Contribution of calcareous nannoplankton to carbonate deposition: a new approach applied to the Lower Jurassic of central Italy

The Upper Pliensbachian–Lower Toarcian Somma section (Central Italy), which displays alternation of limestone–marl lithotypes, was investigated for its calcareous nannofossil and CaCO₃ contents. In order to evaluate the contribution of calcareous phytoplankton to the carbonate fraction of rocks a new method was applied combining a recently developed technique for nannofossil absolute quantification and measurements of nannofossil size. Our study suggests that nannofossils did not produce the bulk of carbonate mud in the studied section, although they contribute to different extents to the pelagic carbonate fraction. The contribution of both coccoliths and (mainly) the Incertae sedis Schizosphaerella to the carbonate fraction was important only in periods of reduced accumulation rates and/or during clay deposition. The highest estimated carbonate production by nannofossils in the studied section is around 83% of the total carbonate content, but in a sample containing only 12.8 wt% CaCO₃. The observed limestone–marl alternations therefore result from cyclical export of carbonate mud from the shallow Latium–Abruzzi carbonate platform to the Umbria–Marche Basin and, only subordinately, from changes in nannoplankton productivity.     

Calcareous nannofossils of the Cenomanian/Turonian boundary interval from the Boreal Realm (Wunstorf,northwest Germany)

The Cenomanian/Turonian boundary interval (CTBI) is marked by an intense climatic warming presumably caused by large magmatic eruptions. This warming was characterised by one of the most prominent Mesozoic perturbations of the carbon cycle, the Oceanic Anoxic Event 2 (OAE2), which is marked by a well pronounced positive carbon isotope excursion (CIE). Sediments of the OAE2-interval often consist of organic rich black shales suggesting widespread bottom water anoxia during the CTBI. This study focuses on calcareous nannofossils from the CTBI of a European section (Wunstorf Core; northwest Germany). A total of 105 samples were examined for calcareous nannofossils using the settling technique. Eight bioevents (last occurrences: Corollithion kennedyi, Lithraphidites acutus; first occurrences: Rotelapillus biarcus, Corollithion exiguum, Eprolithus octopetalus, Eprolithus eptapetalus, Quadrum gartneri, Eiffellithus eximius) have been recognised throughout the middle Cenomanian to middle Turonian interval. With the exception of eleven samples preservation is moderate to good. Calcareous nannofossils are abundant (mean 2.0 billion specimens/g sediment) and highly diverse (mean 58 species/sample). Assemblages are dominated by Watznaueria spp. (32.3%), Prediscosphaera spp. (13.4%), Zeugrhabdotus spp. (11.2%) and Biscutum spp. (10.5%). Pre-OAE2 and also post-OAE2 nannofossil assemblages show high abundances of Biscutum spp. (～ 20%) indicative for stable mesotrophic conditions. The assemblages of the OAE2 itself are marked by high values for Watznaueria spp. and low frequencies of Biscutum spp. making oligotrophic conditions during the OAE2 likely. High absolute abundances of organic walled dinoflagellates and the occurrence of frequent stress tolerant nannofossil genera like Retecapsa spp. in the organic rich intervals suggest, however, a deposition of black shales enhanced by high primary productivity. Thus dinoflagellates and calcareous nannofossils are interpreted to reflect different seasonal signals during the time of black shale deposition. Short-term high fertile seasons allowed the blooming of the organic walled dinoflagellates whereas calcareous nannofossils dominated the longer oligotrophic seasons. The black shale deposition was supported by the formation of large amounts of organic matter during fertile seasons as well as by surface water stagnation during oligotrophic seasons.     

Calcareous nannofossil biostratigraphy of the Thomel Level (OAE2) in the Lambruisse section, Vocontian Basin, southeast France

