Fibrous Aragonite Research Articles

Architecture of Anoteropora latirostris (Bryozoa, Cheilostomata) and implications for their biomineralization

Cheilostome Bryozoa Anoteropora latirostris, a colonial marine invertebrate, constructs its skeleton from calcite and aragonite. This study presents firstly correlated multi-scale electron microscopy, micro-computed tomography, electron backscatter diffraction and NanoSIMS mapping. We show that all primary, coarse-grained platy calcitic lateral walls are covered by fine-grained fibrous aragonite. Vertical lateral walls separating autozooid chambers have aragonite only on their distal side. This type of asymmetric mineralization of lateral walls results from the vertical arrangement of the zooids at the growth margins of the colony and represents a type of biomineralization previously unknown in cheilostome bryozoans. NanoSIMS mapping across the aragonite-calcite interface indicates an organic layer between both mineral phases, likely representing an organic template for biomineralization of aragonite on the calcite layer. Analysis of crystallographic orientations show a moderately strong crystallographic preferred orientation (CPO) for calcite (7.4 times random orientation) and an overall weaker CPO for aragonite (2.4 times random orientation) with a high degree of twinning (45%) of the aragonite grains. The calculated Young’s modulus for the CPO map shows a weak mechanical direction perpendicular to the colony’s upper surface facilitating this organism’s strategy of clonal reproduction by fragmentation along the vertical zooid walls.

Formation of tubular carbonate conduits at Athina mud volcano, eastern Mediterranean Sea

Abstract Tubular carbonate conduits (TCC) represent the termination of fluid plumbing systems in environments of hydrocarbon seepage and play a relevant role in the discharge of methane from sub-seafloor sediments to the water column. However, the biogeochemical reactions and biological activities involved in their formation are not fully understood. To address this, TCC samples were collected with a remotely operated vehicle from the seabed on the SW flank of the Athina mud volcano in the eastern Mediterranean Sea. Petrographic, mineralogical, stable carbon and oxygen isotope and lipid biomarker analyses were performed to elucidate the formation processes of the tubular carbonates. Clotted and fibrous aragonite form the internal lining of the cavities, while the outer portion of the tubes is formed by micritic Mg-calcite cementing hemipelagic sediment. 13C-depleted Mg-calcite and aragonite (as low as −14.4‰ V-PDB) and lipid biomarkers (archaeol, −89.8‰ V-PDB) indicate that carbonate precipitation was influenced by sulfate-dependent anaerobic oxidation of methane (AOM). AOM locally enhances aragonite precipitation, thereby facilitating early lithification of the conduits within the mud volcano sediments. The size and morphology of the TCC comparable with the buried portion of tubeworm colonies found in the proximity of the sampling site. However, our results suggest that TCC likely formed by the action of burrowing organism rather than being mineralizations of the tubeworm colonies. This study provides new insights into the interpretation and understanding of TCC, highlighting the role of macrofaunal activity in the formation of migration pathways for hydrocarbon-rich fluids on the flank of a mud volcano.

Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico

AbstractOil seeps from the southern Gulf of Mexico can be regarded as natural laboratories where the effect of crude oil seepage on chemosynthesis‐based communities and carbonate precipitation can be studied. During R/V Meteor cruise 114 the seep sites UNAM (Universidad Nacional Autónoma de México) Ridge, Mictlan Knoll and Tsanyao Yang Knoll (Bay of Campeche, southern Gulf of Mexico) were investigated and sampled for authigenic carbonate deposits containing large amounts of liquid oil and solid asphalt. The δ13C values of individual carbonate phases including: (i) microcrystalline matrix aragonite and calcite; (ii) grey, cryptocrystalline to microcrystalline aragonite; and (iii) clear, fibrous aragonite cement, are between −30‰ and −20‰, agreeing with oil as the primary carbon source. Raman spectra reveal that residual heavy oils from all sites are immature and most likely originate from the same reservoir. Geochemical batch modelling using the software code PHREEQC demonstrates how sulphate‐driven oxidation of oil‐derived low‐molecular to high‐molecular weight hydrocarbons affects carbonate saturation state, and shows that the oxidation state of carbon in hydrocarbon compounds and oxidation rates of hydrocarbons control carbonate saturation and precipitation at oil seeps. Phase‐specific trace and rare earth element contents of microcrystalline aragonite and calcite, grey cryptocrystalline aragonite and clear aragonite were determined, revealing enrichment in light rare earth elements for grey aragonite. By comparing trace element patterns of carbonates with those of associated oils, it becomes apparent that liquid hydrocarbons constitute an additional source of trace metals to sedimentary pore waters. This work not only demonstrates that the microbial degradation of oil at seeps may result in the precipitation of carbonate minerals, it also elucidates that trace metal inventories of seep carbonates archive diagnostic elemental patterns, which can be assigned to the presence of heavy hydrocarbons in interstitial pore waters.

