Calcite Layer Research Articles

Diagenetic Environment of Sandstones from the Ermaying Formation (Middle Triassic), North-eastern Margin of the Ordos Basin, China: Implications for the Erosion Vulnerability of the Pi Sandstone

A hard layer of calcite cemented sandstones overlaid weakly consolidated mudstones occur in the Middle Triassic, Ermaying Formation in the eastern margin of the Ordos Basin, China, were examined to reveal the diagenetic environment of Pi sandstone. The Pi sandstone refers to a type of terrigenous clastic rock assemblage composed mainly of sandstone, siltstone and mudstone with red and white color, which is characterized by exposing or being covered with sand or loess, vulnerable to weathering and erosion, distributed in the contiguous area of Shaanxi province, Inner Mongolia autonomous region and Shanxi province in China, and formed during Late Paleozoic–Mesozoic. The “Pi” comes from the folk name (pishuang) of arsenic in Chinese, the appellation of the “Pi sandstone” means that it is harmful to soil and water conservation. The Pi sandstone samples were collected from a sheet-like sandstones deposited in ephemeral river. Petrographic observation show that the samples studied carry a semi-arid climate signal, are cemented by mictrie (microcrystalline calcite), sparry (coarse-crystalline) calcite as well as quartz overgrowths. The δ13C and δ18O values of calcite in the studied are –6.2 to – 3.8 ‰ PDB and from–10.2 to –8.9‰ PDB, respectively. The evidence from petrography and stable isotopes suggests that the cementation of the sandstone mainly occurred in the vadose - phreatic environment.

Volume Fraction, Thickness, and Permeability of the Sealing Layer in Microbial Self-Healing Concrete Containing Biogranules

Autonomous repair systems in construction materials have become a promising alternative to current unsustainable and labor-intensive maintenance methods. Biomineralization is a popular route that has been applied to enhance the self-healing capacity of concrete. Various axenic microbial cultures were coupled with protective carriers, and their combination appears to be useful for the development of healing agents for realizing self-healing concrete. The advantageous traits of non-axenic cultures, such as economic feasibility, self-protection, and high specific activity have been neglected so far, and thus the number of studies investigating their performance as healing agents is scarce. Here we present the self-healing performance of a mortar containing a healing agent consisting of non-axenic biogranules with a denitrifying core. Mortar specimens with a defined crack width of 400 μm were used in the experiments and treated with tap water for 28 days. Self-healing was quantified in terms of the crack volume reduction, the thickness of the sealing layer along the crack depth and water permeability under 0.1 bar pressure. Complete visual crack closure was achieved in the bio-based specimens in 28 days, the thickness of the calcite layer was recorded as 10 mm and the healed crack volume was detected as 6%. Upon self-sealing of the specimens, the water permeability decreased by 83%. Overall, non-axenic biogranules with a denitrifying core shows great potential for development of self-healing bioconcrete.

Microstructural overlap of Macroelongatoolithus eggs from Asia and North America expands the occurrence of colossal oviraptorosaurs

ABSTRACTThe first intact North American Macroelongatoolithus specimen was excavated from sediments in the Wayan Formation of southeastern Idaho and initially reported by Krumenacker et al. (2017, Historical Biology 29:170–186). Here, we present a description of the specimen, including a zonal analysis of microstructural variation and comparison with an egg pair from the Liangtoutang Formation of Zhejiang, China. We assign egg pair IMNH 2428\\49608 from the Albian–Cenomanian Wayan Formation of Idaho and egg pair ZMNH M8705 from the Liangtoutang Formation of China to the oospecies Macroelongatoolithus carlylei. The eggs are 39.8–41.7 cm long and 10.8–14.3 cm wide, and the eggshell consists of two structural layers of calcite demarcated by a distinct, undulating boundary. Eggshell thickness is 1.01–2.17 mm, and the continuous layer to mammillary layer ratio (CL:ML) ranges from 2.05:1 to 7.68:1. Identification of intact Macroelongatoolithus specimens from North America supports synonymy of M. xixiaensis with M. carlylei, as proposed by Zelenitsky et al. (2002) on the basis of eggshell fragments. Due to the substantial microstructural variation of the specimens reported here, we also regard M. goseongensis and M. qiongensis as junior synonyms of M. carlylei. The degree of microstructural variation within the new specimens expands the expected range of variation typically used in diagnosing oospecies and has implications for the naming of new oospecies on the basis of fragments or egg portions alone. The discovery of Macroelongatoolithus eggs from the latest Early Cretaceous of Idaho confirms the presence of this extraordinary oogenus outside of Asia and provides nonskeletal evidence of a Gigantoraptor-sized oviraptorosaur in western North America.

