric acid as an example. The crystal structure of barbituric acid form IV solved and refined by the PDF‐Global‐Fit is in excellent agreement with the published crystal structure data.
    Keywords: 548 ; pair distribution function analysis ; structure determination ; total scattering technique ; similarity measures ; PDF‐Global‐Fit
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    Structural basis of chitin utilization by a GH20 β‐N‐acetylglucosaminidase from Vibrio campbellii strain ATCC BAA‐1116 (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-07-01
    Description: Vibrio species play a crucial role in maintaining the carbon and nitrogen balance between the oceans and the land through their ability to employ chitin as a sole source of energy. This study describes the structural basis for the action of the GH20 β‐N‐acetylglucosaminidase (VhGlcNAcase) in chitin metabolism by Vibrio campbellii (formerly V. harveyi) strain ATCC BAA‐1116. Crystal structures of wild‐type VhGlcNAcase in the absence and presence of the sugar ligand, and of the unliganded D437A mutant, were determined. VhGlcNAcase contains three distinct domains: an N‐terminal carbohydrate‐binding domain linked to a small α+β domain and a C‐terminal (β/α)8 catalytic domain. The active site of VhGlcNAcase has a narrow, shallow pocket that is suitable for accommodating a small chitooligosaccharide. VhGlcNAcase is a monomeric enzyme of 74 kDa, but its crystal structures show two molecules of enzyme per asymmetric unit, in which Gln16 at the dimeric interface of the first molecule partially blocks the entrance to the active site of the neighboring molecule. The GlcNAc unit observed in subsite −1 makes exclusive hydrogen bonds to the conserved residues Arg274, Tyr530, Asp532 and Glu584, while Trp487, Trp546, Trp582 and Trp505 form a hydrophobic wall around the −1 GlcNAc. The catalytic mutants D437A/N and E438A/Q exhibited a drastic loss of GlcNAcase activity, confirming the catalytic role of the acidic pair (Asp437–Glu438).
    Description: Crystal structures of a GH20 β‐N‐acetylglucosaminidase from V. campbellii reveal substrate specificity in chitin utilization.
    Keywords: 577.14 ; GH20 β‐N‐acetylglucosaminidase ; chitin recycling ; Vibrio spp ; marine bacteria
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    Resolution of a bent‐crystal spectrometer for X‐ray free‐electron laser pulses: diamond versus silicon (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-09-29
    Description: The resolution function of a spectrometer based on a strongly bent single crystal (bending radius of 10 cm or less) is evaluated. It is shown that the resolution is controlled by two parameters: (i) the ratio of the lattice spacing of the chosen reflection to the crystal thickness and (ii) a single parameter comprising crystal thickness, its bending radius, distance to a detector, and anisotropic elastic constants of the chosen crystal. The results allow the optimization of the parameters of bent‐crystal spectrometers for the hard X‐ray free‐electron laser sources.
    Description: The resolution function of a bent‐crystal spectrometer for pulses of an X‐ray free‐electron laser is evaluated. Under appropriate conditions, the energy resolution reaches the ratio of the lattice spacing to the crystal thickness. image
    Keywords: 548 ; X‐ray free‐electron lasers ; X‐ray spectroscopy ; bent crystals ; diamond crystal optics ; femtosecond X‐ray diffraction
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  • 35
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    Separation of the inelastic and elastic scattering in time‐of‐flight mode on the pinhole small‐angle neutron scattering diffractometer KWS‐2 (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-09-29
    Description: To study and control the incoherent inelastic background in small‐angle neutron scattering, which makes a significant contribution to the detected scattering from hydrocarbon systems, the KWS‐2 small‐angle neutron scattering diffractometer operated by the Jülich Centre for Neutron Science (JCNS) at Heinz‐Maier Leibnitz Zentrum (MLZ), Garching, Germany, was equipped with a secondary single‐disc chopper that is placed in front of the sample stage. This makes it possible to record in time‐of‐flight mode the scattered neutrons in the high‐Q regime of the instrument (i.e. short incoming wavelengths and detection distances) and to discard the inelastic component from the measured data. Examples of measurements on different materials routinely used as standard samples, sample containers and solvents in the experiments at KWS‐2 are presented. When only the elastic region of the spectrum is used in the data‐reduction procedure, a decrease of up to two times in the incoherent background of the experimentally measured scattering cross section may be obtained. The proof of principle is demonstrated on a solution of bovine serum albumin in D2O.
    Description: A new chopper has been installed at the sample position in front of the sample stage at the KWS‐2 small‐angle neutron‐scattering diffractometer of the Jülich Centre for Neutron Science. The pulsed beam and the time‐of‐flight data acquisition enable the separation of elastic and inelastic scattering from hydrogenous samples.
    Keywords: 548 ; TOF‐SANS ; incoherent neutron scattering ; inelastic neutron scattering ; hydrocarbon systems
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    Czochralski growth of mixed cubic sesquioxide crystals in the ternary system Lu2O3–Sc2O3–Y2O3 (2021)
    International Union of Crystallography | 5 Abbey Square, Chester, Cheshire CH1 2HU, England
    Details
    Publication Date: 2021-09-29
    Description: Cubic rare‐earth sesquioxide crystals are strongly demanded host materials for high power lasers, but due to their high melting points investigations on their thermodynamics and the growth of large‐size crystals of high optical quality remain a challenge. Detailed thermal investigations of the ternary system Lu2O3–Sc2O3–Y2O3 revealing a large range of compositions with melting temperatures below 2200°C and a minimum of 2053°C for the composition (Sc0.45Y0.55)2O3 are presented. These reduced temperatures enable for the first time the growth of high optical quality mixed sesquioxide crystals with disordered structure by the conventional Czochralski method from iridium crucibles. An (Er0.07Sc0.50Y0.43)2O3 crystal is successfully grown and characterized with respect to its crystallographic properties as well as its composition, thermal conductivity and optical absorption in the 1 µm range.
    Description: The phase diagram of the ternary system Lu2O3–Sc2O3–Y2O3 is investigated and compositions with melting points below 2200°C are found. This allows for the first successful growth of the mixed cubic sesquioxide crystal (Er0.07Sc0.50Y0.43)2O3 by the Czochralski method from an iridium crucible. image
    Keywords: 548 ; crystal growth ; optical materials ; phase diagrams ; melting points ; rare earth sesquioxides
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    Magma storage at Ocean Islands: insights from Cape Verde (2021)
    AGU (American Geophysical Union)
    In:  In: Crustal Magmatic System Evolution: Anatomy, Architecture, and Physico‐Chemical Processes. , ed. by Masotta, M., Beier, C. and Mollo, S. Geophysical Monograph Series, 264 . AGU (American Geophysical Union), Hoboken, New Jersey, pp. 45-78. ISBN 978-1-119-56445-4
    Details
    Publication Date: 2021-07-14
    Description: The Cape Verde archipelago is a group of Ocean Islands in the Central Atlantic that forms two chains of islands trending Northwest and Southwest. Several of the islands are considered to be volcanically active, with frequent eruptions on Fogo. We examine the mineral chemistry and thermobarometry of the southern islands; Santiago, Fogo and Brava together with the Cadamosto Seamount. Our objective is to explore the magmatic storage system and implications for volcanic eruptions and associated hazards at Cape Verde. The volcanic rocks at Cape Verde are alkaline and dominantly mafic, whereas the island of Brava and the Cadamosto Seamount are unusually felsic. Clinopyroxene compositions range from 60 to 90 Mg# at Santiago and Fogo. In contrast, at Brava and the Cadamosto Seamount the clinopyroxene compositions are 5 to 75 Mg#. Mineral chemistry and zonation records fractional crystallization, recharge, aggregation of crystals, magma mixing and variations in thermal conditions of the magma at temperatures from 925 to 1250C. Magma storage depths at Santiago, Fogo, Brava and the Cadamosto Seamount are between 12 and 40 km, forming deep sub-Moho magma storage zones. Transient magma storage in the crust is suggested by fluid inclusion re-equilibration and pre-eruption seismicity. A global compilation of magma storage at Ocean Islands suggests deep magma storage is a common feature and volcanic eruptions are often associated with rapid magma ascent through the crust. Shallow magma storage is more variable and likely reflects local variations in crustal structure, sediment supply and tectonics. Petrological constraints on the magma plumbing system at Cape Verde and elsewhere are vital to integrate with deformation models and seismicity in order to improve understanding and mitigation of the volcanic hazards.