The PteropodCreseis aciculaForms Its Shell through a Disordered Nascent Aragonite Phase

Shelled pteropods are holoplanktonic mollusks that build lightweight shells containing aragonite crystals. The complex shell architecture is composed of well-aligned, curved aragonitic fibers. Each...

Genetic link between Miocene seafloor methane seep limestones and underlying carbonate conduit concretions at Rocky Knob, Gisborne, New Zealand

ABSTRACTMethane-derived authigenic carbonates (MDACs) in Miocene bathyal mudstones in North Island, New Zealand are typically expressed as either sub-seafloor conduit concretions or as seafloor seep limestones, but rarely are both types exposed in outcrop at one locality. Consequently, any potential genetic link between them is usually inferred. This also appears to be the case for global occurrences of MDAC. At the Rocky Knob seep complex near Gisborne both seep limestones and conduit concretions co-occur. The petrography and stable carbon (δ13C) and oxygen (δ18O) isotope compositions of their various authigenic carbonate components (automicrite, fibrous aragonite crystals, and granular, blocky and bladed calcite crystals) show that distinctive isotope and petrographic groupings for precipitates within the conduit concretions match or “correlate” with several of those in the seep limestones. This corroborates their genetic tie and derivation from the same fluids, albeit in different parts (i.e. sub-seafloor vs. seafloor) of the seep complex.

Beachrock Cementation Patterns Along the Gulf of Aqaba Coast, Saudi Arabia

Beachrocks crop out along the Saudi coast of the Gulf of Aqaba with beds lying at elevations up to 0.8 m above the present sea level and with seaward extents that repeatedly submerge and emerge during high and low water levels. This study discusses the cement composition, petrography and bedding features of beachrocks at 24 different locations, of which a total of eight sites were sampled. The study focuses on the petrographic and microtextural characteristics of the beachrocks to elucidate their cementation environment and diagenetic evolution. The results revealed the predominance of three main cement types: (1) fibrous aragonite cement with morphologies including isopachous rims around grains with or without micritic substrates, pore-filling radial aggregates, pseudospherulite and randomly oriented interlocking aragonite fibers; (2) microcrystalline low-Mg calcite (LMC) cement binding grains and filling inter- and intra-granular pore spaces; and (3) micritic cement in the form of envelopes and/or pore-filling micrite in some samples with meniscus bridges. The last two types also contain infiltrated silt-sized clasts and skeletal remains. The cements indicate precipitation mainly in pore-filled marine phreatic diagenetic zone and, rarely, in the marine vadose zone with the aid of microbial and algal activity. Petrographic investigations and energy-dispersive X-ray spectroscopy of selected samples showed the occurrences of aragonite and LMC.

Functional Analyses of MMPs for Aragonite Crystal Formation in the Ligament of Pinctada fucata

The mollusk class, Bivalvia, plays an important role in the formation of calcium carbonate in the ocean. The bivalve hinge ligament is hard but pliant tissue that resists the stress placed on the hinge during opening and closing. The ligament comprises a fine microstructure of fibrous aragonite crystals surrounded by a dense organic matrix. The matrix consists of organic fibers that are aligned with the fibrous aragonite crystals. Previous studies identified a tissue inhibitor of metalloproteinase (Pf-TIMP) in the organic fibers of Pinctada fucata ligaments. This enzyme exhibited strong inhibition of matrix metalloproteinase (MMP) activity in vitro, suggesting that MMPs also play a role in formation of the organic fibers of the ligament. Using transcriptome data, we identified MMP genes from the mantle isthmus, which is a soft tissue attached to the ligament. To investigate the function of MMP genes in vivo, we performed RNA interference experiments. The expression of MMP14973 and MMP07860 genes was inhibited after injection of each dsRNA. Cumulative injection of MMP07860 dsRNA induced disordered aragonite fiber orientation, whereas the injection of MMP14973 had no effect. When the decalcified ligament was incubated in a solution saturated with calcium carbonate, aragonite fibers aligned along the surface. To investigate MMP function in calcium carbonate crystallization in detail, we precipitated aragonite crystals in collagen gels treated with or without recombinant MMP-1. Treatment with MMP-1 increased the interaction between collagen gels and calcium carbonate. These results suggest that MMPs degrade extracellular matrices in the ligament to produce the fine organic fibers that regulate the orientation of fibrous aragonite crystals.

