Carbonate Mineralogy Research Articles

Mineralogy, geochemistry and petrography of methane-derived authigenic carbonates from Enza River, Northern Apennines (Italy)

A remarkable exposure of carbonate pipe-like and slab concretions occurs along the Enza riverbanks on the Northern Apennines foothills (Italy). Based upon geochemical and field evidences, their genesis has been attributed to microbial-governed carbonate precipitation from hydrocarbon-enriched fluids. The pipe-like concretions are thus interpreted as former conduits (chimneys) marking sites of methane ascent onto the seafloor. The resulting Enza River chimney field is arranged in a palisade fashion, a rare example of such. Mineralogical and petrographic analyses of some chimneys and slabs document that the dominant cement is dolomite. Although the chimneys show a rather homogeneous texture, a clear zonation is observed in the relative distribution of major and minor elements in their internal and external parts. The occurrence of sulphide minerals in the stratigraphically upper samples indicates possible renewal of fluids leakage after a major erosive event.

The carbonate mineralogy and distribution of habitat-forming deep-sea corals in the southwest pacific region

Habitat-forming deep-sea scleractinian and alcyonacean corals from around the southwest Pacific were analysed for their calcium carbonate mineralogy. Scleractinian coral species Solenosmilia variabilis, Enallopsammia rostrata, Goniocorella dumosa, Madrepora oculata and Oculina virgosa were all found to be 100% aragonitic, while some members of the alcyonacean taxa Keratoisis spp., Lepidisis spp., and Paragorgia spp. were determined to be high magnesium (Mg) calcite (with 8–11mol% MgCO3) and Primnoa sp. is bimineralic with both aragonite and Mg calcite. The majority of these habitat-forming deep-sea corals are found at intermediate depths (800–1200m) in the Antarctic Intermediate Waters (AAIW) with low salinities (~34.5), temperatures of 4–8°C and high oxygen concentrations (>180μmol/kg) and currently sitting above the aragonite saturation horizon (ASH). However, habitat-forming corals have been recorded from greater depths, in cooler waters (2–4°C) that are undersaturated with respect to aragonite (Ωaragonite<1), but with oxygen levels still >160μmol/kg. To address the sampling depth bias the coral records were normalised by the number of benthic stations (sampling effort) in the same depth range. This shows that the highest number of corals per sampling effort is between 1000 and 1400m with corals present in over 5% of the stations at these depths. The normalised records and Boot Strap analyses suggests that scleractinian corals, especially S. variabilis should be present in >1% of stations down to 1800m water depth, with E. rostrata, M. oculata and G. dumosa slightly shallower. While alcyonacean corals are found in >1% down to 2600m, with Keratoisis spp. the deepest down to 2600m, while Lepidisis spp. and Paragorgia spp. found down to 1800m. This suggests that most species can probably tolerate some undersaturation of aragonite (Ωaragonite=0.8–0.9), with several species/genera (S. variabilis; Keratoisis spp.) even more tolerant of lower carbonate concentrations ([CO32−]), down to Ωaragonite of 0.7. With this tolerance for some carbonate undersaturation it is unclear how deep sea habitat-forming corals might respond to future ocean acidification. It is likely that some species/genera will cope better than others. However, future changes in oxygen concentrations and food availability, are also going to have a strong influence on the depth and spatial distribution of deep-sea corals in the southwest Pacific.

Carbonate isotopes from high altitude Tso Moriri Lake (NW Himalayas) provide clues to late glacial and Holocene moisture source and atmospheric circulation changes