Characterization of contact properties in biocemented sand using 3D X-ray micro-tomography

The mechanical efficiency of the biocementation process is directly related to the microstructural properties of the biocemented sand, such as the volume fraction of calcite, its distribution within the pore space (localized at the contact between grains, over the grain surfaces) and the contact properties: coordination number, contact surface area, contacts orientation and types of contact. In the present work, all these micromechanical properties are computed, for the first time, from 3D images obtained by X-ray tomography of intact biocemented sand samples. The evolution of all these properties with respect to the volume fraction of calcite is analyzed and compared between each other (from untreated sand to highly cemented sand). Whatever the volume fraction of calcite, it is shown that the precipitation of the calcite is localized at the contacts between grains. These results are confirmed by comparing our numerical results with analytical estimates assuming that the granular medium is made of periodic simple cubic arrangements of grains and by considering two extreme cases of precipitation: (1) The calcite is localized at the contact, and (2) the grains are covered by a uniform layer of calcite. In overall, the obtained results show that a small percentage of calcite is sufficient to get a large amount of cohesive contacts.

Marine Bacteria Provide Lasting Anticorrosion Activity for Steel via Biofilm-Induced Mineralization

Steel corrosion is a global problem in marine engineering. Numerous inhibitory treatments have been applied to mitigate the degradation of metallic materials; however, they typically have a high cost and are not environmental friendly. Here, we present a novel and "green" approach for the protection of steel by a marine bacterium Pseudoalteromonas lipolytica. This approach protects steel from corrosion in seawater via the formation of a biofilm followed by the formation of an organic-inorganic hybrid film. The hybrid film is composed of multiple layers of calcite and bacterial extracellular polymeric substances, exhibiting high and stable barrier protection efficiency and further providing an in situ self-healing activity. The process involving the key transition from biofilm to biomineralized film is essential for its lasting anticorrosion activity, which overcomes the instability of biofilm protection on corrosion. Therefore, this study introduces a new perspective and an option for anticorrosion control in marine environments.

Connecting pattern to process: Growth of spiral shell sculpture in the gastropod Nucella ostrina (Muricidae: Ocenebrinae).

Shell morphology is a well-suited and underused system to examine the development of novel forms. The three-dimensional structure produced (the shell) is separate from the largely two-dimensional tissue that secretes it (the mantle), allowing us to disentangle the pattern from the process. Despite knowing a great deal about the mechanics of shell secretion (process), and the variety of shell shapes that exist (pattern), no effort has been made to understand how the mantle changes to produce different shell shapes. We investigated this question in the dimorphic snail Nucella ostrina, which exhibits both smooth and ribbed shells to determine how ribs are formed by the mantle. Rib thickenings are produced only in the outer calcitic shell layer and secreted by the distal Outer Mantle Epithelium (OME) with increased acid phosphatase activity. The evenly thick inner aragonitic layers are secreted by the proximal OME which expresses acid phosphatase. Here we show that locally thicker ribs in N. ostrina are produced by changing the dimensions of the distal OME: elongation in the direction of growth and increased cell height. This should increase the amount of shell material secreted, producing locally thicker shell (ribs). Preliminary evidence suggests this mechanism may be widespread in gastropods.