The Thomel Level of the Lambruisse section in the Vocontian Basin (southeast France), which is marked by intercalations of black shales and organic-rich marls, accumulated during the oceanic anoxic event 2 (OAE2) occurring in the Cenomanian-Turonian (C-T) boundary interval. Calcareous nannofossil biostratigraphic investigation of this interval revealed a total of five nannofossil zones, corresponding to the UC3-UC8 zones (Middle Cenomanian-Middle Turonian) as defined by Burnett. Biostratigraphically important taxa observed in the section include Cretarhabdus striatus, Axopodorhabdus albianus, Lithraphidites acutus, Corollithion kennedyi, Helenea chiastia, Quadrum gartneri, Q. intermedium, Eiffellithus eximius, Eprolithus octopetalus and E. eptapetalus. The two nannofossil events commonly used in the delineation of the C-T boundary, namely the LO of H. chiastia and the FO of Q. gartneri, occur less than 2 m apart in the studied section. These two bioevents define the limits of the UC6 nannofossil Zone and occur within the Whiteinella archaeocretacea foraminifer Zone. Previous litho- and chemostratigraphic analyses indicate that the δ¹³C profile of the section corresponds well with changes in lithofacies and fluctuations in the total organic carbon (TOC) and calcium carbonate content of the section. Initial increase in the δ¹³C values occurs within the UC3-UC4a undifferentiated zone, coinciding with the onset of the deposition of the organic-rich sediments of the Thomel Level and a drastic decline in the CaCO₃ values. The plateau of high δ¹³C values, on the other hand, occurs within the UC5 zone, between the LO of C. kennedyi and the LO of H. chiastia (and FO of Q. gartneri). This interval of high δ¹³C values also corresponds to the interval of high TOC and low CaCO₃ values. The integrated nannofossil, planktonic foraminifer and δ¹³C data provide a precise biostratigraphic and chemostratigraphic framework of the C-T boundary in the Lambruisse section that can be used in future studies in the Vocontian Basin and allow correlations with other well-studied C-T boundary sections.     

High-resolution chemo- and biostratigraphic records of the Early Aptian oceanic anoxic event in Cantabria (N Spain): Palaeoceanographic and palaeoclimatic implications

During the Early Aptian, major palaeoenvironmental changes occurred leading to an oceanic anoxic event (OAE 1a) and a perturbation of the global carbon cycle. New detailed litho-, bio-, and chemostratigraphic (TOC, δ¹³C_carb, δ¹³C_org) records of two superbly exposed and expanded Lower Aptian sections in Cantabria (La Florida and Cuchía) allow to recognize the expression of the OAE 1a in shallow shelf environments of northern Spain. The succession consists of shallow platform limestones that include a marly unit (Patrocinio Formation), the deposition of which occurred mostly at the onset of the OAE 1a (~ 120.5 Ma). This study presents a new integrated biostratigraphy based on ammonites, planktonic foraminifera, calcareous nannofossils and palynomorphs that allows an accurate age resolution of the succession. The marly unit records an abrupt negative δ¹³C excursion in both bulk organic matter (up to 5‰) and carbonate (up to 6‰, mean 3‰), as has been already observed at the onset of the OAE 1a in other Lower Aptian deposits worldwide. In detail, however, the negative excursion presents two minima in the studied sections. This negative spike is confidently attributed to the upper half of the Hayesites irregularis nannofossil Zone, to the upper part of the Blowiella blowi foraminiferal Zone, and to the middle–upper part of the Deshayesites weissi ammonite Zone. A third negative excursion occurs at the base of the Rhagodiscus angustus nannofossil Zone, which may be correlatable with the Dufrenoyia furcata ammonite Zone. This data set refines the age of the OAE 1a and reveals the existence of a stratigraphic gap in the westernmost margin of the Basque Cantabrian Basin that covers at least a portion of the upper part of the Early Aptian. Sedimentary facies and quantitative analysis of palynomorphs and nannofossils document significant environmental changes associated with the OAE 1a: compositional changes of neritic carbonates and calcareous nannofossils data indicate the occurrence of a biocalcification crisis inferred to have been related to CO₂-induced changes in seawater chemistry, and palynomorphs identify a thermal maximum followed by a cooling phase. The latter show a Classopollis maximum during the OAE 1a, which is followed by a decrease in Classopollis and an increase of bisaccate pollen after the event.     