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  • 38
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    Geologie der Tiefsee (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 46-50. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-05-28
    Description: Die Geowissenschaft betrachtet die Erde und damit auch die Entstehung von Tiefsee-Ebenen und untermeerischen Hochgebirgen aus einem anderen Blickwinkel. Um diese Herangehensweise nachzuvollziehen, hilft es, sich die Erde ohne Wasser vorzustellen. Wie die übrigen inneren Planeten unseres Sonnensystems, Merkur, Venus und Mars, ist auch die Erde von einer festen äußeren Schale, der sogenannten Erdkruste, umgeben. Dabei ist unsere Erde jedoch einzigartig darin, dass hier nicht nur eine, sondern zwei unterschiedliche Krustentypen auftreten, die kontinentale Kruste auf der einen und die ozeanische Kruste auf der anderen Seite.
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  • 39
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    Comparative interevent time statistics of degassing and seismic activity at Villarrica Volcano (Chile) (2020)
    AGU (American Geophysical Union) | Wiley
    In:  (Submitted) Journal of Geophysical Research: Solid Earth .
    Details
    Publication Date: 2021-01-07
    Description: It is generally assumed that seismic activity at volcanoes is closely connected to degassing processes. Intuitively, one would therefore expect a good correlation between degassing rates and seismic amplitude. However, both examples and counterexamples of such a correlation exist. In this study on Villarrica volcano (Chile), we pursued a different approach to relate gas flux and volcanic seismicity using 3 months of SO$_2$ flux rate measurements and 12 days of seismic recordings from early 2012.〈br /> We analyzed the statistical distributions of interevent times between transient seismic waveforms commonly associated with explosions and between peaks in the degassing time series.〈br /> Both event types showed a periodic recurrence with a mode of 20-25 s and around 1 h for transients and degassing, respectively. The normalized interevent times were fitted by almost identical log-normal distributions. Given the actually very different time scales, this similarity potentially indicates a scale-invariant phenomenon. We could reproduce these empirical findings by modelling the occurrence of transients as a renewal process from which the degassing events were derived recursively with increasing probability since the previous degassing event. In this model, the seismic transients could be either produced by degassing processes within the conduit or by gas release at the lava lake surface while the longer intervals of the degassing events may be explained by accumulation of gas either in the magma column or in the juvenile gas plume.〈br /> Additionally, we analyzed volcano-tectonic events, which behaved very differently from the transients. They showed the clustered occurrence of tectonic earthquakes.
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  • 40
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    Hotspots des Lebens - die geologischen Voraussetzungen (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 51-55. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-07-01
    Description: Geologische Prozesse und dabei entweichende wässrige Lösungen sind in vielen Bereichen der Ozeane für die Entstehung von Oasen in der Tiefsee verantwortlich. Das Leben dort ist an die spezifischen Eigenschaften der auftretenden Gesteine und chemischen Lösungen angepasst und unterscheidet sich daher erheblich von dem der normalen Tiefsee-Ebenen oder Seeberge. Zu den außergewöhnlichen Lebensräumen gehören die sogenannten Schwarzen Raucher, die sich in der Umgebung vulkanisch aktiver Bereiche bilden und an denen heiße (hydrothermale) Lösungen austreten. Dazu gehören aber auch Austrittsstellen von Gasen und Lösungen mit geringerer Temperatur, die sich häufig in den Sehelfgebieten der Kontinente oder in sehr stark sedimentierten Bereichen der Tiefsee finden. Diese werden unter dem Begriff „Cold Seeps" (,.kalte Sickerstellen") zusammengefasst, obwohl diese Stellen sehr unterschiedlich sein können.
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  • 41
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    Exploring the Drivers of Global and Local Sea‐Level Change over the 21st Century and Beyond (2020)
    AGU (American Geophysical Union)
    In:  Earth's Future, 8 (9). Art.Nr. e2019EF001413.
    Details
    Publication Date: 2021-01-08
    Description: We present a new set of global and local sea‐level projections at example tide gauge locations under the RCP2.6, RCP4.5 and RCP8.5 emissions scenarios. Compared to the CMIP5‐based sea‐level projections presented in IPCC AR5, we introduce a number of methodological innovations, including: (i) more comprehensive treatment of uncertainties; (ii) direct traceability between global and local projections; (iii) exploratory extended projections to 2300 based on emulation of individual CMIP5 models. Combining the projections with observed tide gauge records, we explore the contribution to total variance that arises from sea‐level variability, different emissions scenarios and model uncertainty. For the period out to 2300 we further breakdown the model uncertainty by sea‐level component and consider the dependence on geographic location, time horizon and emissions scenario. Our analysis highlights the importance of variability for sea‐level change in the coming decades and the potential value of annual‐to‐decadal predictions of local sea‐level change. Projections to 2300 show a substantial degree of committed sea‐level rise under all emissions scenarios considered and highlights the reduced future risk associated with RCP2.6 and RCP4.5 compared to RCP8.5. Tide gauge locations can show large (〉 50%) departures from the global average, in some cases even reversing the sign of the change. While uncertainty in projections of the future Antarctic ice dynamic response tends to dominate post‐2100, we see a substantial differences in the breakdown of model variance as a function of location, timescale and emissions scenario.
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  • 42
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    Planning a Sustainable Future for Earth’s Oceans (2020)
    AGU (American Geophysical Union)
    In:  Eos, 101 . pp. 1-8.
    Details
    Publication Date: 2021-01-08
    Description: Ocean experts are engaged in a long-term effort to envision, develop, and implement best practices for meeting today’s needs while preserving ocean resources for future generations
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  • 43
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    Methanhydrate (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt ind der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 156-158. ISBN 978-3-510-61415-8
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    Publication Date: 2020-06-03
    Type: Book chapter , NonPeerReviewed
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  • 44
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    Massivsulfide (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 153-155. ISBN 978-3-510-61415-8
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    Publication Date: 2020-06-03
    Type: Book chapter , NonPeerReviewed
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  • 45
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    Manganknollen (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 149-152. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-06-03
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  • 46
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    Mineralische Rohstoffe der Tiefsee (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 147-149. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-06-03
    Description: Die Erde hält eine Vielzahl natürlicher mineralischer Rohstoffe bereit, die wir in unserer hochtechnisierten Gesellschaft benötigen. Zurzeit werden fast alle diese Rohstoffe auf einer Fläche abgebaut, die weniger als ein Drittel unseres Planeten ausmacht -der Landfläche. Hier wird es jedoch immer schwieriger, reiche Vorkommen zu finden. Dies zwingt die Bergbauindustrie dazu, Lagerstätten mit geringeren Konzentrationen abzubauen bzw. nach Lagerstätten in abgelegenen Regionen der Erde oder in großer Tiefe zu suchen. Dies ist jedoch mit einem erheblichen Anstieg des Flächenverbrauchs und mit zusätzlichen Umweltbeeinträchtigungen verbunden.
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  • 47
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    Vom Telegraphenkabel zum Internet-Glasfaserkabel in der Tiefsee (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt ind der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 165-167. ISBN 978-3-510-61415-8
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    Publication Date: 2020-07-01
    Description: Bereits 1811 schickte Samuel Thomas von Sömmering elektrische Signale durch einen von Kautschuk isolierten Draht, welcher bei München durch die Isar verlegt wurde. Diese frühen Versuche scheiterten an der ungeeigneten Isolierung. Erst die Erfindung der Gutapercha-Presse 1847 durch Werner von Siemens ermöglichte die Unterwasserverlegung gut isolierter Kabel.
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  • 48
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    Reply to Comment by A. Argnani on "Geometry of the Deep Calabrian Subduction from Wide‐Angle Seismic Data and 3‐D Gravity Modeling" (2020)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 21 (8). e2020GC009223.
    Details
    Publication Date: 2020-08-11
    Description: Keypoints This contribution is a reply on a comment submitted by A. Argnani. The alternate interpretation of the wide-angle seismic model is discussed. The Alfeo Fault system is proposed to be the current location of STEP fault. Abstract Andrea Argnani in his comment on Dellong et al., 2020 (Geometry of the deep Calabrian subduction (Central Mediterranean Sea) from wide‐angle seismic data and 3D gravity modeling), proposes an alternate interpretation of the wide-angle seismic velocity models presented by Dellong et al., 2018 and Dellong et al., 2020 and proposes a correction of the literature citations in these paper. In this reply, we discuss in detail all points raised by Andrea Argnani.