Abstract High resolution isotopic (δ 18 O and δ 13 C) investigations on endogenic carbonates (calcite/aragonite) from Tso Moriri Lake, NW Himalaya show dramatic fluctuations during the late glacial and the early Holocene, and a persistent enrichment trend during the late Holocene. Changes in this lake are largely governed by the [input (meltwater + monsoon precipitation)/evaporation] (I/E) ratio, also reflected in changes in the carbonate mineralogy with aragonite being formed during periods of lowest I/E. Using new isotopic data on endogenic carbonates in combination with the available data on geochemistry, mineralogy, and reconstructed mean annual precipitation, we demonstrate that the late glacial and early Holocene carbonate δ 18 O variability resulted from fluctuating Indian summer monsoon (ISM) precipitation in NW Himalaya. This region experienced increasing ISM precipitation between ca. 13.1 and 11.7 cal ka and highest ISM precipitation during the early Holocene (11.2–8.5 cal ka). However, during the late Holocene, evaporation was the dominant control on the carbonate δ 18 O. Regional comparison of reconstructed hydrological changes from Tso Moriri Lake with other archives from the Asian summer monsoon and westerlies domain shows that the intensified westerly influence that resulted in higher lake levels (after 8 cal ka) in central Asia was not strongly felt in NW Himalaya.

A methane-derived carbonate build-up at a cold seep on the Crimean slope, north-western Black Sea

Abstract A unique chimney-shaped carbonate build-up was produced by microbially mediated anaerobic oxidation of methane at a deep-sea cold seep. The build-up was sampled from 1600 m water depth in the area of the Lomonosov Rise (NW Black Sea, Crimean shelf slope). The carbonate chimney grew free into the anoxic water column, with its base attached to a steep slope composed of plagiogranite and void of sediments. The perfectly preserved 1.5 m high chimney stores reliable records of the diversity of mineralogy, geochemistry, and stable isotope composition of deep-sea methane-related carbonates never studied before. The build-up consists of Mg-calcite (MgCO 3 = 9–13 mol%) with minor aragonite. The carbonate matrix encloses organic matter, and Emiliania huxleyi coccoliths, as well as minor framboidal pyrite, gypsum, barite, and diatomite fragments. The contribution of detrital silicate material is negligible. Micritic Mg-calcite in the inner zone of the chimney forms obtuse rhombohedrons clustered in hemispherical aggregates (clots) and hosts isolated prismatic aragonite (≪ 1 vol.%). In the outer zone, Mg-calcite exists as foliated crystals and spherulites, while aragonite spherulites are restricted to the top surface. The carbon isotope compositions of carbonates (δ 13 С = − 46.5 to − 33.0‰ VPDB) and remnant bacterial mats (δ 13 С = − 76.9 to − 81.6‰ VPDB) provide evidence of a biogenic methane source for the build-up formation. Oxygen came mainly from sea water, judging by a narrow range of δ 18 O values in carbonates (+ 0.2 to 1.3‰ VPDB). The prevalence of Mg-calcite throughout the build-up indicates an ANME-2/SBR consortium being the main component of the bacterial mats living at the seep. Aragonite grew mostly late in the seep history, which hints to environmental changes and the ensuing predominance of the ANME-1/SBR consortium. The observed relationship between the prokaryotic communities and the main carbonate phases is common to the microbial mats of the Black Sea, but is actually opposed to the pattern of AOM-community and AOM-related carbonates found in sediments. Different morphologies of carbonates in the outer and inner zones of the build-up point to variations in the methane flux gradient, which controlled the biomass density of the AOM consortia. The REE + Y patterns of Mg-calcite from the inner zone are typical of carbonates crystallised in a freshened sea-water environment, while both Mg-calcite and aragonite from the outer zone show distinct sea-water signatures. Inasmuch as the time span of build-up growth (radiocarbon ages from 9000 ± 180 to 7500 ± 180 year BP) overlaps the period when the Black Sea ‘Lake’ reconnected to the Mediterranean Sea, the progressive arrival of sulphate-rich waters can be inferred to be another possible control of REE + Y patterns in seep carbonates besides the methane flux gradient and related activity of AOM consortia.