Paleoenvironmental changes across the Cenomanian/Turonian Boundary Event (Oceanic Anoxic Event 2) as indicated by benthic foraminifera from the Demerara Rise (ODP Leg 207)

This study is based on Cenomanian to lower Turonian sediments of Ocean Drilling Program (ODP) Sites 1258, 1259, 1260, and 1261 from the Demerara Rise (Leg 207, western tropical Atlantic, off Suriname) that are oriented along a paleodepth transect. Studied sediments include the Cenomanian/Turonian Boundary Event (CTBE) or Oceanic Anoxic Event 2 (OAE 2) and consist of laminated black shales with TOC values between 5% and 10% below and above OAE 2 and up to 29% within the OAE 2 interval. Benthic foraminiferal assemblages in this eutrophic environment are generally characterized by low diversities and strong fluctuations of abundances, indicating oxygen depletion and high organic matter fluxes. Dominant taxa at all sites are Bolivina anambra, Gavelinella dakotensis, Tappanina sp., Praebulimina prolixa, and Neobulimina albertensis. Based on the positive stable carbon isotope excursion characteristics of OAE 2 we subdivided the studied successions into three intervals: (1) the interval below OAE 2; (2) the carbon isotope excursion reflecting OAE 2; and (3) the interval above OAE 2. In the bathymetrically shallower Sites 1260 and 1261 benthic foraminiferal assemblages indicate anoxic to sometimes slightly dysoxic bottom-water conditions below the OAE 2 interval. The bathymetrically deepest Site 1258, in contrast, reflects more oxygenated bottom waters with an almost continuous occurrence of benthic foraminifera. It is therefore suggested that the shallower sites were located within the oxygen minimum zone (OMZ), whereas Site 1258 was below the OMZ. During OAE 2 anoxic conditions prevailed at the shallower sites. At Site 1258 benthic foraminifera indicate severe dysoxic but not anoxic conditions. This pattern is proposed to reflect a strengthening of the OMZ possibly related to increased primary production during OAE 2. A short-term repopulation event of benthic foraminifera in the lower third of the OAE 2 interval was observed at all sites, reflecting a brief bottom-water oxygenation event. This short-lived event parallels a surface-water cooling and is probably equivalent to the “Plenus Cool Event” in Europe and the “benthonic zone” in the U.S. Western Interior. The benthic foraminifera of a ~0.5 Ma interval following OAE 2 still indicate oxygen depletion of bottom waters. Subsequently, however, a strong increase in benthic foraminiferal abundance and diversity reflects better oxygenation of the bottom-water masses, probably related to a weakening of the OMZ due to decreasing organic matter flux.     

Données paléoenvironnementales sur le Vraconnien/Cénomanien de la région de Tébessa (Atlas Saharien,nord-est Algérie). Caractérisation de l’OAE2

The micropalaeontological content of five sections, located in Northeastern Algeria (Saharan Atlas) was investigated by means of washing/counting of microfossils in marly levels, and microfacies analyses of calcareous levels. In these levels of Upper Cretaceous age, hundred species of foraminifera were identified but only about 15 species of ostracoda and about ten of radiolaria. This work allows, first of all, to establish a rather precise stratigraphic frame, in particular by means of studying planktonic foraminifera. Two to five biozones were defined, between the Vraconnian (Th. Appenninica biozone) and the early Turonian (Whiteinella archaeocretacea biozone, then Helvetoglobotruncana Helvetica biozone for certain sections). Secondly, the quantitative analyses led on foraminifera allowed the definition of palaeoenvironment. The ratio P/P+B, generally very high, coupled with a little diversified benthonic microfauna, indicates a calm and deep environment, of external platform or slope type. Furthermore, at numerous levels, various indications give evidence of the existence of a strong surface productivity (presence of upwellings), responsible for the proliferation of radiolaria (late Vraconnian/early Cenomanian especially) or of globular planktonic foraminifera (hedbergellids/heterohelicids); being both associated with low-oxygen deep waters. Two anoxic events were also revealed, the first one at the end of Vraconnian (OAE1d) and the second at the end of Cenomanian (OAE2). This last event in particular was characterized on all the sections, in a more or less detailed way, thanks to the identification of certain indicator: Heterohelix “bloom”, “filament” event, disappearance of rotaliporids, presence of “blackshales” strongly enriched in organic matter (Bahloul levels).     