Rare earth elements as tracers for microbial activity and early diagenesis: A new perspective from carbonate cements of ancient methane-seep deposits

Ancient methane-seep carbonates are geological archives of environmental conditions and record past microbial activity. To better understand the information stored in this archive, a comparison was made of phase-specific major, trace, and rare earth element (REE) patterns, mineralogies, and textures of two early diagenetic carbonate phases in five ancient methane-seep deposits including Oligocene limestones from the Satsop and Canyon rivers (Washington State), the Cretaceous Tepee Buttes (Colorado), and the Carboniferous Ganigobis limestones (Namibia). (1) Cryptocrystalline, yellow aragonite and (2) banded and botryoidal aggregates of clear, fibrous aragonite typify many Phanerozoic seep limestones, and are common features of authigenic seep carbonates forming close to the sediment-water interface today. Light REE enrichment in yellow aragonite is observed together with high organic matter contents, indicated by intense autofluorescence of yellow aragonite, and occurs in combination with low Mg/Sr ratios. Conversely, heavy REE enrichment is observed in neomorphic yellow calcite spar typified by high Mg/Sr ratios, indicating progressive diagenetic alteration during early diagenesis in marine pore waters affected by a succession of biogeochemical reactions. These observations suggest that REE contents and patterns of early diagenetic carbonate phases are a function of the quality of preservation. Transformation of primary aragonite to calcite resulting from changing pore water compositions during later-stage early diagenesis is apparently capable to significantly modify elemental composition. The new results reveal that rare earth elements do not necessarily behave conservatively if early diagenesis is controlled to a large extent by evolving pore water compositions in environments strongly affected by biogeochemical processes. For such settings a careful assessment of diagenetic alteration is consequently required before calcite resulting from aragonite transformation can be used for reliable paleoenvironment reconstruction.

UNALTERED NACRE FROM THE PENNSYLVANIAN BUCKHORN ASPHALT, AND IMPLICATIONS FOR THE ARMS RACE BETWEEN MOLLUSKS AND THEIR PREDATORS

Nanoscale details of original aragonite crystals and organic inclusions are preserved in shells from the Pennsylvanian Buckhorn Asphalt of Oklahoma, USA. Exceptional preservation occurred because, either during or shortly after deposition, oil migrated along wrench faults generated during the simultaneous Ouachita Orogeny. The early sealing by oil (later converted into asphalt) prevented diagenetic alteration of shell material. Field Emission Scanning Electron Microscopy (FE-SEM), Electron Backscatter Detection (EBSD), and Atomic Force Microscopy (AFM) reveal a striking high fidelity of preservation, including the oldest known unaltered nacre tablets in gastropods, bivalves, and cephalopods. These nacre tablets are indistinguishable from modern representatives in nanoscale morphology and crystallographic orientations. Fossils from the Buckhorn Asphalt show that by the Pennsylvanian Period, nacre and crossed lamellar were the dominant microstructures in the inner shell layer of the Mollusca. Calcitic microstructures and loosely organized horizontal bundles of aragonite fibers were common among Cambrian mollusks and problematic lophotrochozoans (e.g., hyoliths). Through the early to middle Paleozoic the dominance changed to more fracture-resistant textures nacre and crossed lamellar. This transition reflects the importance of these two types of shell microstructure in deterring predation, and it is clear that the ability to produce crossed lamellar and nacreous microstructures contributed to molluscan success during the Mesozoic and Cenozoic eras.