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  • 49
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    Die Lebenswelt der Tiefsee (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in dder Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 74-79. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-07-01
    Description: Die Tiefsee beginnt in einer Tiefe von 200 m. Der darüberliegende Bereich lichtdurchfluteter Wasserschichten wird „photische Zone" genannt, da hier genügend Licht zur Verfügung steht, um die Fotosynthese zu unterstützen, die für das Wachstum von Pflanzen und Algen erforderlich ist. Unterhalb von 200 m kommt nur wenig Licht an, nicht genug für die Fotosynthese. Dieser Bereich heißt Dämmerungszone oder auch „mesopelagische Zone".
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  • 50
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    Cold-Seep-Ökosysteme - Oasen oder Leben am Limit (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 62-68. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-07-01
    Description: Die uns vertraute Grundlage des Lebens ist die Fotosynthese, die Erzeugung organischer Kohlenstoffverbindungen mithilfe des Sonnenlichts durch Pflanzen, Algen und Cyanobakterien (Primärproduktion). Diese Organismen werden als „autotroph" ("sich selbst ernährend") bezeichnet und dienen uns Menschen und anderen heterotroph ("sich von anderen ernährend") lebenden Tieren als Nahrung. In den lichtlosen Tiefen der Ozeane wurden allerdings Ökosysteme entdeckt, die gänzlich anders funktionieren.
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  • 51
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    Zur Geschichte der Meeres- und Tiefseeforschung in Westeuropa und den USA (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 14-22. ISBN 978-3-510-61415-8
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    Publication Date: 2020-07-03
    Description: Lange galt das Meer als unendlicher Ressourcenraum, den wir Menschen nicht nachhaltig beeinflussen könnten. Heute wissen wir, dass auch das Meer endlich ist. Es wird diskutiert, ob der Mensch das gigantisch große „Ökosystem Meer" kollabieren lassen könnte. Gleichzeitig wird die Bedeutung der Meere für das Leben auf der Erde nicht mehr bestritten und die Relevanz der Meeresforschung für nachhaltiges gesellschaftliches, politisches und wirtschaftliches Handeln deutlich.
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  • 52
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    Bedingungen der Tiefsee (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 42-45. ISBN 978-3-510-61415-8
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    Publication Date: 2020-07-01
    Description: Je nach Definition beginnt die Tiefsee bei 200, 500 oder 1.000 m unter der Wasseroberfläche. Die Tiefsee ist dunkel, kalt, nährstoffarm, spärlich besiedelt und steht unter dem enormen Druck der auf ihr lastenden Wassersäule. Sämtliche Tiefseeorganismen müssen diesen z. T. lebensfeindlichen Bedingungen standhalten bzw. an diese angepasst sein. An einigen Stellen, den sogenannten Hotspots, ist die Artenvielfalt jedoch trotz dieser Extrembedingungen sehr hoch.
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  • 53
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    Lebensraum Tiefsee (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 40-41. ISBN 978-3-510-61415-8
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    Publication Date: 2020-07-03
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  • 54
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    Effect on Subduction of Deeply Buried Seamounts Offshore of Kodiak Island (2020)
    AGU (American Geophysical Union) | Wiley
    In:  Tectonics, 39 (7). e2019TC005710.
    Details
    Publication Date: 2021-01-08
    Description: Seamounts are ubiquitous on the oceanic plate; those situated near convergent margins will eventually undergo subduction. Using six prestack depth migrated MCS profiles transecting the Aleutian Trench, we investigate deeply buried seamounts offshore Kodiak Island, within 145–155°W and 55–58°N. A distinct sedimentary horizon exists in all six seismic profiles, at or above the average height of seamounts, which appears to be the preferred structural detachment zone. Where drilled, this horizon contains gravel‐sized debris interpreted to be ice rafted and marks the onset of intensification of Northern Hemisphere glaciation at ~2.7 Ma. Beneath this horizon, sediments prior to the Surveyor Fan development were deposited, all or the majority of these sediments will eventually be subducted. Despite the subducted seamounts being deeply buried, these features cause enhanced surface slope of the accretionary prism. Our observations lead us to propose a model for the stages of subduction for deeply buried seamounts. These stages include the following: (1) Prior to subduction, the protothrust zone undergoes enhanced shortening, (2) frontal thrust steepening and enhanced backthrusting occurs during subduction with a potential décollement step down seaward and a steeping outward of the deformation front to the limit of the protothrust zone, and (3) further subduction results in a pattern of uplift farther into the wedge resulting in enhanced out‐of‐sequence thrusting and persistence of the more seaward deformation front position. This pattern is distinct from the dominance of embayments and effective removal of prism material during seamount subduction described along margins with less deeply buried edifices.
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  • 55
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    Die menschengemachte Klimaerwärmung und die Tiefsee (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 184-185. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-07-01
    Description: Durch das Verbrennen von Kohle, Gas und Öl hat der Mensch die Erde seit Beginn der Industrialisierung um ca. 1 °C erwärmt, mit bereits deutlich spürbaren Auswirkungen -auch für uns Menschen.
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  • 56
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    Meerestechnik für die Tiefseeforschung (2020)
    Schweizerbart
    In:  In: Tiefsee - Vielfalt in der Dunkelheit. , ed. by Müller, T. and Hoffmann-Wieck, G. Senckenberg-Buch, 83 . Schweizerbart, Stuttgart, pp. 31-37. ISBN 978-3-510-61415-8
    Details
    Publication Date: 2020-07-03
    Description: Die Tiefsee wird als eines der letzten Grenzgebiete in der Erforschung unseres blauen Planeten angesehen. Zu den großen Herausforderungen, die uns den Zugang zu diesem Lebensraum erschweren, zählen neben der Dunkelheit tiefe Temperaturen (im Mittel um 2 °C), hohe Korrosion im salzigen Meerwasser, hoher Umgebungsdruck, biologischer Bewuchs (..biofouling"), stürmische See mit hohem Wellengang sowie die schwierige Datenübertragung ohne Kabel, die nur auf die Akustik beschränkt ist. Dies macht die Tiefseetechnik zu einer Spitzentechnologie, die sehr viele Analogien zur Weltraumforschung aufweist.
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  • 57
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    Confronting Arctic Troposphere, Clouds, and Surface Energy Budget Representations in Regional Climate Models With Observations (2020)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Atmospheres, 125 (6). e2019JD031783.
    Details
    Publication Date: 2021-01-08
    Description: A coordinated regional climate model (RCM) evaluation and intercomparison project based on observations from a July–October 2014 trans‐Arctic Ocean field experiment (ACSE‐Arctic Clouds during Summer Experiment) is presented. Six state‐of‐the‐art RCMs were constrained with common reanalysis lateral boundary forcing and upper troposphere nudging techniques to explore how the RCMs represented the evolution of the surface energy budget (SEB) components and their relation to cloud properties. We find that the main reasons for the modeled differences in the SEB components are a direct consequence of the RCM treatment of cloud and cloud‐radiative interactions. The RCMs could be separated into groups by their overestimation or underestimation of cloud liquid. While radiative and turbulent heat flux errors were relatively large, they often invoke compensating errors. In addition, having the surface sea‐ice concentrations constrained by the reanalysis or satellite observations limited how errors in the modeled radiative fluxes could affect the SEB and ultimately the surface evolution and its coupling with lower tropospheric mixing and cloud properties. Many of these results are consistent with RCM biases reported in studies over a decade ago. One of the six models was a fully coupled ocean‐ice‐atmosphere model. Despite the biases in overestimating cloud liquid, and associated SEB errors due to too optically thick clouds, its simulations were useful in understanding how the fully coupled system is forced by, and responds to, the SEB evolution. Moving forward, we suggest that development of RCM studies need to consider the fully coupled climate system.
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  • 58
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    Frictionally modified flow in a deep ocean channel: Application to the Vema Channel (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C9). 21,123-21,136.