Bathymetric distribution of fossil foraminifera within marine sediment cores from the eastern part of Lützow-Holm Bay, East Antarctica, and its paleoceanographic implications

Faunal analysis of fossil foraminifera from marine gravity and piston cores collected by the Japanese Antarctic Research Expeditions (1981 and 1992) is used to estimate the impact of the latest Quaternary paleoceanography on coastal environments of the eastern part of Lützow-Holm Bay, East Antarctica.Accelerator Mass Spectrometry (AMS) carbon-14 ages produced from sedimentary organic carbon were less than 16 ka (non-corrected). Detailed correlation among submarine cores and Holocene elevated marine deposits exposed on the eastern shore of the embayment is difficult due to the indefinite reservoir correction value for marine organic matter and to upward-increasing abnormal ages for some cores.A local carbonate dissolution level can be delineated around the present depth of 300–400 m or shallower in the eastern part of Lützow-Holm Bay during the Holocene, based on distributional trends of arenaceous, calcareous benthic, and planktonic foraminifera recognized within a depth less than 600 m. Downcore recovery of calcareous foraminifera containing Bulimina aculeata from two cores obtained in a drowned glacial trough deeper than 600 m situated far beyond the dissolution depth of CaCO₃ indicates the incursion of warm, high-nutrient, and CaCO₃-saturated Circumpolar Deep Water (CDW) from the offshore area along the trough toward the southeastern coast of Lützow-Holm Bay during the Holocene. The intrusion of CDW impacted on the marine environments of the southeastern coast, thereby contributing to peripheral retreat of the ice sheet as well as increasing calcareous benthic foraminiferal productivity along the southeastern coast of Lützow-Holm Bay.     

The glacial oceans

The accumulation of carbonate and carbonaceous matter increased during the glacial age due to intensified sink water formation in the northernmost Atlantic Ocean and the intensified upwellings in the Indo-Pacific Oceans. As a consequence of that there was a decrease of the ΣCO₂ concentration in the ocean as well as of the CO₂ content in the atmosphere.As a whole the accumulation area of calcareous matter was enlarged by ca. 20% during glacial times. The decreased accumulation area in the North Atlantic was well compensated by a large increase in the Indo-Pacific oceans.The northernmost part of the Atlantic Ocean was quasi-stagnant during the most intense deglacial times, thus causing a reduced exchange between surface water and deep water which, in its turn, led to a decrease of the accumulation of carbon (as CaCO₃ and organic matter). The concentration of ΣCO₂ rose in the seawater and the pCO₂ increased as well.     

Deposition and cycling of carbon and nitrogen in carbonate mud of the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef

The dynamics of carbon and nitrogen in carbonate mud were examined in the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef. Most (89–93%) of the organic carbon and total nitrogen depositing to the carbonate mud zones was mineralized over a sediment depth of 1 m, with ∼50% of CO₂ produced during microbial decomposition involved in carbonate precipitation/dissolution reactions. There was proportionally little burial of organic carbon (10–11%) or nitrogen (7–10%). Nitrogen budgets suggest rapid turnover of porewater inorganic N pools on the order of hours to a few days. Incubation experiments indicate carbonate dissolution in surface deposits (≤20 cm depth) and carbonate precipitation in deeper sediments. Depth-integrated reaction rates indicate net carbonate precipitation of 7–10 mol CaCO₃ m² year⁻¹ over a depth of 1 m. Budget calculations at the whole-reef scale imply that deposition of CaCO₃ in the mud zones of both lagoons may equate to 50–90% of total reef carbonate production, with organic carbon fluxes equating to nearly all net primary production on each reef. These biogeochemical estimates point to the functional importance of carbonate mud zones in the lagoons of the shelf reefs of the Great Barrier Reef.     

The Cenomanian–Turonian boundary in the northwestern part of the Adriatic Carbonate Platform (Ćićarija Mtn., Istria,Croatia): characteristics and implications