Magnesium-Rich Nanometric Layer in the Skeleton of Pocillopora damicornis With Possible Involvement in Fibrous Aragonite Deposition

The complex structures and morphologies of coral skeletons make it difficult to study the construction of individual skeleton components. This is especially true regarding micro-structures deposited during different stages of skeletogenesis. As such structures serve as the basis for subsequent skeleton development, they are often obscured by additional skeletal deposition and growth. Recently we described a new model system, coral-on-a-chip, for studying coral biology and skeletogenesis. This system utilizes micropropagates of the Indo-Pacific coral Pocillopora damicornis maintained within a microfluidic environment, allowing us to follow early stages of skeletogenesis over time and under controlled environmental conditions. Our findings reveal that, following settlement onto glass slides, the micropropagates initially form a thin, almost two dimensional skeleton, which subsequently develops into a robust three-dimensional form with features resembling those of the mother colony. Studying the early stages of skeleton accretion in our micropropagates using high-resolution scanning electron microscopy and Energy Dispersive X-ray Spectroscopy (EDS) revealed a magnesium-rich layer deposited directly on the glass surface. This layer, which has a typical Mg/Ca ratio of 1.43±0.78, forms a dense lattice with a typical pore size of 100 nm. Microscopic observations indicate that this lattice serves as the basis for subsequent growth of fibrous aragonite. Examination of the underside of a skeleton from a small P. damicornis colony growing on a glass surface revealed a similar high-magnesium lattice at the interface between the glass and aragonitic skeleton in association with fibrous aragonite deposition. These observations suggest a role for this magnesium-rich lattice in the deposition of the fibrous aragonite forming the bulk of the coral skeleton.

Pleistocene sea‐floor fibrous crusts and spherulites in the Danakil Depression (Afar, Ethiopia)

AbstractPleistocene fibrous aragonite fabrics, including crusts and spherules, occur in the Danakil Depression (Afar, Ethiopia) following the deposition of two distinctive Middle and Late Pleistocene coralgal reef units and pre‐dating the precipitation of evaporites. Crusts on top of the oldest reef unit (Marine Isotope Stage 7) cover and fill cavities within a red algal framework. The younger aragonite crusts directly cover coralgal bioherms (Marine Isotope Stage 5) and associated deposits. Their stratigraphic position between marine and evaporitic deposits, and their association to euryhaline molluscs, suggest that the crusts and spherules formed in restricted semi‐enclosed conditions. The availability of hard substrate controls crust formation with crusts more often found on steep palaeo‐slopes, from sea level up to at least 80 m depth, while spherules mainly occur associated with mobile substrate. Crusts reach up to 30 cm in thickness and can be microdigitate, columnar (branching and non‐branching) or non‐columnar, with laminated and non‐laminated fabrics. Two different lamination types are found within the crystalline fabrics: (i) isopachous lamination; and (ii) irregular lamination. These two types of lamination can be distinguished by the organization of the aragonite fibres, as well as the lateral continuity of the laminae. Scanning electron microscopy with energy dispersive X‐ray spectroscopy analyses on well‐preserved samples revealed the presence of Mg‐silicate laminae intercalated with fibrous aragonite, as well as Mg‐silicate aggregates closely associated with the fibrous aragonite crusts and spherules. The variety of observed fabrics results from a continuum of abiotic and microbial processes and, thus, reflects the tight interaction between microbially mediated and abiotic mineralization mechanisms. These are the youngest known isopachously laminated, digitate and columnar branching fibrous crusts associated with a transition from marine to evaporitic conditions. Understanding the context of formation of these deposits in Afar can help to better interpret the depositional environment of the widespread Precambrian sea‐floor precipitates.

Minerals in the pre-settled coral Stylophora pistillata crystallize via protein and ion changes

Aragonite skeletons in corals are key contributors to the storage of atmospheric CO2 worldwide. Hence, understanding coral biomineralization/calcification processes is crucial for evaluating and predicting the effect of environmental factors on this process. While coral biomineralization studies have focused on adult corals, the exact stage at which corals initiate mineralization remains enigmatic. Here, we show that minerals are first precipitated as amorphous calcium carbonate and small aragonite crystallites, in the pre-settled larva, which then evolve into the more mature aragonitic fibers characteristic of the stony coral skeleton. The process is accompanied by modulation of proteins and ions within these minerals. These findings may indicate an underlying bimodal regulation tactic adopted by the animal, with important ramification to its resilience or vulnerability toward a changing environment.