Lipid biomarkers and their specific carbon isotopic compositions of cold seep carbonates from the South China Sea

Cold seep carbonates were studied from two locations (Dongsha and Chiasian) in the northern South China Sea and on Taiwan island. Lipid biomarkers and their stable carbon isotopic compositions extracted from these seep carbonate samples were analyzed in this study and the results pointed at the anaerobic oxidation of methane (AOM) as responsible for carbonate formation. Distinct AOM communities were indicated by isotopically highly depleted archaeal biomarkers (with δ13C values as low as −140.8‰ VPDB) and bacterial biomarkers (with δ13C values as low as −128.6‰ VPDB), which confirmed that biogenic methane was the dominant carbon source. Lipid biomarker distributions indicated that ANME-2 archaea were prevalent in sample TVG3-C2g from Dongsha and sample JX8 from Chiasian. In addition, differences of the AOM communities between the carbonates appeared to be caused by the respective biogeochemical environments. ANME-1 archaea tended to be abundant in high-Mg calcite carbonates, and their formation might be linked to low methane fluxes. In contrast, ANME-2 consortia were prevalent in aragonite carbonates and appeared to have prospered at sites with high methane supply. Our data imply that the mineralogy of the seep carbonates precipitated in the northern South China Sea appears to be influenced by the distinct ANME consortia, which are induced by methane flux.

RESEARCH FOCUS: Fuzzy seas

The more we learn about something, the more complex it tends to become. This is nicely demonstrated by the example of calcite-aragonite seas. Secular changes in the prevalent mineralogy of abiotic calcium carbonate precipitates have long been known, since [Sandberg’s (1983)][1] discovery of

Evidence of compositional and ultrastructural shifts during the development of calcareous tubes in the biofouling tubeworm, Hydroides elegans.

The serpulid tubeworm, Hydroides elegans, is an ecologically and economically important species whose biology has been fairly well studied, especially in the context of larval development and settlement on man-made objects (biofouling). Nevertheless, ontogenetic changes associated with calcareous tube composition and structures have not yet been studied. Here, the ultrastructure and composition of the calcareous tubes built by H. elegans was examined in the three early calcifying juvenile stages and in the adult using XRD, FTIR, ICP-OES, SEM and Raman spectroscopy. Ontogenetic shifts in carbonate mineralogy were observed, for example, juvenile tubes contained more amorphous calcium carbonate and were predominantly aragonitic whereas adult tubes were bimineralic with considerably more calcite. The mineral composition gradually shifted during the tube development as shown by a decrease in Sr/Ca and an increase of Mg/Ca ratios with the tubeworm's age. The inner tube layer contained calcite, whereas the outer layer contained aragonite. Similarly, the tube complexity in terms of ultrastructure was associated with development. The sequential appearance of unoriented ultrastructures followed by oriented ultrastructures may reflect the evolutionary history of serpulid tube biominerals. As aragonitic structures are more susceptible to dissolution under ocean acidification (OA) conditions but are more difficult to be removed by anti-fouling treatments, the early developmental stages of the tubeworms may be vulnerable to OA but act as the important target for biofouling control.

Dissolved CO2 and Alkane Gas in Clay Formations

Characterization of dissolved CO2 and alkane gas in clayrocks may help assessing the confinement properties of geological barriers considered as potential host rocks for a deep geological disposal as well as for caprocks of gas storages.A monitoring of alkanes with CO2, combined with carbon isotopes was performed on core samples coming from Underground ResearchLaboratories (Bure, Mont Terri, Tournemire) and the Schlattingen borehole in France and Switzerland. Compositionof hydrocarbon gas and δ13Cof methane strongly suggest a dominant thermogenic origin of methane which ismixed with a bacterial origin for the Toarcian shales, Pliensbachien and Callovian-Oxfordian clayrocks. Results also evidence the contrasted behavior of CO2, which is controlled by chemical equilibrium between pore water and carbonate mineralogy, compared to the alkanes which are present in the porosity as a stock of dissolved gases which can be depleted during degassing experiments.

Authigenic Carbonate Mineralogy, South China Sea and Its Relationship with Cold Seep Activity

Authigenic Carbonate Mineralogy, South China Sea and Its Relationship with Cold Seep Activity

The ca 2.74 Ga Mopoke Member, Kylena Formation: a marine incursion into the northern Fortescue Group?