An avian eggshell from the Brule Formation (Oligocene) of North Dakota

Avian remains are rare from Paleogene sediments in the Great Plains region of North America, especially those that can be identified taxonomically. Here, we describe avian eggshell fragments from the Fitterer Ranch locality within the Oligocene Brule Formation of North Dakota and assign them to ootaxonomy. The specimen examined (NDGS 1934) exhibits low tubercles that form undulating surface ornamentation; 650–900-µm-thick eggshell composed of three structural layers of calcite; abrupt transition between mammillary and continuous layers; prisms visible in continuous layer; squamatic texture primarily visible in lower half of continuous layer; and mammillary-to-total shell thickness ratio of 1:2.70–2.75. Due to this unique combination of characters, we establish the new oospecies Metoolithus jacksonae, oosp. nov. This new oospecies extends both the geographic and temporal ranges of the oogenus Metoolithus and demonstrates the importance of oological research in revealing the diversity of Oligocene avifauna in this region.http//zoobank.org/urn:lsid:zoobank.org:pub:96D66B54-9FBC-4373-B66B-C2CD8C627CCSUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Lawver, D. R., and C. A. Boyd. 2018. An avian eggshell from the Brule Formation (Oligocene) of North Dakota. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1486848.

Recognizing incipient epizootic shell disease lesions in the carapace of the American lobster, Homarus americanus

Causal factors leading to epizootic shell disease (ESD) lesions in American lobster, Homarus americanus H. Milne-Edwards, 1837, are not well understood. We explore the structural and physiological bases for development of ESD from preclinical stages invisible to unaided eye to early visible stages. We present a lobster shell model, which develops structural functional vulnerability and suggests plausible routes to ESD. Medallions of carapace cuticle were obtained from carapace fixed with protocols to minimize movement of mineral and macromolecular components. Rapid processing of medallions was used to encourage large sample sizes compatible with environmental surveys. One- and two-dimensional analytic maps of polished sections of the cuticle, obtained with an electron microprobe, described the composite mineral and polymeric structures. Micro-Raman spectroscopy was used to identify bond properties of phosphates and carbonates, as well as signatures of organic structures. The frequency and properties of structures identified can be monitored through the lobster molting cycle using a high throughput application of micro-computed tomography (μCT). We observed density differences in the calcite layer, exocuticle, and endocuticle, and the frequency and structure of CaCO3 structures in the endocuticle and membranous layer of carapace cuticle during chosen stages of the molting cycle. The correlative microscopy and μCT of shell structures provides improved understanding of the lobster cuticle structure. Detailed structural differences quantified through development and under different environmental conditions can provide insight into causes and vulnerabilities associated with ESD.

In-depth proteomic analyses of Haliotis laevigata (greenlip abalone) nacre and prismatic organic shell matrix

BackgroundThe shells of various Haliotis species have served as models of invertebrate biomineralization and physical shell properties for more than 20 years. A focus of this research has been the nacreous inner layer of the shell with its conspicuous arrangement of aragonite platelets, resembling in cross-section a brick-and-mortar wall. In comparison, the outer, less stable, calcitic prismatic layer has received much less attention. One of the first molluscan shell proteins to be characterized at the molecular level was Lustrin A, a component of the nacreous organic matrix of Haliotis rufescens. This was soon followed by the C-type lectin perlucin and the growth factor-binding perlustrin, both isolated from H. laevigata nacre, and the crystal growth-modulating AP7 and AP24, isolated from H. rufescens nacre. Mass spectrometry-based proteomics was subsequently applied to to Haliotis biomineralization research with the analysis of the H. asinina shell matrix and yielded 14 different shell-associated proteins. That study was the most comprehensive for a Haliotis species to date.MethodsThe shell proteomes of nacre and prismatic layer of the marine gastropod Haliotis laevigata were analyzed combining mass spectrometry-based proteomics and next generation sequencing.ResultsWe identified 297 proteins from the nacreous shell layer and 350 proteins from the prismatic shell layer from the green lip abalone H. laevigata. Considering the overlap between the two sets we identified a total of 448 proteins. Fifty-one nacre proteins and 43 prismatic layer proteins were defined as major proteins based on their abundance at more than 0.2% of the total. The remaining proteins occurred at low abundance and may not play any significant role in shell fabrication. The overlap of major proteins between the two shell layers was 17, amounting to a total of 77 major proteins.ConclusionsThe H. laevigata shell proteome shares moderate sequence similarity at the protein level with other gastropod, bivalve and more distantly related invertebrate biomineralising proteomes. Features conserved in H. laevigata and other molluscan shell proteomes include short repetitive sequences of low complexity predicted to lack intrinsic three-dimensional structure, and domains such as tyrosinase, chitin-binding, and carbonic anhydrase. This catalogue of H. laevigata shell proteins represents the most comprehensive for a haliotid and should support future efforts to elucidate the molecular mechanisms of shell assembly.