The Cenomanian–Turonian boundary (CTB) in the ?i?arija Mountain region (northern Istria, Croatia) is characterized by calcisphere limestone successions with a firmground and glauconite horizon, bioturbated intervals, tempestites, and slumped structures as well as microbially laminated and organic-rich interbeds deposited in the northwestern part of the intra-Tethyan Adriatic Carbonate Platform (AdCP). Compilation of the results from three studied sections (Vodice–Jelovica, Martinjak and Planik) of litho-, bio-, and microfacies analyses, X-ray diffraction, SEM, EDS, and stable isotope analyses allowed reconstruction of marine paleoenvironmental conditions during this time period. Shallow-marine carbonate deposits of the Milna Formation underlie a drowned-platform succession of the Sveti (Sv.) Duh Formation. The contact between these two formations is sharp and commonly marked by slumped deposits. The Sv. Duh Formation consists of about 100 m of calcisphere wackestone enriched in organic matter. The results of preliminary δ¹³C and δ¹⁸O stable isotope analyses indicate the influence of the global Oceanic Anoxic Event (OAE2) on the deposition of this carbonate succession. Anoxic and hypoxic conditions in the water column lead to major changes in the shallow-marine carbonate system of the AdCP. Numerous benthic foraminifera declined during that time, but planktonic foraminifera and calcareous dinoflagellates diversified and expanded greatly. The results of this research provide new insights into the character of the CTB interval in this part of the Tethyan realm. Local and regional synsedimentary tectonics combined with global upper Cretaceous sea-level dynamics allows the correlation of the investigated deeper-marine lithostratigraphic units with OAE2.     

Skeletal growth in the echinoderm Asterias rubens L. (Asteroidea,Echinodermata) estimated by 45Ca-labelling

The skeletal dry weight of the 4.4 ± 0.2 cm size class of Asterias rubens L. from Kiel Bay in the western Baltic is 0.34 ± 0.08 g. The sum of calcium and magnesium carbonates in the skeleton amounts to 94.0 ± 1.3% while the individual concentrations are 86.9 ± 1.3% CaCO₃ and 7.1 ± 0.7% MgCO₃. The MgCO₃ is 4% lower than expected for a magnesium calcite precipitated under Baltic temperature conditions (8°C). Strontium was not determined but strontium carbonate is known to be in the region of 0.4%. The remainder is organic matter and this gives rise to 2.9 ± 1.3% organic carbon.Animals studied were at the ‘waiting stage’ and their actual growth was minimal. Uptake of ⁴⁵Ca in the skeleton consists of a fast step followed by a slow step. The fast step is attributed to saturation of exchangeable skeletal pools while the slow step is due to net deposition of CaCO₃. Skeletal growth at the waiting stage calculated from the second rate constant was found to be 0.76 μg CaCO₃j-mg skeleton^?1 · day^?1 or 0.09% · day^?1 compared with 9.3 μg CaCO₃ · mg skeleton^t1̄ · day^?1 or 1.1% · day^?1 at log phase. The isotope method is considered superior to size-frequency analysis in that it is capable of detecting differences in growth rate in individuals of the same size class and thus provides an insight into asteroid population structure.     

Sulphur fractionation in calcareous soils and bioavailability to plants

Sulphur fractionation and availability to plants are poorly understood in calcareous soils. Sixty-four calcareous soils containing varying amounts of CaCO₃ were collected from ten provinces in China and their S fractions determined. Organic S was the predominant fraction of S, accounting for on average 77% of the soil total S. The amounts of adsorbed sulphate were found to be negligible. 1 M HCl extracted substantially more sulphate than either 0.01 M CaCl₂ or 0.016 M KH₂PO₄, indicating the existence of water-insoluble but acid-soluble sulphate, probably in the form of sulphate co-precipitated with CaCO₃. The concentrations of water-insoluble sulphate correlated positively with the contents of CaCO₃ and accounted for 0.03–40.3% (mean 11.7%) of soil total S. To test the bioavailability of water-insoluble sulphate, a sulphate-CaCO₃ co-precipitate labelled with ³⁵S was prepared and added to a calcareous soil in a pot experiment with either NH₄⁺ or NO₃^– as the N source. In 29 days, wheat plants took up 10.6% and 3.0% of the ³⁵S added to the soil in the NH₄⁺ and NO₃^– treatments, respectively. At the end of the pot experiment, the decrease of water-insoluble, acid-soluble, sulphate was more apparent in the NH₄⁺ than in the NO₃^– treatment. The results indicate that sulphate co-precipitated with CaCO₃ in calcareous soils may become partly available for plant uptake, depending on rhizosphere pH, if the field precipitate is similar to the laboratory prepared sample studied.     