    Details
    Publication Date: 2018-04-17
    Description: The modification of the exchange flow in a deep southern hemisphere passage, resembling the Vema Channel, by frictionally induced secondary circulation is investigated numerically. The hydrostatic primitive equation model is a two-dimensional version of the sigma-coordinate Princeton Ocean Model. The time dependent response of a stratified along-channel flow, forced by barotropic or baroclinic pressure gradients, is examined. Near the bottom, where the along-channel now is retarded, there is cross-channel Ekman nux that is associated with downwelling on the eastern side and upwelling on the western side of the channel. In the presence of stratification the cross-channel flow rearranges the density structure, which in turn acts on the along-channel velocity via the thermal wind relation. Eventually the cross-isobath Ekman flux is shut down. In the case of baroclinically driven flow of Antarctic Bottom Water through the Vema Channel the model reproduces the observed shape of the deep temperature profiles and their cross-channel asymmetry. The model offers an explanation that is alternative or supplementary to inviscid multilayer hydraulic theory that;was proposed in earlier studies. It explains the extremely thick bottom boundary layers in the center and on the western slope of the channel. The deep thermocline is spread out in the west and sharpened in the east, and the coldest water is found on the eastern side of the deep trough; The modified density field reduces the along-channel flow near the bottom and focuses it into a narrow jet on the eastern side of the channel.
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  • 59
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    On the water masses and mean circulation of the South Atlantic Ocean (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 . pp. 20863-20833.
    Details
    Publication Date: 2018-04-17
    Description: We examine recent observations of water mass distribution and circulation schemes at different depths of the South Atlantic Ocean to propose a layered, qualitative representation of the mean distribution of flow in this region. This furthers the simple upper layer geostrophic flow estimates of Peterson and Stramma [1991]. In addition, we assess how well ocean general circulation models (GCMs) capture the overall structure of flow in the South Atlantic in this regard. The South Atlantic Central Water (SACW) is of South Atlantic origin in the subtropical gyre, while the SACW in the tropical region in part originates from the South Indian Ocean. The Antarctic Intermediate Water in the South Atlantic originates from a surface region of the circumpolar layer, especially in the northern Drake Passage and the Falkland Current loop, but also receives some water from the Indian Ocean. The subtropical South Atlantic above the North Atlantic Deep Water and north of the Antarctic Circumpolar Current (ACC) is dominated by the anticyclonic subtropical gyre. In the eastern tropical South Atlantic the cyclonic Angola Gyre exists, embedded in a large tropical cyclonic gyre. The equatorial part of the South Atlantic shows several depth-dependent zonal current bands besides the Angola Gyre. Ocean GCMs have difficulty capturing this detailed zonal circulation structure, even at eddy-permitting resolution. The northward extent of the subtropical gyre reduces with increasing depth, located near Brazil at 16°S in the near-surface layer and at 26°S in the Antarctic Intermediate Water layer, while the tropical cyclonic gyre progresses southward. The southward shift of the northern part of the subtropical gyre is well resolved in global ocean GCMs. However, high horizontal resolution is required to capture the South Atlantic Current north of the ACC. The North Atlantic Deep Water in the South Atlantic progresses mainly southward in the Deep Western Boundary Current, but some water also moves southward at the eastern boundary.
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    The intermediate depth circulation of the western South Atlantic (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 . 21,3329-21,3332.
    Details
    Publication Date: 2018-02-13
    Description: The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally-buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 10(6)m(3)s(-1)), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.
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  • 61
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    Equatorial currents and transports in the upper central Indian Ocean: Annual cycle and interannual variability (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C7). 15,495-15,514.
    Details
    Publication Date: 2018-04-17
    Description: The zonal circulation south of Sri Lanka is an important link for the exchange of water between the Bay of Bengal and the Arabian Sea. Results from a first array of three moorings along 80 degrees 30'E north of 4 degrees 10'N from January .1991 to March 1992 were used to investigate the Monsoon Current regime [Schott et al., 1994]. Measurements from a second array of six current meter moorings are presented here. This array was deployed along 80 degrees 30'E between 45'S and 5 degrees N from July 1993 to September 1994 to investigate the annual cycle and interannual variability of the equatorial currents at this longitude. Both sets of moorings contribute to the Indian Ocean current meter array ICM8 of the World Ocean Circulation Experiment. The semiannual equatorial jet (EJ) was showing a large seasonal asymmetry, reaching a monthly mean eastward transport of 35 Sv (1 Sv = 1 x 10(6) m(3) s(-1)) in November 1993, but just 5 Sv in May 1994. The Equatorial Undercurrent (EUC) had a maximum transport of 17 Sv in March to April 1994. Unexpectedly, compared to previous observations and model studies, the EUC was reappearing again in August 1994 at more than 10 Sv transport and was still flowing when the moorings were recovered. In addition, monthly mean ship drifts near the equator are evaluated to support the interpretation of the moored observations. Interannual variability of the EJ in our measurements and ship drift data appears to be related to the variability of the zonal winds and Southern Oscillation Index. The output of a global numerical model (Parallel Ocean Climate Model) driven by the winds for 1993/1994 is used to connect our observations to the larger scale. The model reproduces the EJ asymmetry and shows the existence of the EUC and its reappearance during summer 1994.
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  • 62
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    Open-ocean convection: observations, theory and models (1999)
    AGU (American Geophysical Union)
    In:  Reviews of Geophysics, 37 (1). pp. 1-64.
    Details
    Publication Date: 2019-01-23
    Description: We review what is known about the convective process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth's rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convective and geostrophic scales are not very disparate from one another. Detailed observations of the process in the Labrador, Greenland, and Mediterranean Seas are described, which were made possible by new observing technology. When interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection, great progress in our description and understanding of the processes at work is being made.
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  • 63
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    Carbonate sedimentation through the late Precambrian and Phanerozoic (1999)
    Schweizerbart
    In:  Zentralblatt für Geologie und Paläontologie / Teil 1, 1998 (5-6). pp. 435-445.
    Details
    Publication Date: 2018-02-06
    Description: The global sediment mass-age distribution indicates large variations in the rates of carbonate sedimentation through time. The largest mass of carbonate deposited during the entire history of the earth was produced during the Cambrian, possibly following on an episode of phosphogenesis in the Late Precambrian. A second major episode occurred during the Late Devonian, probably reflecting the invasion of land by plants that altered the rock-weathering and soil-forming regimes. Other lesser pulses of carbonate deposition occurred in the Late Permian, Triassic, and Cretaceous. A shift in the locus of carbonate deposition from shallow waters to the deep sea occurred during the Cretaceous.
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  • 64
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    An eddy permitting coupled physical-biological model of the North Atlantic. Part I: Sensitivity to advection numerics and mixed layer physics (1999)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 13 (1). pp. 135-160.
    Details
    Publication Date: 2017-06-06
    Description: Physical influences on biological primary production in the North Atlantic are investigated by coupling a four-component pelagic ecosystem model with a high-resolution numerical circulation model. A series of sensitivity experiments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusivity implicit in upstream advection approximately doubles primary production when compared with a less diffusive, higher-order, positive-definite advection scheme.This is of particular concern in the equatorial upwelling region where upstream advection leads to a considerable increase of upper ocean nitrate concentrations. Counteracting this effect of unrealistically large implicit diffusion by changes in the biological model could easily lead to misconceptions in the interpretation of ecosystem dynamics. Subgrid-scale diapycnal diffusion strongly controls biological production in the subtropical gyre where winter mixing does not reach the nutricline. The parameterization of vertical viscosity is important mainly in the equatorial region where friction becomes an important agent in the momentum balance.
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  • 65
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    Role of iron, light, and silicate in controlling algal biomass in subantarctic waters SE of New Zealand (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (13). 13395-13408 .
    Details
    Publication Date: 2017-11-24
    Description: Phytoplankton processes in subantarctic (SA) waters southeast of New Zealand were studied during austral autumn and spring 1997. Chlorophyll a (0.2–0.3 μg L−1) and primary production (350–650 mg C m−2 d−1) were dominated by cells 〈2 μm (cyanobacteria) in both seasons. The photochemical efficiency of photosystem II (Fυ/Fm) of cells was low (0.3), indicating physiological stress. Dissolved Fe (DFe) levels in surface waters were subnanomolar, and the molecular marker flavodoxin indicated that cells were iron stressed. In contrast, Subtropical Convergence (STC) and subtropical waters had higher algal biomass/production levels, particularly in spring. In these waters, DFe levels were 〉1 nmol kg−1, there was little evidence of Fe-stressed algal populations, and Fυ/Fm approached 0.60 at the STC. In addition to these trends, waters of SA origin were occasionally observed within the STC and north of the STC, and thus survey data were interpreted with caution. In vitro Fe enrichment incubations in SA waters resulted in a switch from flavodoxin expression to that of ferredoxin, indicating the alleviation of Fe stress. In another 6-day experiment, iron-mediated increases in chlorophyll a (in particular, increases in large diatoms) were of similar magnitude to those observed in a concurrent Si/Fe enrichment; ambient silicate levels were 4 μM. A concurrent in vitro Fe enrichment, at irradiance levels comparable to the calculated mean levels experienced by cells in situ, resulted in relatively small increases (approximately twofold) in chlorophyll a. Thus, in spring, irradiance and Fe may both control diatom growth. In contrast, during summer, as mean irradiance increases and silicate levels decrease, Fe limitation, Fe/Si colimitation, or silicate limitation may determine diatom growth.