The foraminiferal genus Neotrocholina Reichel, 1955 and its less known relatives: A reappraisal

In the foraminiferal tree, the genus Neotrocholina Reichel, 1955 appears as an oddity. It is the only known post-Paleozoic enrolled tubular foraminifer with a test described as exclusively hyaline-radial calcitic. Its taxonomic affinity and phylogenetic relationships have been a matter of debate for over 60 years. Regarded as a potential ancestor for rotaliid foraminifers, it has been alternatively positioned in the orders Spirillinida Hohenegger & Piller, 1975 and Involutinida Hohenegger & Piller, 1977, which are yet defined as calcitic monocrystalline and aragonitic, respectively.In spite of its great potential to solve part of the foraminiferal macroevolution, little is known about the genus Neotrocholina. Based on the study of various excellently preserved materials, including type specimens, we prove that Neotrocholina possesses a composite test made of two calcitic layers, with distinct optical characteristics. In cross-polarized light, the outer part of the test shows a radial crystalline arrangement but the tube wall displays unit extinction, a feature characteristic of monocrystalline spirillinid foraminifers. Similar wall types are observed in Bancilina Neagu, 1995, Hungarillina Blau & Wernli, 1999, Ichnusella Dieni & Massari, 1966, Radiospirillina Blau & Wernli, 1999, and Spirilliconus Blau & Wernli, 1999, which are here encompassed in the subfamily Neotrocholininae n. subfam., together with Neotrocholina.Through an in-depth systematic revision, including comprehensive synonymy lists and the description of two new species (Ichnusella senerae n. sp. and Neotrocholina theodori n. sp.), this manuscript provides new taxonomic, phylogenetic, and stratigraphic schemes and keys for representatives of the here introduced subfamily Neotrocholininae n. subfam.

Analysis of growth and form in Aerothyris kerguelenensis (rhynchonelliform brachiopod) - Shell spiral deviations, microstructure, trace element contents and stable isotope ratios

Brachiopods are usually attached benthic marine invertebrates with a shell (exoskeleton) that archives modifications of the environment via chemical proxies. Growth lines regularly occurring (i.e. excluding random ones) at the shell's surface reflect phenotypically controlled interruptions or changes in secretion (Williams et al., 1997). Modifications in the secretion rate and reorientation of shell elements within the thickness of the shell induce marked changes in shell morphology providing elements for estimating age throughout ontogeny for specimens of Aerothyris kerguelenensis (Davidson 1880, formerly known as Waldheimia kerguelensis Davidson, 1978), from the Southern Indian Ocean. Maxima and minima of deviations from underlying shell spiral growth (SSD), plus maps (SEM-EDS) and Electron Microprobe (WDS) of Mg, S, Ca, Mn, Fe that are components of shells along with Scanning Electron Microscopy (SEM) all show evidence of rhythms at different time scales, including annual ones. Growth rates differ in young and adult forms as well as between dorsal and ventral valves; additionally the amplitudes of Mg peaks are higher early in ontogeny and in the primary shell layer, progressively decreasing with sporadic peaks. Anterior regions of shells are more informative regarding external growth (i.e. major growth marks) and mainly record later events in the life of the individual. Posterior regions are informative for internal morphology (cumulative growth) and reveal Mg bands and related concentrations that are consistent with the main extrema (maxima and minima) observed when using SSD and WDS analyses. Mg/Ca ratios indicate fluctuations in temperature and food supply during the growth period. Seawater temperatures calculated from the oxygen isotope compositions of the secondary layer of calcite in shells of A. kerguelenensis are close to those measured in situ in the environment in which they live. However, the difference in carbon isotope composition between the two studied shells reveal that these specimens record isotopic compositions most likely resulting from a combination of kinetic ‘vital’ isotope effects that may, sometimes, mimic equilibrium values.

