MgCO3 Content Research Articles

The role of marine fish-produced carbonates in the oceanic carbon cycle is determined by size, specific gravity, and dissolution rate

Rising CO2 emissions have heightened the necessity for increased understanding of Earth's carbon cycle to predict future climates. The involvement of marine planktonic species in the global carbon cycle has been extensively studied, but contributions by marine fish remain poorly characterized. Marine teleost fishes produce carbonate minerals (‘ichthyocarbonates’) within the lumen of their intestines which are excreted at significant rates on a global scale. However, we have limited understanding of the fate of excreted ichthyocarbonate. We analyzed ichthyocarbonate produced by three different marine teleosts for mol%MgCO3 content, size, specific gravity, and dissolution rate to gain a better understanding of ichthyocarbonate fate. Based on the species examined here, we report that 75 % of ichthyocarbonates are ≤0.91 mm in diameter. Analyses indicate high Mg2+ content across species (22.3 to 32.3 % mol%MgCO3), consistent with previous findings. Furthermore, ichthyocarbonate specific gravity ranged from 1.23 to 1.33 g/cm3, and ichthyocarbonate dissolution rates varied among species as a function of aragonite saturation state. Ichthyocarbonate sinking rates and dissolution depth were estimated for the Atlantic, Pacific, and Indian ocean basins for the three species examined. In the North Atlantic, for example, ~33 % of examined ichthyocarbonates are expected to reach depths exceeding 200 m prior to complete dissolution. The remaining ~66 % of ichthyocarbonate is estimated to dissolve and contribute to shallow water alkalinity budgets. Considering fish biomass and ichthyocarbonate production rates, our results support that marine fishes are critical to the global carbon cycle, contributing to oceanic alkalinity budgets and thereby influencing the ability of the oceans to neutralize atmospheric CO2.

Efficient Production of Succinic Acid from Sugarcane Bagasse Hydrolysate by Actinobacillus succinogenes GXAS137

Sugarcane bagasse (SCB) is an abundant agricultural waste, rich in cellulose and hemicellulose, that could be used as an ideal raw material for succinic acid (SA) production. A two-step chemical pretreatment, involving alkali extraction and alkaline hydrogen peroxide treatment, was utilized to treat SCB, followed by multi-enzyme hydrolysis to obtain a reducing sugar hydrolysate mainly composed of glucose and xylose. Optimization of the multi-enzyme hydrolysis of pretreated SCB resulted in a final reducing sugar concentration of 78.34 g/L. In order to enhance the bioconversion of SCB to SA and to reduce the production costs, the initial reducing sugar concentration, nitrogen source, and MgCO3 content were further optimized. The results demonstrated that the inexpensive corn steep liquor powder (CSLP) could be utilized as an alternative nitrogen source to yeast extract for the production of SA; and the optimal concentrations of initial reducing sugar, CSLP, and MgCO3 were 70 g/L, 18 g/L, and 60 g/L, respectively. When fed-batch fermentation was conducted in a 2 L stirred bioreactor, approximately 72.9 g/L of SA was produced, with a yield of 83.2% and a productivity of 1.40 g/L/h. The high SA concentration, yield, and productivity achieved in this study demonstrate the potential of SCB, an agricultural waste, as a viable alternative substrate for Actinobacillus succinogenes GXAS137 to produce SA. This lays a solid foundation for the resource utilization of agricultural waste and cost-effective industrial-scale production of SA in the future.

Role of bottom water chemistry in the formation of fibrous magnesium calcite at methane seeps in the Black Sea