A negative influence of manure on calcareous soils

Summary On a soil rich in CaCO₃ in a semiaride climate in Algeria a hard soil layer, impermeable for plant roots, was formed in a depth of 20–40 cm after farmyard manure application and irrigation. To find the reason soil samples of this field were taken and leaching experiments were carried out in the laboratory, with the result that much more Caions and HCO₃-ions were leached out of the soils with farmyard manure application than from the soils without manure. Probably the high amount of CO₂, being liberated by the organic matter, and the irrigation water dissolved the CaCO₃ in the soil, and the formed Ca- and HCO₃-ions followed the movement of water in the soil. Where the Ca(HCO₃)₂ reached soil layers with a less amount of CO₂, CaCO₃ precipitated and formed the hard soil layer. In order to avoid the formation of such calcareous crusts on irrigated, limy soils in a dry climate it is recommended to fertilize rather often small quantities instead of rarely big quantities of farmyard manure.     

Insights into the composition,morphology, and formation of the calcareous shell of the serpulid Hydroides dianthus

To date, the calcareous tubes of serpulid marine worms have not been studied extensively in a biomineralization context. The structure and composition of the tube shell and adhesive cement of the marine tubeworm Hydroides dianthus were studied using a variety of characterization techniques, including powder XRD, FTIR, SEM, EDX, and AFM. The tube and cement were determined to be inorganic–organic composite materials, consisting of inorganic aragonite (CaCO₃) and Mg-calcite ((Ca_0.8Mg_0.2)CO₃) crystals, and both soluble and insoluble organic matrices (SOM and IOM). SEM imaging revealed a variety of crystal morphologies. AFM nanoindentation of the inorganic components yielded Young’s moduli of ～20 GPa in the wet state, and ～50 GPa in the dry state. Amino acid analysis of the SOM indicated substantial amounts of acidic and non-polar neutral amino acids. Part of the insoluble organic tube lining was identified as being composed of collagen-containing fibres aligned in a criss-crossed structure. The SOM and organic tube lining were found to contain carboxylated and sulphated polysaccharides. In an artificial seawater solution, the SOM and the organic tube lining mediated CaCO₃ mineralization in vitro.     

An Alternative Biotechnological Tool for Magnesite Enrichment: Lactic Acid Bacteria Isolated from Soil

Abstract

Calcium carbonate (CaCO₃) is found in different polymorph structures such as aragonite, vaterite, and calcite. The most common and stable form of CaCO₃, calcite, which is abundant in sedimentary rocks as magnesite ore. Magnesite has application areas in many industrial fields including paper, pharmaceutical and refractory materials. Magnesite is theoretically formulated MgCO₃, but contains many impurities (silicium, iron, and also calcite), that limits its usability and applicability. In this research, we aimed to investigate the decalcification possibility of the raw magnesite material through application of Enterococcus feacelis (EF) with CaCO₃ dissolution ability. The exact mechanism of CaCO₃ dissolution was investigated by carbonic anhydrase enzyme assay and HPLC analysis of organic acids produced by EF. Consequently, EF reduced the amount of CaCO₃ from 2.94% to 0.49% which means a reduction (≈83.33%) in the rate of CaCO₃ percentage. As a result of the experiments, it was observed that different organic acids produced by bacteria reacted with CaCO₃ and removed the lime of magnesite ore. The bacteria used in the study did not show any pathogenic properties in rats, thus, it can be used safely for the industrial applications.     

Calcium carbonate budgets for two coral reefs affected by different terrestrial runoff regimes, Rio Bueno, Jamaica

A process-based carbonate budget was used to compare carbonate framework production at two reef sites subject to varying degrees of fluvial influence in Rio Bueno, Jamaica. The turbid, central embayment was subjected to high rates of fluvial sediment input, framework accretion was restricted to ≤30 m, and net carbonate production was 1,887 g CaCO₃ m⁻² year⁻¹. Gross carbonate production (GCP) was dominated by scleractinians (97%), particularly by sediment-resistant species, e.g. Diploria strigosa on the reef flat (<2 m). Calcareous encrusters contributed very little carbonate. Total bioerosion removed 265 g CaCO₃ m⁻² year⁻¹ and was dominated by microborers. At the clear-water site, net carbonate production was 1,236 g CaCO₃ m⁻² year⁻¹; the most productive zone was on the fore-reef (10 m). Corals accounted for 82% of GCP, and encrusting organisms 16%. Bioerosion removed 126 g CaCO₃ m⁻² year⁻¹ and was dominated by macroborers. Total fish and urchin grazing was limited throughout (≤20 g CaCO₃ m⁻² year⁻¹). The study demonstrates that: (1) carbonate production and net reef accretion can occur where environmental conditions approach or exceed perceived threshold levels for coral survival; and (2) although live coral cover (and carbonate production rates) were reduced on reef-front sites along the North Jamaican coast, low population densities of grazing fish and echinoids to some extent offset this, thus maintaining positive carbonate budgets.     