Reading the Fine Print: Ultra-Microstructures of Foraminiferal Calcification Revealed Using Focused Ion Beam Microscopy

The elemental composition of calcite is of critical value in paleoceanographic reconstructions, yet little is known about biological processes underlying elemental uptake by foraminifers during calcification. Especially crucial in the understanding of elemental composition and distribution is the involvement of organic templates separating different layers of calcite forming the wall of a foraminiferal chamber. In this study, we applied the focused ion beam (FIB) scanning electron microscopy (SEM) technique to the site of calcification (SOC) in a newly growing chamber of Ammonia beccarii, a benthic foraminifer, to reveal the ultra- and microstructure during calcification. This allowed cross-sections of both soft and hard tissues, allowing detailed observation of the SOC across a series of calcification stages. For the first time, we show that numerous voids of calcareous layers and internal organic structures are present within the SOC during the calcification process. The series of SEM observations suggest that organic layers are actively involved in calcite precipitation. We provide the first evidence that the SOC is isolated from surrounding seawater during calcification. Our findings improve the understanding of foraminiferal biomineralization and characterize key conditions under which element partitioning and isotope fractionation occur.

The potential action of single functionalization treatments and combined treatments for the consolidation of carbonate stones

The lack of relevant chemical bond between carbonate minerals and TEOS-based consolidants was addressed by using a fine-grain porous carbonate stone to investigate alternative single treatments and functionalization pre-treatments for a commercial TEOS-based product (BS) (combined treatments). Solutions of tartaric (T) and citric (C) acids with different aggressiveness (by varying the concentration, pH and number of applications) and one solution of an amino-functional alkoxysilane solution (SiD) were developed and tested. The 0.035 M (pH ≈ 4.5) solutions of T and C applied 3 times revealed the best potential to act as single treatments while more aggressive treatments showed to have negative outcomes: formation of a superficial dense layer (T case) and excessive dissolution of calcite gains (C case). Although the formation of new crystal phases was confirmed, advantages on their use as pre-treatments for BS were hardly unveiled. The SiD was capable to form a thin layer of an ORMOSIL material covering calcite and showed potential to act as single treatment, while also having some potential to improve the superficial consolidation action of BS.

Structure and composition of Unio pictorum shell: arguments for the diversity of the nacroprismatic arrangement in molluscs.

Mollusc shells are complex organomineral structures, the arrangement and composition depending on the species. Most studies are dedicated to shells composed of an aragonite nacreous and a calcite prismatic layer, so the nacreous prismatic model based on Pinctada and Atrina-Pinna. Here, we studied the micro- and nanostructure, the mineralogy and composition of a nacroprismatic bivalve species: Unio pictorum. The prismatic layer of Unio is aragonite, and the inner structure of the prismatic units strongly differs from those of the calcitic layers. The shape of the prisms varies depending on their growth stage. The first layers of nacre are similar to those of gastropods (columnar nacre), then evolve towards the typical bivalve arrangement (sheet nacre). Na, Sr, Mg, P and S are present in both prisms and nacre. The organic prismatic envelopes are rich in sulphur amino acids, whereas organic sulphate is present within the prisms and the nacreous tablets. P is present as phosphate, probably a mixture of organic and mineral complex. Chemical distribution maps confirm the absence of an organic membrane between the nacre and the prisms. The comparison of the structure, mineralogy and composition of Unio pictorum and different species show the diversity of nacroprismatic shells, and that these features are taxonomically dependent.