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    A box model for the paleoceanography of the Black Sea (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters (26). pp. 497-500.
    Details
    Publication Date: 2018-02-13
    Description: The evolution of the Black Sea's salinity after the opening of the Bosporus about 7500 years ago is investigated using a simple two-box model. The model consists of watermass and salt conservation equations, and allows for changes in halocline depth. The paleoceanographic box model is forced by present-day Mediterranean inflow and outflow, and atmospheric forcings. Analytic solutions for the evolution of the box volumes are given. Model salinities reach 90% of their the present-day values in both boxes about 2,500 years after the opening of the Bosporus. The evolution of the salinities is shown to be almost independent of the evolution of the box volumes, and the results are compared with the existing paleoceanographic proxy records.
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  • 67
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    Volatile halogenated hydrocarbons over the western Pacific between 43° and 4°N (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Atmospheres, 104 . pp. 1663-1678.
    Details
    Publication Date: 2017-11-24
    Description: A spectrum of halogenated hydrocarbon compounds in marine air masses were surveyed over an area in the western Pacific between 43°N, 150°E and 4°N, 113°E in September 1994. The ship's track between northern Japan and Singapore traversed three climatic zones of the northern hemisphere. Recently polluted air, clean marine air derived from the central Pacific Ocean from different latitudes, and marine air from the Indonesian archipelago were collected. Tetrachloroethene and trichloroethene of anthropogenic origin, brominated halocarbons as tribromomethane, dibromochloromethane and bromodichloromethane of anthropogenic and natural sources, and other trace gases were measured in the air samples. Very sparse data on the distribution of these compounds exist for the western Pacific atmosphere. The distribution patterns of the compounds were related to synoptic-scale meteorology, spatial conditions, and origin of the air masses. Anthropogenic and natural sources for both chlorinated and brominated substances were identified. Tetrachloroethene and trichloroethene concentrations and their ratios identify anthropogenic sources. Their mixing ratios were quite low compared to previously published data. They are in agreement with expected low concentrations of photochemically active substances during autumn, with an overall decrease in concentrations toward lower latitudes, and with a decrease of emissions during recent years. Strong evidence for a natural source of trichloroethene was discovered in the tropical region. The concentrations of naturally released brominated species were high compared to other measurements over the Pacific. Gradients toward the coasts and elevated concentrations in air masses influenced by coastal emissions point to significant coastal sources of these compounds. The trace gas composition of anthropogenic and natural compounds clearly identified the air masses which were traversed during the cruise.
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  • 68
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    The Importance of flow in the Mozambique Channel to seasonality in the greater Agulhas Current system (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 . pp. 3321-3324.
    Details
    Publication Date: 2018-02-13
    Description: The temporal variability of the greater Agulhas Current system has important climatological consequences. Some recent results have suggested that this variability contains a large seasonal component, due to changes in the circulation at latitudes poleward of Madagascar only. A model simulation shows that the contribution of Tropical Surface Water to Agulhas Current waters, via the Mozambique Channel, also has a distinct seasonal characteristic that is brought about by the seasonal wind stress over the tropical Indian Ocean. This simulated flow through the Channel contributes substantially to the seasonality of the Agulhas Current. This model result is shown to be not inconsistent with available hydrographic observations.
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    Introduction to special section: World Ocean Circulation Experiment: South Atlantic results (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C9). pp. 20859-20861.
    Details
    Publication Date: 2018-04-17
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  • 70
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    Verteilung und Artenzusammensetzung benthischer Ostracoden der Islandsee im Spätquartär [Distribution and abundance of benthic ostracods in the Iceland Sea during the Late Quaternary] (1999)
    Schweizerbart
    In:  Zentralblatt für Geologie und Paläontologie / Teil 1, 1998 (5-6). pp. 353-365.
    Details
    Publication Date: 2017-06-22
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    Indium geology, mineralogy, and economics - a review (1999)
    Schweizerbart
    In:  Zeitschrift für Angewandte Geologie, 45 . pp. 164-169.
    Details
    Publication Date: 2016-06-16
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  • 72
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    Interbasin deep water exchange in the western Mediterranean (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C10). 23,495-23,508.
    Details
    Publication Date: 2018-04-17
    Description: Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean, The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Provencal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Provencal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600-1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10(6) m(3) s(-1)), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero-Provencal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Provencal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior
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    On the determination of characteristics of the interior ocean dynamics from radar signatures of oceanic internal solitary waves (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 . 30,039-30,046.
    Details
    Publication Date: 2018-04-17
    Description: In this paper we discuss two different methods of inferring characteristics of the interior ocean dynamics from radar signatures of internal solitary waves visible on synthetic aperture radar (SAR) images. The first one consists in the recognition and the interpretation of sea surface patterns of internal solitary waves; the second one consists in the analysis of the modulation depth of the normalized radar backscattering cross section (NRCS) associated with internal solitary waves. For this purpose we consider a data set composed of SAR and in situ measurements carried out from 1991 to 1997 in the region of the Strait of Messina. The recognition and the interpretation of sea surface patterns of internal solitary waves in the Strait of Messina can be used to study characteristics of the density distribution in the area: The internal wave field varies with seasonal variations in the vertical density stratification and with remotely induced variations, i.e., variations induced by the larger-scale circulation, in the horizontal density distribution. In order to inquire into the possibility of inferring parameters of the interior ocean dynamics by analyzing the modulation of the NRCS associated with internal solitary waves, several numerical simulations are carried out using a radar imaging model. These simulations are performed by assuming different wind conditions and internal wave parameters. It is shown that an accurate knowledge of wind conditions is crucial for deriving internal wave parameters and hence parameters of the interior ocean dynamics from the modulation of measured NRCS associated with internal solitary waves.
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  • 74
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    Increased seasonality in the gulf of Aquaba, Red Sea, recorded in the Oxygen Isotope Record of a Porites lutea coral (1999)
    Schweizerbart
    In:  Senckenbergiana Maritima, 30 . pp. 17-26.
    Details
    Publication Date: 2019-08-08
    Description: At Aqaba in the Northern Gulf of Aqaba, Red Sea, the stable oxygen isotope record of a Por#es lutea coral colony was studied. From a vertical and a horizontal core two parts from the base of the colony (10 years of coral growth), and two parts from the living surface (12 years of coral growth) were analysed with a monthly sampling resolution. The results show that seasonality of sea surface temperatures (SST) in the northem GulfofAqaba has increased since the early 19th century, mainly due to higher summer temper- atures. 8~80-ratios indicate that SST increased by at least 1.3~ Horizontal and vertical cores display dif- ferent variations in the modern parts, probably caused by the very shallow water depth of the youngest part of the vertical core.
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    On the influence of submarine ridges on translation and stability of Agulhas rings (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C4). pp. 7897-7906.
    Details
    Publication Date: 2019-09-23
    Description: A series of experiments with a quasi‐geostrophic model have been carried out to investigate the influence of topographic obstacles on the translatory movement of Agulhas rings. The rings were initialized as Gaussian‐shaped anomalies in the stream function field of a two‐layer ocean at rest. Bottom topography consisted of a meridional ridge of constant height in the middle of the quadratic model domain. The vertical ring structure, the initial ring position, and the height of the ridge were varied. The general northwestward movement of the model eddies has been shown to be modified toward a more equatorward direction by encountering the upslope of the ridge. Sufficient topographic heights and strong slopes can even block the eddies and force them toward a pure meridional movement. During their translation the eddies lose their vertical coherence. After about 150 days the eddy can only be detected by the surface signal, while the lower layer eddy is dispersed by the radiation of Rossby waves. The passage of “young” (regarding the time between their initialization and their contact with the ridge) and energetic eddies is accompanied by the observation of along‐slope currents of significant strength. These may be due to the rectification of radiated Rossby waves at the topographic slope. Only eddies with a significant dynamic signal in the lower layer are influenced by the bottom topography. Strong, shallow eddies over deep lower layers can cross the ridge without strong modification of their translatory movement.