Redox conditions and authigenic mineralization related to cold seeps in central Guaymas Basin, Gulf of California

Highlights • In cold seeps of Guaymas Basin, aragonite, barite and pyrite precipitated from modified seawater. • Aragonite is highly depleted in 13C suggesting formation via anaerobic oxidation of methane. • Barite formed through mixing of reducing, Ba-rich seep fluids with a 34S-rich sulfate pool. • Pyrite framboids formed under anoxic-sulfidic water via microbial sulfate reduction. Abstract Authigenic carbonate crusts, surface muds and bivalve shell fragments have been recovered from inactive and active recently discovered cold seep sites in central Guaymas Basin. In this study, for first time, redox conditions and fluid sources involved in mineral precipitation were investigated by analyzing the mineralogy and textures of surface samples, along with skeletal contents, and C, O and S isotopes variations. The δ13C values of aragonitic bivalve shells and non-skeletal carbonate from some surface muds (1‰ to −3.7‰ V-PDB) suggest that carbonate precipitated from ambient dissolved inorganic carbon, whereas fibrous aragonite cement and non-skeletal carbonate from other sites are highly depleted in 13C (down to −47.6‰ V-PDB), suggesting formation via anaerobic oxidation of methane, characteristic of methane seepage environments. δ18O in most of the carbonates varies from +1.4‰ to +3.2‰ V-PDB, indicating that they formed from slightly modified seawater. Some non-skeletal carbonate grains from surface muds have lower δ18O values (−12.5‰ to −8.2‰ V-PDB) reflecting the influence of 18O-depleted pore water. Size distribution of pyrite framboids (mean value: 3.1 μm) scattered within diatomaceous sinter suggests formation from anoxic-sulfidic bottom waters. δ34S in pyrite is of −0.3‰ V-CDT compared to +46.6‰ V-CDT in barite, thus implying a fluid sulfate−sulfide fractionation of 21.3‰ that argues in favor of microbial sulfate reduction as the processes that mediated pyrite framboid formation, in a semi-closed system. Barite formation occurred through the mixing of reducing and Ba-rich seep fluids with a 34S-enriched sulfate pool that resulted from microbial sulfate reduction in a semi-closed system. The chemical composition of aragonite cement, barite and pyrite suggest mineral precipitation from modified seawater. Taken together, our data suggest that mineralization at the studied seep sites is controlled by the mixing of seawater with minor amounts of hydrothermal fluids, and oxygen-depleted conditions favoring anaerobic microbial processes.

Multiple Dolomitization and Fluid Flow Events in the Precambrian Dengying Formation of Sichuan Basin, Southwestern China

AbstractThe Precambrian Dengying Formation is a set of large‐scale, extensively dolomitized, carbonate reservoirs occurring within the Sichuan Basin. Petrographic and geochemical studies reveal dolomitization was a direct result of precipitation by chemically distinct fluids occurring at different times and at different intensities. Based on this evidence, dolomitization and multiple fluid flow events are analyzed, and three types of fluid evolution models are proposed. Results of analysis show that Precambrian Dengying Formation carbonates were deposited in a restricted peritidal environment (630–542 Ma). A high temperature and high Mg2+ concentration seawater was a direct result of dolomitization for the micrite matrix, and for fibrous aragonite in primary pores. Geochemical evidence shows low δ18O values of micritic dolomite varying from −1.29± to −4.52± PDB, abundant light rare earth elements (REEs), and low dolomite order degrees. Microbes and meteoric water significantly altered dolomite original chemical signatures, resulting in algal micritic dolomite and the fine‐grained, granular, dolosparite dolomite having very negative δ18O values. Finely crystalline cement dolomite (536.3–280 Ma) and coarsely crystalline cement dolomite have a higher crystallization degree and higher order degree. The diagenetic sequence and fluid inclusion evidence imply a linear correlation between their burial depth and homogenization temperatures, which closely resemble the temperature of generated hydrocarbon. Compared with finely crystalline dolomite, precipitation of coarsely crystalline dolomite was more affected by restricted basinal fluids. In addition, there is a trend toward a more negative δ18O value, higher salinity, higher Fe and Mn concentrations, REE‐rich. Two periods of hydrothermal fluids are identified, as the exceptionally high temperatures as opposed to the temperatures of burial history, in addition to the presence of high salinity fluid inclusions. The early hydrothermal fluid flow event was characterized by hot magnesium‐ and silicon‐rich fluids, as demonstrated by the recrystallized matrix dolomite that is intimately associated with flint, opal, and microcrystalline quartz in intergranular or intercrystalline pores. This event was likely the result of a seafloor hydrothermal chimney eruption during Episode I of the Tongwan Movement (536.3±5.5 Ma). In contrast, later hydrothermal fluids, which caused precipitation of saddle dolomite, were characterized by high salinity (15–16.05wt% NaCl equivalent) and homogenization temperatures (250 to 265°C), δ18O values that were more enriched, and REE signatures. Geochemical data and the paragenetic sequence indicate that this hydrothermal fluid was related to extensive Permian large igneous province activity (360–280 Ma). This study demonstrates the presence of complicated dolomitization processes occurring during various paleoclimates, tectonic cycles, and basinal fluids flow; results are a useful reference for these dolomitized Precambrian carbonates reservoirs.