ABSTRACTClimate change poses a significant challenge for life on Earth. Different climate modes have been shown to come along with changes of the magnesium/calcium (Mg/Ca) ratio of seawater, and such changes are believed to control the primary mineral phase of marine authigenic carbonates. However, factors controlling marine carbonate phases other than seawater Mg/Ca ratios exist. Fibrous cements forming at methane seeps in the Black Sea provide new insight into the factors governing elemental and mineral phase compositions of fibrous carbonates. In this study, the distribution of aragonite and fibrous Mg calcite cements from three seep sites in the Black Sea is described as a function of water depth. The Mg/Ca ratio of seawater, as well as the ratio in shallow pore water, is close to four at the examined sites. Fibrous Mg calcite post‐dated aragonite cement in seep carbonates from shallow water depth of 120 to 190 m, whereas Mg calcite is the only cement at a greater depth of ca 2000 m. The primary formation of fibrous Mg calcite is confirmed by its zonation under cathodoluminescence, crystal morphologies agreeing with competitive growth, uniformly distributed MgCO3 contents and precipitation in equilibrium with local conditions calculated from δ18O values. The MgCO3 contents (4.5 to 12.2 mol%) are negatively correlated with δ13C values, indicating that the incorporation of Mg into the calcite crystal structure was favoured by high concentrations of sulphide generated by sulphate‐driven anaerobic oxidation of methane. Unlike open oceanic basins, stratification in the Black Sea leads to euxinic conditions in the deeper water column, favouring fibrous Mg calcite formation. This observation is consistent with sulphide catalysis as a critical agent for the formation of low‐Mg calcite to very high‐Mg calcite at high Mg/Ca ratios and is possibly relevant to carbonate cements forming during times of oceanic euxinia.

Fibrous dolomite formation at a Miocene methane seep may reflect Neoproterozoic aragonite-dolomite sea conditions

Fibrous dolomite widely formed in Neoproterozoic marine sedimentary environments, but apparently disappeared in the Phanerozoic. Here, fibrous dolomite is recognised in a Miocene methane seep limestone (Marmorito, Italy) by synchrotron X-ray diffraction, length slow crystal optics and primary zonation under cathodoluminescence, which is unexpected. Low δ13C values and their negative correlation with MgCO3 contents indicate a formation driven by highly alkaline pore waters and catalysis of dissolved sulphide generated by sulphate-driven anaerobic oxidation of methane. Cementing cavities of reefal carbonate, Neoproterozoic fibrous dolomite might have formed under sulphate-reducing conditions like Quaternary reef microbialites. Since the cavities of Neoproterozoic reefs were restricted microenvironments, the formation of fibrous dolomite was possibly favoured by catalysis similar to its Miocene seep counterpart. Our findings reinforce the concept of penecontemporaneous dolomite formation by sulphide catalysis and contribute to our understanding of the environmental conditions of the Neoproterozoic.

Structural and Geochemical Assessment of the Coralline Alga Tethysphytum antarcticum from Terra Nova Bay, Ross Sea, Antarctica

Crustose coralline algae (CCA) occur from the tropics to the poles in photic benthic environments. Here, we report on some of the world’s southernmost and coldest CCA sites in Terra Nova, Ross Sea, Antarctica at 74°41′ S. The recently described red alga Tethysphytum antarticum is investigated for its skeletal architecture, its mineralogical and geochemical composition, as well as for its taxonomic classification. A phylogenetic analysis based on molecular genetics and the sequencing of the photosystem II protein D1 (psbA) gave a perfect match with T. antarcticum. Histological sections and micro-CT-scans provide new diagnostic details for the conceptacles (the reproductive organs of the alga). X-ray diffractometry and electron-microprobe measurements yielded a clear high-Mg calcite (~8 mol%) composition of the skeletal parts. Detailed back-scattered electron imaging of polished petrographic thin sections revealed a two-layered thallus (vegetative plant tissue), comprising an organic-rich irregularly calcified basal layer with rectangular cells, overlain by the main thallus. Elemental maps show relatively increased sulphur in the basal layer, clearly tied to organic cell walls. MgCO3 and SrCO3 were targeted with semiquantitative elemental mappings and in an ontogenetic quantitative spot transect. Compared with temperature (−1.95 °C to +1.08 °C), the MgCO3 (mol%) reflects this world’s coldest CCA site temperature with the lowest MgCO3 content of 7.9 ± 1.6 mol%. The along transect variability, however, shows with ~6 mol% a larger MgCO3 variability than expected for the 3 °C intra-annual temperature amplitude in Terra Nova Bay. This implies that in low amplitude settings the biomineralisation control on Mg/Ca ratios can outcompete its temperature sensitivity. Mark-recapture studies, next to the environmental logger station La Zecca are suggested, to perform a detailed growth rate and biomineralisation quantification.