Testing whether early diagenesis of skeletal carbonate is different in non-marine settings: Contrasting styles of molluscan preservation in the Upper Jurassic of Portugal

This study tests the hypothesis that the early diagenesis of aragonite shells should differ fundamentally between marine and freshwater environments. This is predicted to be the case because aragonite is highly susceptible to dissolution in the TAZ (Taphonomically Active Zone) in low energy marine settings due to acidity caused largely by the oxidation of H2S generated by sulphate-reducing bacteria, but reduced sulphide activity in freshwater settings should result in less early dissolution of aragonite. To test this hypothesis a range of fresh-brackish-hypersaline and marine limestones were sampled from the Upper Jurassic (mid Oxfordian) Cabaços Formation of central western Portugal. In these freshwater and brackish deposits, molluscs are preserved mostly as sparite shell replacements indicating that the original aragonite was preserved through the TAZ and was later replaced during subsequent burial by calcite cement. In limestones deposited in more marine to hypersaline settings, molluscan remains mostly consist of the calcitic layers of bimineralic bivalves, as shell where the original was wholly calcitic, or as gastropod steinkerns. Exceptions occur and reflect other factors such as higher energy conditions during deposition whereby organic matter, as the drive for microbial decay processes, was removed. The implications for molluscan preservation including some hydrocarbon reservoirs are discussed.

Revisiting the role of calcite in Spondylus crassisquama shell

Spondylus crassisquama (princeps) is a bivalve mollusc inhabiting the coast of Central and South Americas. Its shell uses a two-layer armour system to protect the inner soft tissues from the environment. The external layer is composed of a calcite-based composite, and the internal layer is composed of an aragonite-based composite. For the first time, the structural characterisation of the S. crassisquama shell is reported. The results show similarities with those for other shells such as that of red abalone. As expected, microhardness tests demonstrate that the internal nacreous layer is stiffer and harder than the external calcitic layer. However, the amount of energy dissipated during the indentation tests of the calcitic layer is similar to the amount of energy dissipated by the nacreous layers. Despite the highly ordered structure of the nacreous layer, the characterisation conducted shows that the calcitic layer can also be used as a model for the development of bioinspired materials.

Impact of grain size evolution on necking in calcite layers deforming by combined diffusion and dislocation creep

Natural pinch-and-swell structure in centimetre-thick calcite layers shows a reduction of grain size from swell towards pinch. However, the impact of grain size evolution on necking and pinch-and-swell formation is incompletely understood. We perform zero-dimensional (0D) and 2D thermo-mechanical numerical simulations of calcite layer extension to quantify the impact of grain size evolution on necking for bulk extension rates between 10−12 s−1 and 10−14 s−1 and temperatures around 350 °C. For a combination of diffusion and dislocation creep we calculate grain size evolution according to the paleowattmeter (grain size is proportional to mechanical work rate) or the paleopiezometer (grain size is proportional to stress). Numerical results fit three observations: (i) significant thickness variations along the layer after extension, (ii) grain size reduction from swells towards pinches, and (ii) dislocation creep dominated deformation in swells and significant contribution of diffusion creep in pinches. Modelled grain size in pinches (10–60 μm) and swells (70–800 μm) is close to observed grain size in pinches (21 ± 6 μm) and in swells (250–1500 μm). In the models, grain size evolution has a minor impact on necking, and viscous shear heating and grain size evolution have a negligible thermal impact.

Microstructure of aptychi of Upper Jurassic (Upper Oxfordian) ammonites from Central Russia

To date, bivalve calcitic plates which cover the outer chitinous lamella of the lower jaws of Jurassic and Cretaceous ammonoids (aptychi sensu stricto) have been classified into several morphotypes (form genera) based on shape, surface sculpture and internal microstructure. However, previous works on aptychi microstructure focused mainly on thick morphotypes (e.g. Laevaptychus and Lamellaptychus), whereas little was investigated for thin Praestriaptychi. In this study, the microstructure of Praestriaptychi of Upper Oxfordian Perisphinctes and of recently discovered aptychi of aspidoceratid microconch Mirosphinctes (both belonging to the superfamily Perisphinctoidea) is described. These aptychi are compared with Laevaptychi of Upper Oxfordian macroconchs Euaspidoceras (a dimorphic counterpart of Mirosphinctes). This study demonstrated that the aptychi of Perisphinctes and Mirosphinctes differ from each other and from Laevaptychi in their microstructure, number of calcitic layers and in growth patterns. The aptychi of both aspidoceratid dimorphs are similar in terms of growth pattern, but differ in microstructure and the number of calcitic layers. Neither aptychi of Perisphinctes nor of Mirosphinctes have a tubular layer with a honeycomb surface pattern, which is typical for Laevaptychus. This indicates that aptychi were extremely diverse and their microstructure varied significantly not only within the superfamily, but even within a dimorphic pair of aspidoceratid ammonites. A lack of a tubular layer in Praestriaptychus indicates that it developed independently in Lamellaptychus of Haploceratoidea and Laevaptychus of Aspidoceratidae.