The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a predominantly subaerial succession, and it remains unclear whether data obtained from these intervals should be interpreted in the context of deposition in marine environments, possibly recording changes in the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member δ13Ccarb values are generally slightly positive, but also include some significantly depleted values, which may relate to the reoxidation of 13C-depleted organic matter. δ13Corg values average –36.7‰, consistent with Neoarchean marine units reported from elsewhere, but significantly less 13C-depleted than values reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more work focusing on the overall depositional environments of the Fortescue Group is needed in order to appropriately interpret geobiological data reported from that group.

Tracking the hydro-climatic signal from lake to sediment: A field study from central Turkey

Palaeo-hydrological interpretations of lake sediment proxies can benefit from a robust understanding of the modern lake environment. In this study, we use Nar Golu, a non-outlet, monomictic maar lake in central Turkey, as a field site for a natural experiment using observations and measurements over a 17-year monitoring period (1997–2014). We compare lake water and sediment trap data to isotopic, chemical and biotic proxies preserved in its varved sediments. Nar Golu underwent a 3 m lake-level fall between 2000 and 2010. δ18Olakewater is correlated with this lake-level fall, responding to the change in water balance. Endogenic carbonate is shown to precipitate in isotopic equilibrium with lake water and there is a strong relationship between δ18Olakewater and δ18Ocarbonate, which suggests the water balance signal is accurately recorded in the sediment isotope record. Over the same period, sedimentary diatom assemblages also responded, and conductivity inferred from diatoms showed a rise. Shifts in carbonate mineralogy and elemental chemistry in the sediment record through this decade were also recorded. Intra-annual changes in δ18Olakewater and lake water chemistry are used to demonstrate the seasonal variability of the system and the influence this may have on the interpretation of δ18Ocarbonate. We use these relationships to help interpret the sedimentary record of changing lake hydrology over the last 1725 years. Nar Golu has provided an opportunity to test critically the chain of connection from present to past, and its sedimentary record offers an archive of decadal- to centennial-scale hydro-climatic change

Preservation of primary lake signatures in alkaline earth carbonates of the Eocene Green River Wilkins Peak-Laney Member transition zone

Significant changes in carbonate mineralogy, texture, and stable isotope composition occur at the transition from the Wilkins Peak Member to the Laney Member in the Eocene Green River Formation, Bridger Basin, Wyoming, which reflect evolution of inflow waters, lake waters, and paleoenvironments.The top of the Wilkins Peak Member contains heterogeneous laminae of calcite and dolomite. Evaporites associated with these layers suggest deposition in hypersaline lakes. Diagenetic carbonate mineral textures include euhedral cement overgrowths and interlocking mosaics of calcite and dolomite crystals, 20–70μm in size. Electron microprobe analyses indicate diagenetic overgrowth of Fe-rich dolomite on cloudy Fe-poor cores. δ18O values of carbonate laminae in the upper Wilkins Peak Member vary by ~6‰ with no depth dependent or mineralogic trends, which also suggests diagenetic overprinting.Alternating organic-rich and primary aragonite, calcite, and dolomite laminae were identified from the lower Laney Member. Primary lacustrine aragonite consists of well sorted, prismatic crystals 5–10μm in length, with micro-lamination defined by crystal size variation. Primary precipitated calcite and dolomite laminae are monominerallic, with well sorted polyhedral crystals, ~10μm in size. Primary mineralogy of the lower Laney Member changes from calcite to aragonite and dolomite stratigraphically upward. Along the same 15m thick stratigraphic interval, δ18O values decrease upward by ~3‰, all of which suggests (1) lake waters underwent evaporative concentration, which together with calcite precipitation increased the lake water Mg/Ca ratios and led to formation of aragonite and dolomite, (2) source waters became lower in δ18O, possibly as inflow changed to higher altitude foreland rivers.The results from this study show that understanding the primary lacustrine versus diagenetic origin of Green River carbonate minerals is essential for paleoenvironmental and paleoclimate interpretations.