Biomimetic Mineralization of Ca-Mg Carbonates: Relevance to Microbial Cells and Extracellular Polymeric Substances.

Research on Ca-Mg carbonate crystallization and phase transition regulated by microbial cells and extracellular polymeric substances (EPS) is significant for carbon sequestration, remediation of polluted soil and water, and synthesis of functional biomaterials. This study focused on the mineralogical transformation from amorphism to crystal, and interaction among cells, EPS, and minerals. By tracing the law of mineral growth and morphological evolution, the influences of cells and EPS on mineral formation were investigated. The results revealed that calcification and the template of rod-shaped cells of strain HJ-1 were the basis for the formation of dumbbell-shaped particles, and directional aggregation and differential growth were the keys to the development and stability of dumbbells. Cell participation had a noticeable impact on mineral prototypes, instead of determining the polymorphism. EPS contributed to aragonite formation and stability. The acidic amino acids or proteins in EPS were likely to cause an increase in MgCO3 content in Mg-calcite. EPS promoted aggregation of particles and induced spherical formation. Exopolysaccharides and proteins were the main components of EPS that can affect carbonate mineralization. EPS could influence the morphology and polymorphism by providing nucleation sites, interacting with Mg2+, adsorbing or incorporating mineral lattices, and inducing particle aggregation.

Geochemical and mineralogical studies of the celestite bearing formations and associated sediments in Benghazi Formation, Ar Rajmah Group (Middle Miocene), Al Jabal Al Akhdar, NE Libya

The study area is located in the northeast of Libya, in the areas of Benghazi city and Ar Rajmah village. The purpose of this study is to find out geochemically and mineralogically the source of strontium element. Also the distribution of strontium in allochems, matrix and cement in the Benghazi Formation of the Ar Rajmah Group. Celestite grade 88.3 % (SrSO4) occurs only in the Benghazi Formation at Benghazi Cement Quarry as geodic nodules in the Middle Miocene rocks. Detailed mineralogical and chemical data have been collected on the celestite and associated sediments using XRF, XRD, EPMA, SEM and DSC techniques. Authigenic celestite and strontianite minerals occur both as a cement and as a replaced by other fracture filling minerals such as anhydrite. However, carbonate skeletal grains have quite high MgCO3 contents, reflecting sometimes the presence of dolomite and brucite minerals. The average Sr/Ca ratio of coralline algae is 0.65. Echinoderm fragments have the highest value 1.31 of Sr/Ca and containing 0.47 to 1.8 mole percent of MgCO3 in the Benghazi Formation. Echinoid spines contain higher strontium contents than other echinoderm fragments. Some bivalves are replaced totally by apatite and had a very high absolute concentration of strontium. The high strontium- low magnesium contents and vice versa are related to the diagenesis, mineralogical nature of the sediments and organic remains. Celestite occurs mainly in sedimentary beds of the Benghazi Formation, adjacent to the evaporitic environment (gypsum/anhydrite) and near sites of intense dolomitization

Cold-seep carbonates of the Laptev Sea continental slope: Constraints from fluid sources and environment of formation