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  • 76
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    Dynamics, estimation and impact of South Atlantic inter-ocean exchange (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C9). pp. 20885-20910.
    Details
    Publication Date: 2017-11-24
    Description: Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3 s−1). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales. Modeling studies suggest that the Indian-Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Currents' retroflection. State-of-the-art global and regional “eddy-permitting” models show a reasonably realistic representation of the mean Agulhas system; but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement. In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchanges dynamics and forcing, its representation in state-of-the-art models, and, finally, the impact of the Indian-Atlantic fluxes on regional and global scale both within the Atlantic Ocean and in interaction with the overlying atmosphere.
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    Origins of the midlatitude Pacific decadal variability (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 (10). pp. 1453-1456.
    Details
    Publication Date: 2018-02-13
    Description: Analysis of multiple climate simulations shows much of the midlatitude Pacific decadal variability to be composed of two simultaneously occurring elements: One is a stochastically driven, passive ocean response to the atmosphere while the other is oscillatory and represents a coupled mode of the ocean‐atmosphere system. ENSO processes are not required to explain the origins of the decadal variability. The stochastic variability is driven by random variations in wind stress and heat flux associated with internal atmospheric variability but amplified by a factor of 2 by interactions with the ocean. We also found a coupled mode of the ocean‐atmosphere system, characterized by a significant power spectral peak near 1 cycle/20 years in the region of the midlatitude North Pacific and Kuroshio Extension. Ocean dynamics appear to play a critical role in this coupled air/sea mode.
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    Pacific thermocline bridge revisited (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 (9). p. 1329.
    Details
    Publication Date: 2018-02-13
    Description: The coupling on decadal time scales of the mid‐latitude and tropical Pacific via an oceanic ‘bridge’ in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model. Results indicate that in the tropics decadal anomalies of isopycnal depth are forced by Ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid‐latitudes of either hemisphere. In the coupled model, temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere, but are lag correlated between southern hemisphere mid‐ and low‐latitudes. However, anomaly magnitudes on the equator are small. These results suggest that the oceanic ‘bridge’ to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial Pacific. Coupling to the southern mid‐latitudes via temperature anomalies on isopycnals remains an intriguing possibility.
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  • 79
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    Interdecadal interactions between the tropics and midlatitudes in the Pacific Basin (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 (5). p. 615.
    Details
    Publication Date: 2018-02-13
    Description: Analysis of global climate model simulations and observations suggest decadal, midlatitude changes in and over the North Pacific cause decadal modulation of the El Niño‐Southern Oscillation. This coupling between the two geographic regions is via atmospheric, not oceanographic, teleconnections. In essence, large scale changes in the circulation of the atmosphere over the Pacific Basin, while largest in midlatitudes, have a significant projection onto the wind field overlying the equatorial regions. These low frequency wind changes precondition the mean state of the thermocline in the equatorial ocean to produce prolonged periods of enhanced or reduced ENSO activity. The midlatitude variability that drives equatorial impacts is of stochastic origin and, although the magnitude of the signal is enhanced by ocean processes, likely unpredictable.
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    A comment on the proper use of nutrient ratios in microalgal ecology (1999)
    Schweizerbart
    In:  Archiv für Hydrobiologie, 146 . pp. 55-64.
    Details
    Publication Date: 2019-03-05
    Description: Repeatable species dominance patterns across nutrient ratio gradients are found both in culture competition experiments and in natural waters. However, the mechanisms behind these similar patterns need not to be identical. In chemostat experiments, such patterns are caused by the two-way competitive interactions between contemporaneously occurring organisms. Nutrient competition may be important in situ too, but there is an additional effect of early successional species influencing the nutrient environment for later successional species, without being influenced themselves by their successors.
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    Sea ice dynamics in the Weddell Sea simulated with an optimized model (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C5). pp. 11151-11162.
    Details
    Publication Date: 2019-09-23
    Description: The advection of sea ice and associated freshwater/salt fluxes in the Weddell Sea in 1986 and 1987 are investigated with a large‐scale dynamic‐thermodynamic sea ice model. The model is validated and optimized by comparison of simulated sea ice trajectories with observed drift paths of six buoys deployed on the Weddell Sea ice. The skill of the model is quantified by an error function that measures the deviations of simulated trajectories from observed 30‐day sea ice drift. A large number of sensitivity studies show how simulated sea ice transports and associated freshwater/salt fluxes respond to variations in physical parameterizations. The model reproduces the observed ice drift well, provided ice dynamics parameters are set to appropriate values. Optimized values for the drag coefficients and for the ice strength parameter are determined by applying the error function to various sensitivity studies with different parameters. The optimized model yields a mean northward sea ice volume export out of the southern Weddell Sea of 1693 km3 in 1986 and 2339 km3 in 1987. This shows the important role of sea ice transport for the freshwater budget of the Weddell Sea and gives an indication of its high interannual variability.
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    Anthropogenic CO〈sub〉2〈/sub〉 and CFCs in the North Atlantic Ocean - a comparison of man-made tracers (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 . pp. 2065-2068.
    Details
    Publication Date: 2018-02-13
    Description: We compare estimates of the anthropogenic CO2 content of seawater samples from the subpolar North Atlantic Ocean calculated on the basis of a back-calculation technique with measurements of the chlorofluorocarbon CFC-11. Estimated anthropogenic CO2 concentrations are in the range 10–80 µmol kg-1, while CFC-11 concentrations cover the full range from below detection limit to 〉 5 pmol kg-1 in waters at atmospheric equilibrium. The majority of the data points show a linear correlation between anthropogenic CO2 concentrations and CFC-11 saturation, which can only be explained by the strongly advective nature of the North Atlantic Ocean. Only deep eastern basin samples deviate from this general observation in that they show still significant concentrations of anthropogenic CO2 where CFC-11 is no longer detectable. In order to remove the influence of the Revelle factor reflected in the anthropogenic CO2 concentrations we have calculated 'excess' pCO2, showing an even tighter linear correlation with atmospheric equilibrium concentrations of CFC-11.
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  • 83
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    Anthropogenic CO〈sub〉2〈/sub〉 Inventory of the Indian Ocean (1999)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 13 . pp. 179-198.
    Details
    Publication Date: 2018-03-15
    Description: This study presents basin-wide anthropogenic CO2 inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified ΔC* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO2 and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO2 column inventories are observed south of 50°S. The total anthropogenic CO2 inventory north of 35°S was 13.6±2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1±1 Pg C for the same area. Approximately 6.7±1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ±3 Pg C for 1995. These estimates are compared to anthropogenic CO2 inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61–0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.
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  • 84
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    Methane in the northern Atlantic controlled by microbial oxidation and atmospheric history (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 (5). pp. 587-590.
    Details
    Publication Date: 2018-02-13
    Description: During May - August, 1997, the distributions of dissolved methane and CCl3F (CFC11) were measured in the Atlantic between 50° and 60°N. In surface waters throughout the region, methane was observed to be close to equilibrium with the atmospheric mixing ratio, implying that surface ocean methane is tracking its atmospheric history in regions of North Atlantic Deep Water formation. Despite the different atmospheric history and ocean chemistry of CH4 and CFC11, their spatial distribution patterns in the water column are remarkably similar. One-dimensional distributions have been simulated with an advection-diffusion model forced by the atmospheric histories. The results suggest that the similar patterns result from the increasing input of CH4 and CFC11 to newly formed deep waters over time, combined with the effect of horizontal mixing and the oxidation of methane on a 50 year time scale.
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    Little-studied arc-backarc system in the spotlight (1999)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 80 (32). pp. 353-359.
    Details
    Publication Date: 2016-06-07
    Description: A research cruise has documented changes in rift tectonics, volcanism, and hydrothermalism along the least studied and most enigmatic sector of a crustal complex in the southwest Pacific Ocean. Results from the longitudinal transect are expected to provide insight into processes involving the Kermadec arc-Havre backarc (KAHB) system, a continuum from oceanic spreading to continental rifting at a convergent plate boundary KAHB forms the central sector of an active, 2000-km arc-backarc complex between Tonga and New Zealand (Figure 1). The expedition also engaged in the first comprehensive survey of submarine vents in the Taupo Volcanic Zone (TVZ) at the south end of the KAHB system. Identified in the off-shore segment of TVZ were three major hydrothermal vent areas associated with late Quaternary fault structures. Data from the expedition and from other recent research in the same area addressed questions concerning the type of hydrothermal venting, magmatic heterogeneity along and across KAHB, the style of backarc rifting, and tectonic and magmatic consequences of anomalous terranes colliding with the subduction margin.