Structural and functional analyses of a TIMP and MMP in the ligament of Pinctada fucata.

The bivalve hinge ligament is the hard tissue that functions to open and close shells. The ligament contains fibrous structures consisting of aragonite crystals surrounded by a dense organic matrix. This organic matrix may contribute to the formation of fibrous aragonite crystals, but the mechanism underlying this formation remains unclear. In this study, we identified a novel ligament-specific protein, Pinctada fucata tissue inhibitor of metalloproteinase (PfTIMP), from the fibrous organic matrix between aragonite crystals in the ligament using the amino acid sequence and cDNA cloning methods. PfTIMP consists of 143 amino acid residues and has a molecular weight of 13,580.4. To investigate the activity of PfTIMP, inhibition of matrix metalloproteinase (MMP) activity was measured. PfTIMP strongly inhibited human MMP13 and MMP9. Eight MMP homologs were identified from a P. fucata genomic database by BLAST search. To identify the specific MMP that may contribute to ligament formation, the expression level of each MMP was measured in the mantle isthmus, which secretes the ligament. The expression of MMP54089 increased after scratching of the ligament, while the expressions of other MMPs did not increase after doing the same operation. To identify the role of MMP54089 in forming the ligament structure, double stranded (ds) RNA targeting MMP54089 was injected into living P. fucata to suppress the function of MMP54089. Scanning electron microscopic images showed disordered growing surfaces of the ligament in individuals injected with MMP54089-specific dsRNA. These results suggest that PfTIMP and MMP54089 play important roles in the formation of the fibrous ligament structure.

Study on characteristics and genesis of botryoidal dolostone of the Upper Sinian Dengying Formation: a case study from Hanyuan region, Sichuan, China

Botryoidal dolostone (BD) developed extensively within the Neoproterozoic Upper Sinian Dengying Formation in the Hanyuan region, which is an important gas reservoir. Based on literature review, a study of their outcrop characteristics, petrology, microfabric, degree of order, and genesis reveal that BD mainly comprises botryoidal lumps (BL) originated from depositional-penecontemporaneous seawater and botryoidal and ctenoid cements or fillings. BL occurred in parallel beds during the depositional-syngenetic stage, including core and coating. The core consists of grapestone and thrombolite, which is dominated by muddy and microcrystalline high-magnesium calcite (HMC) resulting from marine deposition and micro-organism capturing and binding effects. The coating, classified as either thin or thick, is mainly isopachous fibrous aragonite (IFA) that formed during the depositional-syngenetic stage. Thin coatings formed in the turbulent sea bottom, while thicker ones formed in a relatively quiet sea bottom. Botryoidal and ctenoid cements and fillings compose IFA and HMC. IFA that includes marine phreatic zone deposition from the syngenetic-penecontemporaneous stage mainly occurs in the spaces between grains of BL and intraclast or on the top of them. The HMC that is the product from meteoric leaching in the penecontemporaneous stage and marine deposition occurs in the dissolved pores and fractures. During the process when BL and the botryoidal, ctenoid cements, or fillings formed, either abiogenetic or microbiogenic aragonite and HMC were mimetically dolomitized and preserved the primary texture and structure. BD was also reworked by the meteoric leaching in the penecontemporaneous and supergenous stages.