Composition and origin of authigenic carbonates in the Krishna–Godavari and Mahanadi Basins, eastern continental margin of India

The mineralogical and stable isotopic composition of authigenic carbonates from the Krishna–Godavari (KG) and Mahanadi Basin provide a deeper insight into the processes inducing carbonate formation in the sediments of the eastern continental margin of India in the Bay of Bengal. Authigenic carbonate cements, (micro) nodules, bioturbation casts and tubes from 12 core locations drilled during the Indian National Gas Hydrate Program (NGHP) Expedition 01 were investigated for this study. Three main processes responsible for authigenic carbonate precipitation are identified: organoclastic sulfate reduction, anaerobic oxidation of methane and methanogenesis. Evidence of vigorous methane seepage is indicated in carbonates recovered at Sites 7, 10, 12 (KG Basin) and 19 (Mahanadi Basin). These methane-derived carbonates display typical paragenetic carbonate mineralogies (aragonite, high-Mg calcite with >15 Mol% MgCO3, Ca-rich dolomite). Two separate horizons of methane derived-carbonates are correlated between 4 drill holes (up to ∼16 km apart). The upper horizon has been dated with 14C (40,100–51,600 a BP 1950) and clearly indicates that methane seepage has been much more vigorous in the past, possibly due to the effect of lower sea level on the gas hydrate stability zone across the margin.

Physicochemical characteristics of drip waters: Influence on mineralogy and crystal morphology of recent cave carbonate precipitates

Speleothems are one of the most intensively explored continental archives for palaeoclimate variability. The parameters, however, that control speleothem petrography and its changes with time and space, specifically calcite crystal morphology and carbonate mineralogy, are still poorly understood. In order to shed light on processes and their products, precipitation experiments of recent carbonate crystals on watch glasses and glass plates were performed in seven selected caves. Drip water sites were analysed for their fluid Mg/Ca molar ratio, pH, degree of saturation for calcite and aragonite and drip rates. Corresponding precipitates were analysed with respect to their mineralogy, calcite crystal morphology and Mg/Ca molar ratio of calcite. The following results are found: High fluid Mg/Ca ratios are found only for caves situated in dolostone, thus the hostrock lithology indirectly controls the carbonate mineralogy and calcite crystal morphology of speleothems. The precipitation of aragonite in place of calcite occurred only in dolostone caves and is bound to very specific conditions. These are: high fluid Mg/Ca ratios (⩾0.5), high fluid pH (>8.2) and low fluid saturation indices for calcite (<0.8). These specific conditions are induced by slow drip rates of <0.2ml/min as often under more arid conditions, causing the precipitation of calcite/aragonite prior to reaching the stalagmite top. Due to this, fluid chemistry is altered, which in turn leads to changes in carbonate mineralogy and geochemistry on the stalagmite top. Calcite growth is inhibited at high fluid Mg/Ca ratios and hence, aragonite precipitation is kinetically stabilised. An increase of the drip water Mg/Ca ratio leads to an increased incorporation of Mg2+ into the calcite crystal lattice and thus, to a change in calcite crystal morphology. Four distinctive changes occur with increasing Mg2+ incorporation: (i) development of new forms (steeper rhombohedra and base pinacoid) at the edges and corners of the crystal seed, (ii) crystal habit tend to elongate along [001] due to slower growth of faces with high Mg2+ densities, (iii) reconstitution of crystal faces with low Mg2+ densities, and (iv) occurrence of calcite crystals with bended faces and edges due to very high Mg2+ (Mg/Ca ratios of 0.009–0.051) incorporation. Growth rates and possibly also organic compounds, however, may also affect the morphology of calcite crystals. Based on the data shown here, the relation of Mg2+ incorporation and the resulting changes in calcite crystal morphologies as well as the conditions of aragonite precipitation are now clearly better understood. Further work should aim at linking the calcite crystal morphology of watch glass precipitates with calcite crystal fabrics in speleothems in order to exploit the petrographic archive of speleothem deposits.