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  • 86
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    Some aspects of ocean heat transport by the shallow, intermediate and deep overturning circulations (1999)
    AGU (American Geophysical Union)
    In:  In: Mechanisms of global climate change at millenial time scales. , ed. by Clark, P. U. Geophysical monograph, 112 . AGU (American Geophysical Union), Washington, D.C., pp. 1-22. ISBN 0-87590-095-X
    Details
    Publication Date: 2020-08-04
    Type: Book chapter , NonPeerReviewed
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  • 87
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    Intermediate water in the Brazil-Malvinas Confluence Zone: A Lagrangian view (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C9). 21,063-21,082.
    Details
    Publication Date: 2018-04-27
    Description: The subsurface flow within the subantarctic and subtropical regions around the Brazil-Malvinas (Falkland) Confluence Zone is studied, using daily hydrographic and kinematic data from four subsurface floats and a hydrographic section parallel to the South American shelf. The float trajectories are mapped against sea surface flow patterns as visible in concurrent satellite sea surface temperature (SST) images, with focus on the November 1994 and October/November 1995 periods. The unprecedented employment of Lagrangian θ-S diagrams enables us to trace the advection of patches of fresh Antarctic Intermediate Water (AAIW) from the Confluence Zone into the subtropical region. The fresh AAIW consists of a mixture of subtropical AAIW and Malvinas Current core water. Within the subtropical gyre, these patches are discernible for extended periods and drift over long distances, reaching north to 34°S and east to 40°W. The cross-frontal migration of quasi-isobaric floats across the Confluence Zone from the subtropical to the subantarctic environment is observed on three occasions. The reverse process, float migration from a subpolar to a subtropical environment was observed once. These events were located near 40°S, 50°W, the site of a reoccurring cold core feature. Subsurface float and SST data comparison reveals similarities with analogous observations made in the Gulf Stream [Rossby, 1996] where cross-frontal processes were observed close to meander crests. The limited number of floats of this study and the complex structure of the Brazil-Malvinas Confluence Zone, however, restricts the analysis to a description of two events.
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  • 88
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    Nitrous oxide in the deep waters of the world's oceans (1999)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 13 . pp. 1127-1135.
    Details
    Publication Date: 2018-03-15
    Description: We present a compilation and analysis Of N2O data from the deep-water zone of the oceans below 2000 m. The N2O values show an increasing trend from low concentrations in the North Atlantic Ocean to high concentrations in the North Pacific Ocean, indicating an accumulation of N2O in deep waters with time. We conclude that the observed N2O accumulation is mainly caused by nitrification in the global deep-water circulation system (i.e., the “conveyor belt”). Hydrothermal and sedimentary N2O fluxes are negligible. We estimate the annual N2O deep-water production to be 0.3 ± 0.1 Tg. Despite the fact that the deep sea below 2000 m represents about 95% of the total ocean volume, it contributes only about 3–16% to the global open-ocean N2O production. A rough estimate of the oceanic N2O budget suggests that the loss to the atmosphere is not balanced by the deep-sea nitrification and pelagic denitrification. Therefore an additional source of 3.8 Tg N2O yr−1 attributed to nitrification in the upper water column (0–2000 m) might exist. With a simple model we estimated the effect of changes in the North Atlantic Deep Water (NADW) formation for deep-water N2O. The upper water N2O budget is not significantly influenced by variations in the N2O deep-water formation. However, the predicted decrease in the NADW formation rate in the near future might lead to an additional source of atmospheric N2O in the range of about 0.02-0.4 Tg yr−1. This (anthropogenically induced) source is small, and it will be difficult to detect its signal against the natural variations in the annual growth rates of tropospheric N2O.
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  • 89
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    A model for growth of steep-sided vent structures on the Endeavour Segment of the Juan de Fuca Ridge: Results of a petrologic and geochemical study (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 104 (B10). pp. 22859-22884.
    Details
    Publication Date: 2018-04-12
    Description: Petrologic and geochemical studies of vent solids from the Main Endeavour Field (MEF) and the High Rise Field (HRF), Juan de Fuca Ridge, demonstrate that the steep‐sided vent structures characteristic of these sites form dominantly by flange growth, combined with diffuse flow through sealed portions of structures, and incorporation of flanges into structures. Geochemical calculations suggest that the prevalence of amorphous silica and flanges in Endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia. At Endeavour, as the temperature of vent fluid decreases, ammonia‐ammonium equilibrium buffers pH and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher pH than conductively cooled ammonia‐poor fluids present at most other unsedimented mid‐ocean ridge vent sites. Deposition of silica stabilizes flanges and allows structures to attain large size. It also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones, thus decreasing entrainment of seawater. Most inactive vent samples recovered from areas peripheral to the HRF and MEF are similar to barite + silica rich samples from the Explorer Ridge and Axial Seamount and likely formed from precipitation of silica and barite on a biological substrate. Active white smoker chimneys from the Clam Bed Field, located south of the HRF, are pyrrhotite rich and likely formed from vent fluids that are depleted in Zn and Cd and enriched in Pb and Ba relative to fluids exiting trans‐Atlantic geotraverse (TAG) and Cleft Segment white smoker chimneys.
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  • 90
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    Use of observed ice crystal sizes and shapes to calculate mean-scattering properties and multispectral radiances: CEPEX April 4, 1993, case study (1999)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Atmospheres, 104 (D24). pp. 31763-31779.
    Details
    Publication Date: 2018-01-24
    Description: During the Central Equatorial Pacific Experiment, ice crystal sizes and shapes were measured in an outflow anvil. A habit (i.e., column, bullet rosette, Koch fractal polycrystal, sphere) was assigned to each particle using a self-organized neural network based on simulations of how the maximum particle dimension and area ratio varied for random orientations of these crystals. Average ice crystal size and shape distributions were calculated for 25 km long segments at six altitudes using measurements from a two-dimensional cloud probe for crystals larger than 90 μm and a parameterization for smaller crystals based on measurements from the Video Ice Particle Sampler (VIPS). Mean-scattering properties were determined by weighting the size and shape dependent single-scattering properties computed with ray-tracing algorithms according to scattering cross-section. Reflectances at 0.664, 0.875, 1.621, and 2.142 μm were then calculated using a Monte Carlo radiative transfer routine. Although these reflectances agree reasonably with those measured by the MODIS airborne simulator (MAS) above the anvil, uncertainties in cloud base and system evolution prevent a determination of whether ray-tracing or anomalous diffraction theory better predict reflectance. The calculated reflectances are as sensitive to the numbers and shapes of crystals smaller than 90 μm as to those of larger crystals. The calculated reflectances were insensitive to the classification scheme (i.e., neural network, discriminator analysis, and previously used classification scheme) for assigning particle shape to observed crystals. However, the reflectances significantly depended on assumed particle shape.
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  • 91
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    Interaction of the North Atlantic Current with the deep Charlie Gibbs Fracture Zone throughflow (1999)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 26 (3). pp. 369-372.
    Details
    Publication Date: 2018-02-13
    Description: The Charlie Gibbs Fracture Zone (CGFZ), a passage of 3600 m sill depth through the Mid‐Atlantic Ridge near 52°N, is a known gateway for the passage of deep waters from the Northeast Atlantic into the western basin. During a shipboard survey of August 1997 deep current profiling yielded eastward deep flow through the passage while geostrophy calculated against an intermediate reference level resulted in westward relative deep transport. The reason was an unusual and deep‐reaching northward excursion of the North Atlantic Current (NAC). Inspection of historical data showed that such interference of the NAC with the CGFZ regime occured occasionally in the past. Relocation of surface circulation patterns by decadal ocean‐climate anomalies may thus be of significance also for the deep circulation.
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  • 92
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    Factors determining cyanobacterial success in aquatic systems: A literature review (1998)
    Schweizerbart
    In:  Archiv für Hydrobiologie Spec. Iss. Advances in Limnology, 51 . pp. 41-62.