Cymbiola nobilis shell: Toughening mechanisms in a crossed-lamellar structure

Natural structural materials with intricate hierarchical architectures over several length scales exhibit excellent combinations of strength and toughness. Here we report the mechanical response of a crossed-lamellar structure in Cymbiola nobilis shell via stepwise compression tests, focusing on toughening mechanisms. At the lower loads microcracking is developed in the stacked direction, and channel cracking along with uncracked-ligament bridging and aragonite fiber bridging occurs in the tiled direction. At the higher loads the main mechanisms involve cracking deflection in the bridging lamellae in the tiled direction alongside step-like cracking in the stacked direction. A distinctive crack deflection in the form of “convex” paths occurs in alternative lamellae with respect to the channel cracks in the tiled direction. Furthermore, a barb-like interlocking mechanism along with the uneven interfaces in the 1st-order aragonite lamellae is also observed. The unique arrangement of the crossed-lamellar structure provides multiple interfaces which result in a complicated stress field ahead of the crack tip, hence increasing the toughness of shell.

An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite

The isotopic composition of boron (δ11B) in marine carbonates is well established as a proxy for past ocean pH, however, its robust application to palaeo-environments relies on the generation of species-specific calibrations. Existing calibrations utilising the deep-sea coral (DSC) Desmophyllum dianthus highlight the potential application of this pervasive species to pH reconstructions of intermediate depth waters. Nevertheless, considerable uncertainty remains regarding the estimation of seawater pH from these bulk skeletal δ11B measurements, likely resulting from microstructural heterogeneities in δ11B of D. dianthus. To circumvent this problem, thus improving the reliability of the D. dianthus δ11B-pH calibration, we present a new δ11B calibration of micro-sampled fibrous aragonite from this species.Modern coral specimens recovered from the Atlantic, Pacific, and Southern Oceans, micro-sampled using microdrilling, micromilling, and laser cutting extraction, were analysed for trace element (B/Ca, Mg/Ca, Sr/Ca, and U/Ca) and boron isotopic composition. We find the best calibration against the δ11B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises δ11B measurements of fibres with likely slow growth rates and minimal contamination from adjacent microstructures (identified by low Mg/Ca) for each coral specimen. This new calibration exhibits a stronger, and better-defined dependence on ambient seawater pH compared to bulk coral δ11B; δ11Bfibre=(0.93±0.17)×δ11Bborate+(12.02±2.63). We suggest that the majority of the variability in measured δ11B between replicate bands of fibrous aragonite from a D. dianthus specimen can be explained by small incorporation of non-fibrous aragonite and surface impurities during microsampling and growth rate effects. This study confirms the utility of D. dianthus as an archive of precise palaeo-pH (±0.07pH units), provided that suitable sampling strategies are applied.

Biological strategy for the fabrication of highly ordered aragonite helices: the microstructure of the cavolinioidean gastropods.

The Cavolinioidea are planktonic gastropods which construct their shells with the so-called aragonitic helical fibrous microstructure, consisting of a highly ordered arrangement of helically coiled interlocking continuous crystalline aragonite fibres. Our study reveals that, despite the high and continuous degree of interlocking between fibres, every fibre has a differentiated organic-rich thin external band, which is never invaded by neighbouring fibres. In this way, fibres avoid extinction. These intra-fibre organic-rich bands appear on the growth surface of the shell as minuscule elevations, which have to be secreted differentially by the outer mantle cells. We propose that, as the shell thickens during mineralization, fibre secretion proceeds by a mechanism of contact recognition and displacement of the tips along circular trajectories by the cells of the outer mantle surface. Given the sizes of the tips, this mechanism has to operate at the subcellular level. Accordingly, the fabrication of the helical microstructure is under strict biological control. This mechanism of fibre-by-fibre fabrication by the mantle cells is unlike that any other shell microstructure.

Highly ordered nano-scale structure in nacre of green-lipped mussel Perna canaliculus

The ability of the complex hierarchical structure of nacre to withstand huge work of fracture beyond its constituent materials has generated much interest in its growth mechanism and biomimicry studies. However, the structural features directly involved in the robust mechanical properties have not been successfully identified or emulate in biomimetic materials. In the current article, we present the characterisation of a novel nanostructure framework in nacre of Perna canaliculus after chemical etching. The inner structure of the nacre platelet reveals a crystalline three-dimensional framework of orientated fibrous aragonite crystals, which shows that the hierarchical structure at the nano-scale level is much more complex than researchers previously thought. Fracture analysis shows that the fibrous aragonite framework remains largely intact, which indicates the strengthening nature of the elusive internal structure. The discovery of the nano-scale highly ordered structure could be the key to understanding the toughening mechanism in nacre and could serve as a guide to the future biomimetic design of nacre-like, high strength, tough, composite materials.