Numerical modelling of the healing process induced by carbonation of a single crack in concrete structures: Theoretical formulation and Embedded Finite Element Method implementation

We consider a model for reactive flows which describes the healing process induced by carbonation of a single crack in concrete structures. The aim of this paper is to study the complex interplay between advection-diffusion mechanisms in a crack-matrix system combined with different chemical reactions taking place (dissolution/precipitation). Carbonated water is first injected through a crack. Then, a diffusion process of calcium ions (Ca2+ ) takes place from the porous matrix to the crack due to the existing calcium ions concentration gradient. Finally, those calcium and carbonates ions (CO32−) from the percolating solution react to form a calcite (CaCO3) layer responsible for the healing of the crack. The developed model takes the form of transport-reaction partial differential equations for both crack and porous matrix. From numerical point of view these equations are discretized by means of the Embedded Finite Element Method (E-FEM). The E-FEM allows to use meshes not necessarily matching the physical interface, defined herein as the crack, while retaining the accuracy of the classical finite element approach. This is achieved by introducing a weak discontinuity in the calcium ions concentration field for finite elements where the crack is present. A numerical solving strategy is presented to efficiently resolve the FE problem both in terms of calcium and carbonate concentration field variables and weak discontinuity parameters. In addition, an analytical model for the computation of the calcite layer width, resulting in the healing process, is suggested. Finally, considering the dependence of the diffusivity and permeability coefficients on the width of the calcite, a coupled model arises for the numerical modelling of the healing process induced by carbonation in a crack.

Evolution and palaeogeographical dispersion of the radiolitid rudist genus Auroradiolites (Bivalvia, Hippuritida) with descriptions of new material from Tibet and archived specimens from Afghanistan

AbstractDiagnostic characters of the recently established new genus of radiolitid rudist Auroradiolites include an entirely compact outer shell layer, a distinctly convex upper (left) valve and a robust myocardinal apparatus surrounding a strongly internally projected ligamentary infolding. Until now, A. biconvexus (previously considered to be exclusively late Albian in age) has been reported only from the Langshan Formation, which crops out along the northern portion of the Lhasa block, Tibet. Here, it is recorded for the first time from the Sangzugang Formation of the Xigaze Forearc Basin, situated on the southern margin of the Lhasa block. A. biconvexus differs from the south‐west Asian type species A. gilgitensis (late Aptian to Albian) by its relatively larger size, more strongly domed left valve and distinct radial undulations of the outer shell layer. The characters of A. gilgitensis are further clarified from archived material from central Afghanistan, also newly identified and described herein. All Auroradiolites records to date are revised. The recognition of examples of A. biconvexus from upper Aptian strata increases both the stratigraphical and geographical ranges of the species, indicating that it had already branched off from A. gilgitensis in the late Aptian. Some new radiolitid specimens that combine both compact and cellular calcitic outer shell layer structures are also described from the Langshan Formation, but a number of internal differences from Auroradiolites cast doubt on their constituting a sister group to the latter and we assign them to Eoradiolites cf. hedini. Rather, the evolution of Auroradiolites directly from the ancestral radiolitid genus Agriopleura is favoured on the grounds of parsimony. So far, the genus Auroradiolites has been recorded from Iran, central and eastern Afghanistan, the type locality of Yasin in north‐western Pakistan, southern and northern Ladakh, the Lhasa block, as well as Hokkaido in northern Japan. During the late Aptian to Albian interval, all of these localities were associated with terranes and blocks that were limited to the north‐eastern margin of Tethys and the western Pacific margin, making Auroradiolites an indicator of a SW Asian to Pacific faunal province.