Inorganic control on original carbonate mineralogy and creation of gas reservoir of the Upper Jurassic carbonates in the Kopet-Dagh Basin, NE, Iran

The Upper Jurassic Mozduran Formation (Oxfordian–Tithonian) is the main petroleum reservoir in the Kopet-Dagh Basin, northeast Iran, which consists predominantly of carbonate rocks with subordinate evaporites and siliciclastics deposited in platform to deep marine settings of a subtropical sea. Detailed field surveys, petrographic investigations, facies and wire line log analyses were carried out at eight surface sections and four wells in the Kopet-Dagh Basin. Integration of petrographic and isotopic data suggests primary low-Mg calcite mineralogy of Oxfordian and Tithonian ooids. On the other hand, in the wells, Kimmeridgian ooids and cements are dominantly aragonitic and high-Mg calcite mineralogy (HMC). Marine cements with isopachous, fibrous and isopachous bladed fabrics indicate original aragonite and HMC mineralogy, respectively. The domination of aragonite mineralogy could be related to increased hypersalinity, evaporite precipitation and consequently an increase in Mg/Ca ratio, which resulted in formation of aragonite in preference to calcite. Preserved ooids with radial and concentric cortices in shallow-water settings that are nearby siliciclastic source, together with aragonitic and HMC ooids accompanied by evaporites in the drilled fields, suggest original mineralogy was probably controlled inorganically following local salinity variations. This study suggests that Kimmeridgian pay zones are mainly controlled by depositional facies, aragonitic and HMC mineralogy, and diagenetic processes such as dolomitization and dissolution.

Geochemistry, microfacies and diagenetic evidences for original aragonite mineralogy and open diagenetic system of Lower Cretaceous carbonates Fahliyan Formation (Kuh-e Siah area, Zagros Basin, South Iran)

The Fahliyan Formation is an important reservoir unit of Cretaceous strata in the south of Iran (Zagros Mountains). This formation with the age of Lower Cretaceous overlies unconformably the Hith Formation and is conformably covered by the Gadvan Formation in the studied area. To evaluate the original carbonate mineralogy, paleotemperature, and diagenetic environment in Kuh-e Siah anticline in Boushehr Province, the Fahliyan Formation was investigated by petrographic and geochemical analyses. Petrographic studies led to the recognition of 25 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environment. Of these microfacies, twenty-one belong to inner ramp and four are located in the mid ramp. Calcareous algae and benthic foraminifera are abundant in the shallow-marine carbonates of the Fahliyan Formation. Evidences such as absence of calciturbidite deposits and reefal facies, the occurrence of widespread tidal flat deposits, gradual facies changes, and abundant micrites indicate that the Fahliyan Formation was deposited in a homoclinal carbonate ramp environment. Abundant aragonite skeletal and non-skeletal components and presence of dolomites in Fahliyan carbonates indicate original aragonite mineralogy. Radial ooids generated in low-energy environments similar to modern aragonitic forms. Deformed and spalled ooids as well as shattered micritic envelopes indicate aragonite dissolution during meteoric diagenesis. Isopachous and fibrous sparry calcite cements resemble modern aragonite morphologies. Micritization, geopetal fabric, bioturbation, neomorphism, compaction, cementation, dissolution, dolomitization, dedolomitization, and fracturing are diagenetic processes in the Fahliyan Formation, occurring in marine to meteoric and burial diagenetic environments. Cementation, dissolution, and dolomitization are the main diagenetic processes that affected the original texture. Values of major (Ca and Mg) and minor (Sr, Na, Fe and Mn) elements (such as high Sr value) and δ18O and δ13C designate that the Fahliyan carbonates were deposited in a shallow warm-water sub-tropical environment and aragonite was the original carbonate mineralogy. Variations of Sr/Ca and δ18O values versus Mn suggest that diagenetic alteration must have occurred in an open diagenetic system, with high water–rock interaction. Dolomitization, dissolution and cementation are the main diagenetic features observed in the Fahliyan Formation of Kuh-e Siah and confirm high water–rock interaction in open diagenetic system. Dissolution as leaching is the most important diagenetic event in the evolution of porosity, particularly where green algae and other benthic foraminifera are abundant. This may indicate that the original carbonate mineralogy of the mentioned bioclasts might be aragonite, rather than low-Mg calcite. Temperature calculation based on the oxygen isotope value of the least-altered sample, using Anderson and Arthur (Stable isotopes in sedimentary geology, vol 10. Society of Economic Paleontologists and Mineralogists. Short Course, pp 1–151, 1983) equation, indicates paleotemperature to be around 29.1 °C.