    Details
    Publication Date: 2017-12-01
    Description: Cyanobacteria are important constituents of most aquatic ecosystems on earth, and planktonic forms frequently show mass development in lakes and estuaries. Despite extensive scientific efforts directed towards research on cyanobacteria, a comprehensive theory explaining their success is lacking. Because cyanobacteria comprise a diverse group of organisms, it is concluded that any analysis of the conditions leading to cyanobacterial dominance in pelagic ecosystems should consider at least three different subgroups, classified based on presence or absence of (i) buoyancy regulation and (ii) the ability to fix molecular nitrogen. In this review, nine single-factor hypotheses regarding regulation of cyanobacterial development are examined against the background of physiological and ecological characteristics of these organisms. Special emphasis has been put on understanding differences in nitrogen metabolism between cyanobacteria and algae which directly relate to a hypothesis conceming inorganic nitrogen forms and cyanobacterial success previously presented by our research group. The review lends support to the theory that cyanobacteria have a low competitive ability for nitrate compared with algae and a high ability to compete for ammonium, particularly under nitrogen-limiting conditions. Finally it is concluded that to understand cyanobacterial devlopment and how high Standing Stocks in pelagic ecosystems are maintained requires parts o f all the single-factor hypotheses initially presented.
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  • 93
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    Age determination of mixed water masses using CFC and oxygen data (1998)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 103 (C9). pp. 18599-18610.
    Details
    Publication Date: 2018-04-17
    Description: We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section I5 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48 degrees-55 degrees E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55 degrees E after 15-20 years. It takes ICW some 25-30 years to reach 110 degrees E.
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  • 94
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    The d13C anomaly in the northeastern Atlantic (1998)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 12 (3). pp. 467-477.
    Details
    Publication Date: 2018-03-14
    Description: The δ13C of dissolved inorganic carbon was measured on samples collected at 49°N in the northeast Atlantic in January 1994. Deeper than 2000 m, δ13C exhibits the same negative correlation versus dissolved phosphate that is observed elsewhere in the deep Atlantic. Upward from 2000 m to about 600 m, δ13C shifts to values more negative than expected from the correlation with nutrients at depth, which is likely due to penetration of anthropogenic CO2. From these data, the profile of the anthropogenic δ13C decrease is calculated by using either dissolved phosphate or apparent oxygen utilization as a proxy for the preanthropogenic δ13C distribution. The shape of the anthropogenic anomaly profile derived from phosphate is similar to that of the increase in dissolved inorganic carbon derived by others in the same area. The reconstruction from oxygen utilization results in a lower estimate of the anthropogenic δ13C decrease in the upper water column, and the vertical anomaly profile is less similar to that of the dissolved inorganic carbon increase. A 13C budget for the atmosphere, ocean, and terrestrial biosphere indicates that within the range of probable ocean CO2 uptake the ratio of δ13C to inorganic carbon change should be mostly influenced by the 13C inventory change of the biosphere. However, the uncertainty in the ratio we derive prevents a strong contraint on the size of the exchangeable biosphere.
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  • 95
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    The temporal evolution of the tracer signal in the Deep Western Boundary Current, Tropical Atlantic (1998)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research, 103 . pp. 15869-15883.
    Details
    Publication Date: 2017-11-24
    Description: Four World Ocean Circulation Experiment (WOCE) repeat cruises (October 1990 to March 1994) in the tropical Atlantic off Brazil are used to study the spatial and temporal evolution of the chlorofluorocarbon (CFC) (components CFC-11 and CFC-12) and tritium signal in the upper North Atlantic Deep Water (NADW). Its shallowest part, located in the tropical Atlantic around 1600-m depth, is the shallow upper North Atlantic Deep Water (SUNADW). It is characterized by a distinct tracer maximum, which is presumably received through winter time convection in the subpolar North Atlantic. Here we discuss the tracer fields and the temporal evolution of the tracer signal of the SUNADW in the tropical Atlantic along two meridional sections at 44 degrees and 35 degrees W and two zonal sections at 5 degrees and 10 degrees S off Brazil. The spatial and temporal development of the tracer field in the tropical Atlantic as well as the correlation with hydrographic parameters show that the temporal tracer change being due to the arrival of "younger" water is disturbed by other processes. In particular, the impact of variable mixing and spreading pathways on the observed tracer variability in the SUNADW is evident in the observations.
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  • 96
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    Longterm increases in western Mediteranean salinities and temperatures: a mixture of anthropogenic and climate sources (1998)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 25 (22). pp. 4209-4212.
    Details
    Publication Date: 2018-02-13
    Description: The deep water of the western Mediterranean Sea is known to have become warmer and saltier since about the 1950s. The causes of these changes have, however, not yet been sactisfactorily determined. Previous studies speculated on decreasing precipitation, greenhouse warming and/or anthropogenic reduction of the freshwater flux into the eastern Mediterranean. Here we report on results from a new oceanographic database of the western Mediterranean Sea together with determinations of longterm changes of the fresh water budget. We analyzed temperature and salinity data of the past 40 years to detect deviations from the longterm average. Certain areas and depth ranges are showing increases in temperature or salinity some of which have been found earlier and some which are new. From the regional and vertical distribution we conclude that the observed increases of temperature and salinity in the western Mediterranean Sea are caused both by changes in atmospheric conditions as described by the NAO‐index and by the regulation of Spanish rivers.
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    Propagation of electromagnetic fields in the coastal ocean with applications to underwater navigation and communication (1998)
    AGU (American Geophysical Union)
    In:  Radio Science, 33 . pp. 967-987.
    Details
    Publication Date: 2016-09-15
    Description: We examine the propagation of low-frequency electromagnetic (EM) waves in the coastal ocean produced by controlled or motional impressed sources. Four important modes are the direct, up-over-down, down-over-up, and “beach” modes. The analyses of these modes are complicated by the varying bathymetry in the coastal region. We derive criteria to determine (1) which modes are important for given parameters; (2) a “matched phase” condition describing both when the up-over-down and down-over-up modes interfere constructively in the shallow zone and when the beach mode becomes important; and (3) a low-frequency cutoff, below which the EM fields are not sensitive to the details of the coastal geometry. We verify the theoretically derived criteria with numerical examples and finally discuss the importance of our results in designing navigation and communications applications for subsurface vehicles and instruments.
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    Float Experiment Studies Interocean Exchanges at the Tip of Africa (1998)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 79 (1). pp. 7-8.
    Details
    Publication Date: 2017-02-10
    Description: A joint research effort is currently focused on the oceanic region south of Africa—the gateway for the exchange of mass, heat, and salt between the Indian and Atlantic Oceans (Figure lb). The name of this collaboration, KAPEX, stands for Cape of Good Hope Experiments, Kap der guten Hoffnung Experimente, or Kaap die Goeie Hoop Eksperimente in the three languages of the participating scientists. This is the first time that scientists are using acoustically tracked floats extensively in ocean regions surrounding southern Africa to measure ocean flow patterns. At the tip of Africa, the Agulhas Current from the Indian Ocean interacts with the South Atlantic Current, contributing to the northwestward flowing Benguela Current, which transports water, heat, and salt to the subtropical and subequatorial South Atlantic (Figure la). This transport increases the heat and salinity of the North Atlantic, preconditioning it for the formation of the global thermohaline circulation cell, a driving force of the world climate [Gordon etal., 1992]. Our objective in the KAPEX is to trace the flow of intermediate water around southern Africa by the Agulhas, Benguela, and South Atlantic Current systems and to answer key questions about the inter-oceanic intermediate circulation.
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    Russian and German scientists explore the Arctic's Laptev Sea and its climate system (1998)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 79 (27). 317+322-323.
    Details
    Publication Date: 2019-09-23
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    The role of salinity in circulation of the Cretaceous ocean (1998)
    Schweizerbart
    In:  Zentralblatt für Geologie und Paläontologie / Teil 1, 1996 (11/12). pp. 1445-1454.
    Details
    Publication Date: 2018-02-06
    Description: The density of seawater is a complex function of temperature, salinity, and pressure. Because of the non-linearity of the equation of state of seawater, the densities of sea waters having the same temperature and the same salinity differences (with respect to the mean salinity of the ocean) will vary with the mean salinity of the ocean. Although this strange property of seawater is evident in a plot of the equation of state, it has never been considered in trying to reconstruct ancient ocean circulation. These differences in the density field may have caused the ocean to respond differently to atmospheric forcing in the past. The different response may hold the key to understanding "ocean anoxic events" and episodes of large-scale burial of organic carbon and production of petroleum source rocks.
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