Early diagenesis in biogenic carbonates of temperate and shoal waters (Cadiz Bay and adjacent continental shelf)

Because of the chemical reactivity of biogenic carbonates in shallow waters, there is increasing interest in those early diagenesis processes that imply an alteration or variation of the composition of original material. This paper presents the preliminary results of a study concerning the compositional variations that have taken place in the skeletons or skeletal remains of bryozoans and echinoids. This variation, which includes an aragonite-calcite transformation, has been checked by means of the Rietveld method, using the Fullprof program. This transformation is caused by an in situ dissolution-recrystallisation process, due to aragonite's metastability. Moreover, the presence of specific authigenic minerals helps to delimit early diagenetic processes, which are, in turn, indicative of the path and the extension of the diagenetic reactions that take place in the sediment column.Debido a que los carbonatos biogénicos de aguas someras son químicamente muy reactivos, existe cada vez mayor interés por conocer los procesos de diagénesis temprana que conllevan una alteración o modificación de la composición del material original. En este trabajo se presentan los resultados iniciales sobre las variaciones de composición que han tenido lugar en los caparazones o restos del esqueleto de equinodermos y briozoos. Esa variación, incluida la transformación aragonito-calcita, ha sido chequeada mediante el método Rietveld, programa Fullprof. Dicha transformación ocurre por un proceso de disolución-recristalización debida a la metaestabilidad del aragonito. Además, la presencia de una asociación de minerales neoformados implica procesos diagenéticos tempranos indicativos de las reacciones que tienen lugar en la columna del sedimento.Instituto Español de Oceanografí

Las glauconitas de la plataforma continental gallega: indicadores geoquímicos del grado de evolución.

La interpretación estratigráfica de unidades que contienen glauconita requiere de una adecuada caracterización del grado de evolución, definido a partir del concepto de madurez (Amorosi, 1995). Su utilización es útil para determinar la duración de periodos asociados a bajas tasas de sedimentación o para caracterizar condiciones paleoceanográficas y paleoambientales en la interfase sedimento-agua del mar. En este tipo de estudios se asume implícitamente que la glauconita actúa como un sistema abierto, susceptible de continuos cambios en la estequiometría, como resultado del intercambio de cationes con el agua del mar que, a su vez, inducen reajustes estructurales. La determinación del grado de evolución de glauconitas está asociada habitualmente a la presencia de determinados marcadores, algunos de tipo cualitativo como es el color o bien asociados a la variación de alguna de sus propiedades físicas, como es la susceptibilidad magnética o composicionales y estructurales, como son la determinación del contenido en K+ o la distancia entre las reflexiones 001, 020 en diagramas de difracción de rayos X. Sin embargo, una caracterización adecuada del proceso evolutivo en glauconita que pueda ser aplicada de forma general debe ser capaz de establecer una relación precisa entre los cambios químicos con el medio durante el proceso de creci 233 S. Fernández-Bastero et al. Las glauconitas de la plataforma continental gallega... miento y la tendencia hacia una mayor estabilidad termodinámica, conectada con modificaciones estructurales, desde una etapa inicial de formación, casi amorfa, hasta alcanzar los últimos términos de la solución sólida, más ordenados. En este sentido, este trabajo se ha planteado con una doble finalidad: en una primera etapa se ha determinado la variabilidad composicional en glauconitas de la plataforma continental gallega y en segundo lugar, estos valores han sido comparados con la variabilidad estructural al objeto de establecer patrones de comportamiento durante las diferentes etapas del proceso evolutivo. El desarrollo de este estudio está basado en el análisis cuantitativo de los diagramas de difracción de rayos X de 50 muestras de sedimento superficial, mediante el método de Rietveld. Con este fin, se ha desarrollado una aplicación particular de este procedimiento de análisis, basada en el ajuste simultáneo de los factores de ocupación para las distintas sustituciones en lugares tetraédricos y octaédricos, así como en las intercapas de la estructura. De este modo, las modificaciones en la estequiometría, inducidas por los procesos de intercambio catiónico entre la superficie mineral y el agua del mar se pueden relacionar con cambios estructurales asociados a las diferentes etapas del proceso evolutivo. Como resultado de este análisis, ha sido posible la elaboración de mapas de contenido geoquímico que reflejan la distribución espacial, tanto para la variabilidad composicional como para la cristalinidad en las glauconitas asociadas a sedimentos superficiales de la plataforma continental gallega