Growth and calcification of marine bryozoans in a changing ocean.

Bryozoans are colonial benthic marine invertebrate calcifiers, important and especially abundant and diverse in southern hemisphere shelf environments. Large heavily calcified colonies can be up to 50 years old, but most longer-lived bryozoans are limited to 10-20 y. Many smaller species are annual. Radial extension in flat encrusting bryozoans is generally on the order of 1-5 mm/y. Erect calcified species generally grow vertically 2-15 mm/y, though articulated species such as Cellaria may reach rates of 40 mm/y. Corresponding calcification rates are generally 10(1)-10(2) mg/y, but there can be an order of magnitude variation in rate among years in high-latitude bryozoans. Multi-branched bryozoans produce up to 24 g of CaCO3/y. The carbonate produced by bryozoans varies from calcite to aragonite and mixtures of both. Skeletal carbonate mineralogy of bryozoans is complex and appears to be strongly genetically controlled. Global climate change, leading to increasing water temperatures, will generally increase marine bryozoan metabolic rates, and may increase Mg in calcite. On the other hand, decreasing pH (ocean acidification) causes corrosion, changes in mineralogy, and decreased survival. This review of bryozoan growth and calcification allows a general perspective, but also reveals gaps in our knowledge which need to be addressed.

RETRACTED: Mineralogy and geochemistry of authigenic carbonates from the Gulf of Cadiz

RETRACTED: Mineralogy and geochemistry of authigenic carbonates from the Gulf of Cadiz

Mineralogy and crystal chemistry of carbonates from the Holocene sediments of Lake Kiran (western Transbaikalia): connection with paleoclimate

We present results of research into the Holocene sediments of Lake Kiran, one of the small saline lakes with carbonate sedimentation in western Transbaikalia. The sediments were studied by X-ray diffraction (XRD) analysis, IR spectroscopy, analysis of stable O and C isotopes, laser granulometry, element analysis (SRXFA), etc. By the mathematical modeling of complex XRD patterns of lacustrine sediments, Mg-calcites with different Mg contents and excess-Ca dolomites have been determined in the assemblage of endogenic carbonate minerals.The obtained carbonate record gives an insight into the stratigraphic distribution of endogenic carbonates of the calcite-dolomite series, in which the number and proportion of phases with different Mg contents are determined by the Mg/Ca ratio, salinity, and total alkalinity of the lake water in the past. The abundance of excess-Ca dolomites in the sediments indicates that the lake is of shallow playa type. The mineral composition and crystallochemical structure of carbonates are in high correlation with the data of lithological analysis, results of determination of stable isotopes (518O and 513C), and distribution of some geochemical indicators of sedimentation conditions. Several stages of the lake evolution depending on the regional climatic changes have been recognized. It has been established that the most arid conditions in southern Buryatia were in the Subatlantic. Despite the highly arid climate in the region in the period under study (from 7ka to the present), it underwent fluctuations toward humidification, in particular, at the Atlantic stage (the time of the lake formation) and in the recent decades, up to the present.

Original Mineralogy and Recognition of Upper Boundary of the Sarvak Formation Based on Geochemistry and Isotope Studies

Carbonate sequence of upper cretaceous (Cenomanian) of Sarvak Formation is a part of Bangestan Group with the thickness of 760 m in Ahvaz oil field (well no. 63). This formation is overlain by the Kazhdumi Formation and is uncomfortably underlain by the Ilam Formation. In this study major and minor elements and carbon and oxygen isotope values and bivariate plot of them indicate that aragonite was the original carbonate mineralogy of the Sarvak Formation. Variations of Sr/Ca and δ18O values versus Mn also illustrate that they were affected by nonmarine diagenesis in a nearly closed system. Recognition of the exact boundary between the Sarvak and Ilam Formations is difficult, due to similar lithologies. So in this study, elemental and oxygen and carbon isotope analysis were used to determine the exact boundary between these Formations. Geochemical data clearly indicate a sub aerial exposure surface, below which meteoric diagenesis effected the sediments.