This study presents results of the petrographic, mineralogical, stable isotopes of oxygen and carbon, and trace element investigation of authigenic carbonates collected at newly discovered active cold seeps on the Laptev Sea continental slope at ∼300 m water depth. These carbonates are mainly represented by Mg-calcite with MgCO3 content from 9.1 mol% to 14.0 mol%. The low δ13C values of carbonates ranging from −50.6 ‰ to −32.4 ‰ (V-PDB) indicate that they were formed from anaerobic oxidation of biogenic methane and minor participation of other carbon sources. The difference between measured (from 4.7 ‰ to 5.5 ‰) and calculated (4.0 ‰) δ18Ocarb values might be inherited from fluids enriched in 18O due to dissociation of gas hydrates, which could be the source of methane. The carbonates exhibit weak enrichment in Co, moderate and strong enrichments in As, Mo, and Sb, and strong enrichment in U. Interestingly, As, Sb, and Co correlate with the pyrite content. This indicates that authigenic iron sulfides promote the immobilization of these redox-sensitive elements in seep sediments. The (Mo/U)EF values and anomalies of concentration of Mo and U probably indicate variations in the redox conditions during carbonate formation due to episodically seepage activity changes. Ascending methane-bearing fluids were the main contributor to the enrichment of cold-seep carbonates in As, Mo, Sb, and U at the Laptev Sea continental slope. However additional input from the particulate shuttle process can not be ruled out.

Enhancing the catalytic performance of calcium-based catalyst derived from gypsum waste for renewable light fuel production through a pyrolysis process: A study on the effect of magnesium content

The thermochemical conversion of abundant renewable resources through pyrolytic catalysis cracking (PCC) is one of the most promising technologies for producing green biofuels. In this study, the pyrolysis of palm oil was investigated over a sustainable CaO-based catalyst derived from waste gypsum. PCC was conducted in a continuous packed-bed reactor under atmospheric pressure without purge gas. The effects of Mg doping and reaction temperature were also examined. A wet ball milling process was used to prepare the well-mixed catalysts and to subsequently form a heterojunction structure between the CaO and MgO particles. CaO was synthesized using the Ca(OH)2 derived from the reaction between gypsum and sodium hydroxide. The pyrolytic oil was separated from the crude oil to remove water and other impurities. The pyrolytic oil was then distilled following ASTM D86, and the three separated products were classified as bio-gasoline, bio-kerosene, and bio-diesel. The highest renewable light fuel volume (bio-gasoline and bio-kerosene) of about 75% (74 %wt.) was obtained at a reaction temperature of 525 °C with 10% MgCO3 content. The percent volume of light fuel increased with increasing reaction temperature. Renewable light fuel production over the Mg-doped CaO-based catalyst was related to both the Mg content and reaction temperature.

Reducing microenvironments promote incorporation of magnesium ions into authigenic carbonate forming at methane seeps: Constraints for dolomite formation

AbstractIn part composed of Mg calcite and dolomite with a nearly continuous spectrum of MgCO3 contents, carbonates forming at marine methane seeps are ideal candidates to study the formation of early diagenetic dolomite at surface conditions. Laboratory experiments, modelling and the co‐variation of mineralogical and geochemical attributes of seep carbonates suggest that sulphide – locally released from sulphate‐driven anaerobic oxidation of methane – drives catalytic dolomite formation at seeps. Direct comparison of the concentration of dissolved sulphide with the MgCO3 content of seep carbonate could test this hypothesis. Although the concentration of sulphide during precipitation of carbonate cannot be determined, sedimentary fabrics probably had an effect on local sulphide concentration. Carbonates from the seabed of the Shenhu seepage area of the South China Sea reveal longitudinal, winding and branched fabrics with a width of 400 to 700 μm, interpreted to represent burrows piercing semi‐consolidated sediment. The abandoned burrows were infilled with fine‐grained sediment, which was subsequently cemented by microcrystalline dolomite and Mg calcite. The degree of cation ordering of dolomite in burrow infills and host sediment is similar, indicating virtually coeval dolomite formation. The Mg/Ca mole ratios of carbonate minerals are lower in burrow infills than in the surrounding sedimentary matrix. Similarly, δ13Ccarbonate values tend to be higher for burrow infills (from −41 to −33‰) than for the matrix (from −43 to −38‰), suggesting stronger seawater influence on pore waters in the burrows. More reducing microenvironments of the sedimentary matrix supposedly came along with higher concentrations of dissolved sulphide, which allowed more Mg2+ ions to enter the crystal lattice and led to more catalytic dolomite formation in the host sediment. The distribution of MgCO3 contents and δ13C values in the authigenic Shenhu carbonates reinforces the hypothesis that sulphide release by sulphate‐driven anaerobic oxidation of methane is a key factor in the formation of dolomite at seeps.