Constraining the preservation of organic compounds in Mars analog nontronites after exposure to acid and alkaline fluids

The presence of organic matter in lacustrine mudstone sediments at Gale crater was revealed by the Mars Science Laboratory Curiosity rover, which also identified smectite clay minerals. Analogue experiments on phyllosilicates formed under low temperature aqueous conditons have illustrated that these are excellent reservoirs to host organic compounds against the harsh surface conditions of Mars. Here, we evaluate whether the capacity of smectites to preserve organic compounds can be influenced by a short exposure to different diagenetic fluids. We analyzed the stability of glycine embedded within nontronite samples previously exposed to either acidic or alkaline fluids (hereafter referred to as “treated nontronites”) under Mars-like surface conditions. Analyses performed using multiple techniques showed higher photodegradation of glycine in the acid-treated nontronite, triggered by decarboxylation and deamination processes. In constrast, our experiments showed that glycine molecules were preferably incorporated by ion exchange in the interlayer region of the alkali-treated nontronite, conferring them a better protection against the external conditions. Our results demonstrate that smectite previously exposed to fluids with different pH values influences how glycine is adsorbed into their interlayer regions, affecting their potential for preservation of organic compounds under contemporary Mars surface conditionsEuropean Commission | Ref. FP7 n. 307496European Commission | Ref. H2020 n. 818602Ministerio de Economía | Ref. MDM-2017-0737Ministerio de Economía | Ref. ESP2017-89053-C2-1-

Chemical factors controlling the steady-state distribution of mixed carbonate-siliciclastic sediments in Bayona Bay (northwest Spain)

The relative space distributions of single mineral constituents in mixed terrigenous-carbonated sediments of the Bayona Bay have been analysed. In order to determine the existence of different mixing mechanisms, a study comparing the general sedimentary trends resulting from the average patterns and the single mineral behaviour, reflected in the single-mineral mapping, was carried out. The relative abundance of all mineral phases in surface sediment samples was first determined by quantitative X-ray powder diffraction data, using the Rietveld method. This procedure enabled us to create specific maps for both the minerals forming the terrigenous fraction and the different calcium carbonate polymorphs involved in the carbonatic component. A cross-linked behaviour between the carbonate facies and the terrigenous minerals involved in the calcium carbonate generation was found, suggesting the existence of a chemical control mechanism which, through dissolution-crystallization processes, constrains the mixed environment's long-term evolution. In the general framework of the region studied, these relations operate for the relative amount of plagioclase to calcium carbonate. The existence of dissolution and growth between both minerals, favoured by the high rate of removal, is proposed, yielding a final crossed pattern, representative of the steady-state. Furthermore, a specific analysis, restricted to the region of low rate of material removal and maximum amount of intermixed sediments, clearly shows an anisotropic distribution for every calcium carbonate polymorph (calcite and aragonite). These correlate with the biotite distribution, and can be connected to local variations in seawater Mg/Ca ratio, induced by biotite weathering.En este trabajo se analiza de forma específica la distribución espacial de cada uno de los minerales involucrados en sedimentos mixtos terrígenos-carbonatados de la bahía de Bayona y se compara con la distribución global de sedimentos, tal como resulta de análisis granulométricos, con el fin de determinar la existencia de diferentes mecanismos de mezcla entre los minerales de ambas fracciones. El contenido porcentual en peso de cada fase mineral, en muestras superficiales de sedimento, se determinó a partir de datos cuantitativos de DRX, empleando el método de Rietveld. Este procedimiento nos ha permitido realizar mapas específicos de distribución, tanto para cada mineral de la fracción terrígena como para cada uno de los diferentes polimorfos de carbonato cálcico en la componente carbonatada. Como resultado de este estudio comparativo, se ha encontrado una correlación, en la distribución espacial y granulométrica, de las facies carbonatadas y los minerales terrígenos implicados en la generación de carbonato cálcico. Un posterior análisis de este resultado sugiere la presencia de mecanismos químicos de control que regulan la interacción entre siliciclastos y carbonatos mediante procesos de disolución-cristalización mediados por el agua del mar. El estudio se ha realizado a dos escalas. En el ámbito general de la bahía de Bayona, las relaciones de distribución observadas entre plagioclasas y carbonato cálcico pueden ser explicadas, dentro del balance global del Ca³⁺ disuelto en el agua del mar, mediante la meteorización química de las plagioclasas y la consiguiente generación de carbonatos. En este caso, los patrones de distribución reflejan la aproximación a un estado estacionario mediante sucesivos eventos de mezcla asociados a la alta removilización material en el medio, tanto para la distribución porcentual en peso como para la distribución granulométrica entre ambos minerales. Finalmente, se realiza un análisis particularizado en una región con baja tasa de removilización y donde la interacción entre sedimentos de ambas fracciones es máxima. El resultado muestra claramente una distribución espacial anisótropa para cada uno de las fases de carbonato cálcico (calcita y aragonito). La abundancia relativa de cada uno de los polimorfos, así como el contenido en Mg³⁺ de la calcita, ha sido correlacionado con la distribución de la biotita y explicado a partir de las diferencias de solubilidad en función de la alta relación Mg³⁺ /Ca³⁺ en el agua de mar que resulta, localmente, de la meteorización química de dicho mineral.Instituto Español de Oceanografí