Spectroscopic studies for identifying the chemical states of the periostracum of the Corbicula species in Lake Biwa

Corbicula clam shells consist of thin periostracum and calcareous layers made of calcium carbonate (CaCO₃). Depending on habitat conditions, the shell exhibits various colorations, such as yellow, brown, and black. The chemical state of the periostracum of the Corbicula species in Lake Biwa was studied by X-ray absorption fine structure (XAFS) and Raman scattering spectroscopies. Fe K-edge X-ray absorption near edge structure (XANES) revealed that the Fe³⁺ intensity increases as the color of the shell changes from yellow to black. Raman spectra suggested that quinone-based polymers cover the yellow shell, and the black shell is further covered by dihydroxyphenylalanine (DOPA) rings of amino acid derivatives. From Fe K-edge extended X-ray absorption fine structure (EXAFS), it was found that Fe³⁺ in the periostracum was surrounded by five to six oxygen atoms with an average Fe-O ligand distance of 2.0 Å. Accordingly, a tris-DOPA-Fe³⁺ complex is formed, which is responsible for the periostracum’s black color.     

Seeded Growth Route to Noble Calcium Carbonate Nanocrystal

A solution-phase route has been considered as the most promising route to synthesize noble nanostructures. A majority of their synthesis approaches of calcium carbonate (CaCO₃) are based on either using fungi or the CO₂ bubbling methods. Here, we approached the preparation of nano-precipitated calcium carbonate single crystal from salmacis sphaeroides in the presence of zwitterionic or cationic biosurfactants without external source of CO₂. The calcium carbonate crystals were rhombohedron structure and regularly shaped with side dimension ranging from 33–41 nm. The high degree of morphological control of CaCO₃ nanocrystals suggested that surfactants are capable of strongly interacting with the CaCO₃ surface and control the nucleation and growth direction of calcium carbonate nanocrystals. Finally, the mechanism of formation of nanocrystals in light of proposed routes was also discussed.     

Effect of calcium carbonate and organic matter on the growth and concentration of iron and manganese in sorghum (Sorghum bicolor)

Summary The effect of CaCO₃ and organic matter on the availability of iron and manganese was studied on sorghum (Sorghum bicolor). Addition of calcium carbonate caused significant decrease in dry matter and grain yield. But yields tended to increase with the addition of organic matter at the rate of 15 t/ha. The iron concentration decreased with the addition of CaCO₃ at all the stages of crop growth. Like iron, concentration of manganese also decreased with the increase in added CaCO₃. Organic matter increased the iron concentration both at seedling and flowering stage. The concentration of manganese was, however, dependent on the stage of the crop.     

Geochemistry of recent marine sediments in the Bardawil lagoon,northern Sinai,Egypt

The concentrations of major (Al, Fe, Mg, Ti, K and Na) and minor (Ba, Sr, V, Cu, Mn, Cr, Ni, Zn, Pb and Mo) elements as well as carbonate, organic carbon and total nitrogen have been determined in surface sediments collected at 12 stations from the Bardawil lagoon. The aim of the study was to characterize the geochemistry of the sediments in three different environments, the lagoon, the salt pans and the inlet between the lagoon and the Mediterranean Sea. Higher CaCO₃ percentage (53.5–70.5%) were found in the salt pans where biogenic calcareous components and carbonate rock fragments were found in sufficient quantities in the sediment fractions. Based on high C/N ratios, the organic carbon fraction of the Bardawil lagoon surface sediments is clearly dominated by terrigenous material. The distribution of Al, Fe, Mg and Ti are essentially controlled by the mineralogy of the sediments. The ratios of Ba, Sr, Cu, Mn, Pb and Mo to Al are all high in the salt pans and reflect changes in mineralogy and sediment texture.