Contribution of selective bacterial extracellular polymeric substances to the polymorphism and morphologies of formed Ca/Mg carbonates

Recently, evidence is accumulating on the role of bacterial extracellular polymeric substances (EPS) to the crystal growth and morphological forms of carbonates. In order to explore the function of EPS on crystal growth, EPS from Curvibacter sp. HJ-1 and Arthrobacter sp. MF-2 (HJEPS and MFEPS) were used in this study to induce carbonate precipitation at different Mg/Ca molar ratios. The obtained results showed that carbohydrates and proteins in EPS were the main functional biomacromolecules to the crystal growth of carbonate, and EPS reduced the δ13C values of minerals (approximately −16.307 and −21.898 for carbonates induced by HJEPS and MFEPS vs. −15.207 for abiotic carbonates). The presence of HJEPS was beneficial for the precipitation of aragonite, while MFEPS induced the formation of vaterite and monohydrocalcite, as well as markedly enhanced the MgCO3 content of calcite (approximately 28.20% for carbonates induced by MFEPS vs. 10.26% for abiotic carbonates). Proteins in MFEPS significantly affected the crystal structure and induced the formation of different morphologies. In contrast, carbohydrates in HJEPS induced crystal aggregation into spherical structures. These findings suggest that the physicochemical property of EPS influences the polymorph selection and the resulting morphology of Ca/Mg carbonates.

The Impact of Ambient Atmospheric Mineral-Dust Particles on the Calcification of Lungs

For the first time, it is shown that inhaled ambient air-dust particles settled in the human lower respiratory tract induce lung calcification. Chemical and mineral compositions of pulmonary calcium precipitates in the lung right lower-lobe (RLL) tissues of 12 individuals who lived in the Upper Silesia conurbation in Poland and who had died from causes not related to a lung disorder were determined by transmission and scanning electron microscopy. Whereas calcium salts in lungs are usually reported as phosphates, calcium salts precipitated in the studied RLL tissue were almost exclusively carbonates, specifically Mg-calcite and calcite. These constituted 37% of the 1652 mineral particles examined. Mg-calcite predominated in the submicrometer size range, with a MgCO3 content up to 50 mol %. Magnesium plays a significant role in lung mineralization, a fact so far overlooked. The calcium phosphate (hydroxyapatite) content in the studied RLL tissue was negligible. The predominance of carbonates is explained by the increased CO2 fugacity in the RLL. Carbonates enveloped inhaled mineral-dust particles, including uranium-bearing oxides, quartz, aluminosilicates, and metal sulfides. Three possible pathways for the carbonates precipitation on the dust particles are postulated: (1) precipitation of amorphous calcium carbonate (ACC), followed by its transformation to calcite; (2) precipitation of Mg-ACC, followed by its transformation to Mg-calcite; (3) precipitation of Mg-free ACC, causing a localized relative enrichment in Mg ions and subsequent heterogeneous nucleation and crystal growth of Mg-calcite. The actual number of inhaled dust particles may be significantly greater than was observed because of the masking effect of the carbonate coatings. There is no simple correlation between smoking habit and lung calcification.

Precipitation of low-temperature disordered dolomite induced by extracellular polymeric substances of methanogenic ArchaeaMethanosarcina barkeri: Implications for sedimentary dolomite formation