Tracking the weathering of basalts on Mars using lithium isotope fractionation models

An edited version of this paper was published by AGU. Copyright (2015) American Geophysical UnionLithium (Li), the lightest of the alkali elements, has geochemical properties that include high aqueous solubility (Li is the most fluid mobile element) and high relative abundance in basalt-forming minerals (values ranking between 0.2 and 12 ppm). Li isotopes are particularly subject to fractionation because the two stable isotopes of lithium - 7Li and 6Li - have a large relative mass difference (∼15%) that results in significant fractionation between water and solid phases. The extent of Li isotope fractionation during aqueous alteration of basalt depends on the dissolution rate of primary minerals - the source of Li - and on the precipitation kinetics, leading to formation of secondary phases. Consequently, a detailed analysis of Li isotopic ratios in both solution and secondary mineral lattices could provide clues about past Martian weathering conditions, including weathering extent, temperature, pH, supersaturation, and evaporation rate of the initial solutions in contact with basalt rocks. In this paper, we discuss ways in which Martian aqueous processes could have lead to Li isotope fractionation. We show that Li isotopic data obtained by future exploration of Mars could be relevant to highlighting different processes of Li isotopic fractionation in the past, and therefore to understanding basalt weathering and environmental conditions early in the planet's historyData supporting our models and calculations are available as supporting information. The research leading to these results is a contribution from the Project ‘icyMARS’’, funded by the European Research Council, Starting Grant no 307496. This work was also partially supported by the European FEDER program and the Spanish Ministry of Science (MICINN) through the project CGL2011–30079. Comments by R. James and four anonymous reviewers helped us to clarify and strengthen our wor

Crystallization by particle attachment in synthetic, biogenic, and geologic environments.

Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments

The mineralogical composition of calcium and calcium-magnesium carbonate pedofeatures of calcareous soils in the European prairie ecodivision in Hungary

Abstract There is little data on the mineralogy of carbonate pedofeatures in the calcareous soils in Hungary which belong to the European prairie ecodivision. The aim of the present study is to enrich these data. The mineralogical composition of the carbonate pedofeatures from characteristic profiles of the calcareous soils in Hungary was studied by X-ray diffractometry, thermal analysis, SEM combined with microanalysis, and stable isotope determination. Regarding carbonate minerals only aragonite, calcite (+ magnesian calcite) and dolomite (+proto-dolomite) were identified in carbonate grains, skeletons and pedofeatures. The values relating, respectively, to stable isotope compositions (C13, O18) of carbonates in chernozems and in salt-affected soils were in the same range as those for recent soils (latter data reported earlier). There were no considerable differences between the values for the carbonate nodules and tubules from the same horizons, nor were there significant variations between the values of the same pedofeatures from different horizons (BC-C) of the same profile. Thus it can be assumed that there were no considerable changes in conditions of formation. Tendencies were recognized in the changes of (i) carbonate mineral associations, (ii) the MgCO3 content of calcites, (iii) the corrected decomposition temperatures, and (iv) the activation energies of carbonate thermal decompositions among the various substance-regimes of soils. Differences were found in substance-regimes types of soils rather than in soil types

Biomineralisation by earthworms: an investigation into the stability and distribution of amorphous calcium carbonate

Background Many biominerals form from amorphous calcium carbonate (ACC), but this phase is highly unstable when synthesised in its pure form inorganically. Several species of earthworm secrete calcium carbonate granules which contain highly stable ACC. We analysed the milky fluid from which granules form and solid granules for amino acid (by liquid chromatography) and functional group (by Fourier transform infrared (FTIR) spectroscopy) compositions. Granule elemental composition was determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) and electron microprobe analysis (EMPA). Mass of ACC present in solid granules was quantified using FTIR and compared to granule elemental and amino acid compositions. Bulk analysis of granules was of powdered bulk material. Spatially resolved analysis was of thin sections of granules using synchrotron-based μ-FTIR and EMPA electron microprobe analysis. Results The milky fluid from which granules form is amino acid-rich (≤ 136 ± 3 nmol mg−1 (n = 3; ± std dev) per individual amino acid); the CaCO3 phase present is ACC. Even four years after production, granules contain ACC. No correlation exists between mass of ACC present and granule elemental composition. Granule amino acid concentrations correlate well with ACC content (r ≥ 0.7, p ≤ 0.05) consistent with a role for amino acids (or the proteins they make up) in ACC stabilisation. Intra-granule variation in ACC (RSD = 16%) and amino acid concentration (RSD = 22–35%) was high for granules produced by the same earthworm. Maps of ACC distribution produced using synchrotron-based μ-FTIR mapping of granule thin sections and the relative intensity of the ν2: ν4 peak ratio, cluster analysis and component regression using ACC and calcite standards showed similar spatial distributions of likely ACC-rich and calcite-rich areas. We could not identify organic peaks in the μ-FTIR spectra and thus could not determine whether ACC-rich domains also had relatively high amino acid concentrations. No correlation exists between ACC distribution and elemental concentrations determined by EMPA. Conclusions ACC present in earthworm CaCO3 granules is highly stable. Our results suggest a role for amino acids (or proteins) in this stability. We see no evidence for stabilisation of ACC by incorporation of inorganic components

Combining μXANES and μXRD mapping to analyse the heterogeneity in calcium carbonate granules excreted by the earthworm Lumbricus terrestris

The use of fluorescence full spectral micro-X-ray absorption near-edge structure (μXANES) mapping is becoming more widespread in the hard energy regime. This experimental method using the Ca K-edge combined with micro-X-ray diffraction (μXRD) mapping of the same sample has been enabled on beamline I18 at Diamond Light Source. This combined approach has been used to probe both long- and short-range order in calcium carbonate granules produced by the earthworm Lumbricus terrestris. In granules produced by earthworms cultured in a control artificial soil, calcite and vaterite are observed in the granules. However, granules produced by earthworms cultivated in the same artificial soil amended with 500 p.p.m. Mg also contain an aragonite. The two techniques, μXRD and μXANES, probe different sample volumes but there is good agreement in the phase maps produced