AbstractA correlation between methanogenesis and dolomite formation has been reported; however, the mechanism underlying this association is not fully understood. In this study, we conducted forced carbonate precipitation experiments at room temperature in calcite-seeded Ca/Mg carbonate solutions containing either purified non-living biomass or bound extracellular polymeric substances (EPS) of the methanogen Methanosarcina barkeri. Purified non-living biomass and bound EPS was used so as to avoid the possible influence of the complex components of the growing microbial culture on carbonate crystallization. Our results demonstrated that non-living biomass of M. Barkeri can enhance the Mg incorporation into calcitic structure and induce the crystallization of disordered dolomite. In the presence of ~113 mg L–1 of non-living biomass, disordered dolomite with ~41 and 45 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. A systematic increase in the MgCO3 contents of the precipitated Ca-Mg carbonates was also observed with the increased non-living biomass concentration. Bound EPS was shown to be the component of non-living biomass that catalyzed the precipitation of disordered dolomite. At only ~25 mg L–1 of bound EPS, disordered dolomite with ~47 and 48 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. We propose that adsorption of bound EPS to growing carbonate surfaces through hydrogen bonding is the key to catalyzing disordered dolomite crystallization, and that this mechanism is also applicable to natural EPS-induced dolomite formation. This study provides significant insight into the formation mechanism of microbial-induced dolomite with high δ13C values.

Summer and winter MgCO3 levels in the skeletons of Arctic bryozoans

In the Arctic, seasonal patterns in seawater biochemical conditions are shaped by physical, chemical, and biological processes related to the alternation of seasons, i.e. winter polar night and summer midnight sun. In summertime, CO2 concentration is driven by photosynthetic activity of autotrophs which raises seawater pH and carbonate saturation state (Ω). In addition, restriction of photosynthetic activity to the euphotic zone and establishment of seasonal stratification often leads to depth gradients in pH and Ω. In winter, however, severely reduced primary production along with respiration processes lead to higher CO2 concentrations which consequently decrease seawater pH and Ω.Many calcifying invertebrates incorporate other metals, in addition to calcium, into their skeletons, with potential consequences for stability of the mineral matrix and vulnerability to abrasion of predators. We tested whether changes in seawater chemistry due to light-driven activities of marine biota can influence the uptake of Mg into calcified skeletons of Arctic Bryozoa, a dominant faunal group in polar hard-bottom habitats. Our results indicate no clear differences between summer and winter levels of skeletal MgCO3 in five bryozoan species despite differences in Ω between these two seasons. Furthermore, we could not detect any depth-related differences in MgCO3 content in skeletons of selected bryozoans. These results may indicate that Arctic bryozoans are able to control MgCO3 skeletal concentrations biologically. Yet recorded spatial variability in MgCO3 content in skeletons from stations exhibiting different seawater parameters suggests that environmental factors can also, to some extent, shape the skeletal chemistry of Arctic bryozoans.

Comparison of carbonate precipitation induced by Curvibacter sp. HJ-1 and Arthrobacter sp. MF-2: Further insight into the biomineralization process

Microorganisms are generally involved in the nucleation, growth and phase transformation of carbonate minerals, and influence the development of their morphology and polymorphism. However, understanding of the process of microbially induced carbonate precipitation (MICP) remains limited. Herein, MICP experiments were carried out using Curvibacter sp. HJ-1 and Arthrobacter sp. MF-2 in M2 medium, and the processes of MICP were monitored. Bacterial cells induced the precipitation of carbonate by creating favorable physicochemical conditions and acting as nucleation templates for carbonate particles and thereby, markedly influenced the morphology and growth of the carbonate structure. The extracellular polymeric substance (EPS) secreted by the bacteria was readily absorbed by the precipitated carbonate, which modified its crystal growth orientation. The MgCO3 content of Mg-calcite, induced by MF-2, was dramatically higher than that induced by HJ-1; HJ-1 promoted the formation and stability of aragonite. Multiple formation mechanisms coexisted during the evolution process of the mineral morphologies in the presence of the bacteria. The spherulites observed mainly evolved from dumbbell-like precursors in the presence of MF-2, whereas aggregate growth was the main formation mechanism of radial spherulites in the presence of HJ-1.

Catalytic effect of microbially-derived carboxylic acids on the precipitation of Mg-calcite and disordered dolomite: Implications for sedimentary dolomite formation

The genesis of dolomite is a contentious issue partly due to the difficulty in its synthesis at ambient temperature. Certain types of microorganisms have been demonstrated to be effective in promoting the precipitation of disordered dolomite, an important precursor of sedimentary ordered dolomite. In contrast to a growing body of research on the catalytic role of microbial exopolymers in the crystallization of disordered dolomite, the role of other microbial exudates (e.g., carboxylic acids) remains unknown. To fill in this knowledge gap, precipitation experiments, mimicking the carbonation process within microbial mats, were conducted in saline solutions containing 0–30 mM succinic acid or citric acid, which are commonly produced by microbes. The starting salinities of experiment solutions were set to 35‰ and 70‰, in order to evaluate the effect of solution salinity on dolomite formation. Our results showed that both succinic acid and citric acid enhanced the incorporation of Mg2+ into growing Ca-Mg carbonates. Solution salinity also played a positive role in enhancing Mg signature in Ca-Mg carbonates. Disordered dolomite with 40.92 mol% MgCO3 was detected under the conditions of 30 mM succinic acid and 70‰ salinity, whereas in other reactors with succinic acid, Mg-calcites formed. Citric acid was more effective in loading Mg2+ into Ca-Mg carbonates compared with succinic acid, as evidenced by the predominant occurrence of disordered dolomite with MgCO3 content ranging from 40.91 mol% to 46.75 mol% in most conditions tested. The results of this study have implications for the formation mechanism of sedimentary dolomite.

Degrading windward patch reefs and processes influencing composition, mineralogy, and stable-isotope record of peri-reefal sediment, San Salvador Island, Bahamas

Patch reefs and their surrounding skeletal–peloidal–intraclast grainstone facies are very abundant on modern (sub-) tropical carbonate platforms. Compared to the barrier and fringing reef facies, the processes influencing composition, mineralogy, and stable-isotope record of peri-reefal sediment have received much less attention, despite a potentially significant volume of such sediment in many Phanerozoic carbonate platforms. To better understand the evolution and sedimentary record of patch-reef settings, this study investigates the patch-reef biota (scuba diving surveys) and composition (petrography), mineralogy (X-ray diffraction) and stable-isotope record (mass spectrometry) of peri-reefal sediments at East Beach on the northeastern, windward margin of San Salvador Island, the Bahamas. The macroalgae-dominated ecology is assessed, and a decadal decline in stony coral coverage with a shift to more opportunistic, stress-tolerant species is documented. The most common stony corals on these shallow-water (3–5 m) patch reefs are Diploria strigosa, Porites astreoides, and P. porites, and Halimeda lacrimosa dominates among green algae. Peri-reefal sediment is pure carbonate sand composed of roughly equal volumes of aragonite and high-Mg calcite (HMC), with mean MgCO3 content in HMC of 15.2%. The bulk-sediment isotope record shows very limited variation (δ13C between + 2.3‰ and + 3.2‰, and δ18O between − 0.8‰ and − 0.6‰) and this is consistent with similar modern settings. A lack of very fine particles and an abundance of intraclasts suggest that the environment is strongly influenced by bio- and physical erosion. Fast-growing algae have likely reached an abundance beyond the grazing capacity, which has drastically reduced the space for coral settlement and recruitment, and thus has compromised the patch-reef resilience in this part of the Bahamian Archipelago.

Difference in pore evolution of calcium aluminate cement-bonded alumina bubble-based castables with micro-sized MgCO3 and Al(OH)3

Micro-sized MgCO3 and Al(OH)3 were used as inorganic porogenic agents for increasing porosity in alumina bubble lightweight castables. The influence of 0–4 wt% micro-sized MgCO3 and 0–8 wt% micro-sized Al(OH)3 additions on the porosity, thermal conductivity and strength of castables after firing was investigated. The results indicate that porosities in castables containing micro-sized Al(OH)3 or MgCO3 after heat-treatment were raised by augmenting micro-sized Al(OH)3 or MgCO3 contents. It is found that MgCO3 favors the porosity formation more than Al(OH)3 in castables because of the different formation behavior of calcium aluminates and MgAl2O4 spinel. The dependence of properties of castables on the micro-sized Al(OH) or MgCO3 addition was discussed with respect to the pore evolution.

Autocorrelation analysis of the infrared spectra of synthetic and biogenic carbonates along the calcite–dolomite join

Autocorrelation analysis of infrared spectra can provide insights on the strain energy associated with cation substitutions along a solid-solution compositional join which to date has been applied primarily to silicate minerals. In this study, the method is applied to carbonates synthesized at 10 mol% increments along the calcite–dolomite (CaCO3–CaMg(CO3)2) join in the range of 1000–1150 °C and 0.6–2.5 GPa for the purpose of determining how the band broadening in both the far- and mid-infrared ranges, as represented by the autocorrelation parameter δΔCorr, compares with the existing enthalpy of mixing data for this join. It was found that the carbonate internal vibration ν2 (out-of-plane bending) in the mid-infrared range, and the sum of the three internal vibration modes ν4 + ν2 + ν3 most closely matched the enthalpy of mixing data for the synthetic carbonates. Autocorrelation analysis of a series of biogenic carbonates in the mid-infrared range showed only a systematic variation for the ν2 band. Using the biogenic carbonate with the lowest Mg content for reference, the trend in δΔCorr for biogenic carbonates shows a steady increase with increasing Mg content suggesting a steady increase in solubility with Mg content. The results from this study indicate that autocorrelation analysis of carbonates in the mid-infrared range provides an independent and reliable assessment of the crystallographic strain energy of carbonates. In particular, inorganic carbonates in the range of 0–17 mol% MgCO3 experience a minimum in strain energy and a corresponding minimum in the enthalpy of mixing, whereas biogenic carbonates show a steady increase in strain energy with increasing MgCO3 content. In the event of increasing ocean acidification, biogenic carbonates in the range of 0–17 mol% MgCO3 will dissolve more readily than the compositionally equivalent inorganic carbonates.

Petrographic and geochemical features of the dolostones at the Gölboğazı Formation (Upper Devonian), Southwest Konya, Turkey

This paper describes the occurrence of dolostone and the mechanism of dolomitization of the Upper Devonian Golbogazi Formation in the allochthonous Taurus Mountains Aladag unit in Turkey. The Upper Devonian Golbogazi Formation carbonates, with dominant ostracod-bearing mudstone and wackestone, formed tidal and subtidal environments, and some of these rocks were dolomitized from shallow to deep burial. On the basis of the field, the petrographic and geochemical features, four different replaceable and cement dolostone phases have been recognized. The replacive dolostones contain (1) very fine to fine crystalline planar-s dolostone (df1), (2) medium to coarse crystalline planar-s to planar-e dolostone (df2), (3) coarse to very coarse crystalline non-planar-a dolostone (df3), and (4) coarse to very coarse crystalline planar dolostone cement (df4). The replacive dolostones are disordered to moderate the ordered and calcium-rich. They are non-stoichiometric and have 46–59 mol% CaCO3 and 41–54 mol% MgCO3 total contents. The df1 dolostones have MgCO3 contents of 41–54 mol%, the df2 dolostones have 41–53 mol%, the df3 dolostones have 49 mol%, and the df4 dolostones have 49–50 mol%, respectively. The Golbogazi dolostones have δ18O values of −9.44 to −2.20‰ Vienna Pee Dee Belemnite (VPDB) and δ13C values of −1.58 to +2.52 VPDB. Sr, Na, Mn, and Fe concentrations of replacive dolostones are 74–184, 148–593, below detection level (bdl)–619, and 1049–9233 ppm, respectively. The petrographic and geochemical data demonstrate that the replacive dolostones occurred prior to the chemical compaction at shallow to intermediate burial depths from Late Devonian seawater and/or seawater lightly modified by water–rock interaction process and later recrystallized by basinal brines at increasing burial depths and temperature. The North American Shale Composite-normalized rare earth element values of both limestone and dolostone show very similar rare earth element patterns characterized by slightly or considerably negative cerium (Ce) anomalies and a clear depletion in all rare earth element species. The dedolomitization observed in the Golbogazi Formation is thought to occur by the oxidizing effect of the meteoric water in the shallow burial environment during the telodiagenesis.