Coupling textural and stable-isotope variations in fluvial stromatolites: Comparison of Pleistocene and recent records in NE Spain

Textural and stable isotopic features of two middle Pleistocene fluvial stromatolite profiles are compared to a recent stromatolite, both formed in the River Piedra system (NE Spain), to test the reliability of climatic, hydrologic and depositional information derived from ancient records. The Pleistocene stromatolites formed in a multi-domed, highly-inclined cascade-barrage. The recent stromatolite also formed in a highly-inclined cascade of the River Piedra, the sedimentary conditions of which were periodically examined between the years 2000 and 2012. The Pleistocene stromatolites are formed of an alternation of 1) thin large-crystal laminae (type A), with elongated crystals up to 1 mm long, and 2) thick small-crystal laminae (type B), consisting of cyanobacterial fan- and bush-shaped bodies. The textural and isotopic comparison with the recent stromatolite shows that each A–B couplet corresponds to one year. The type-A laminae are comparable to the macrocrystalline laminae that occur in the cool-period deposits of the recent stromatolite, and the type-B laminae are comparable to the warm-period deposits of the recent stromatolite. Water temperatures (Tw), calculated from d 18 O calcite and present measures of d 18 O water , were similar in the Pleistocene and recent specimens, and close to the measured river Tw. Thus, the Pleistocene stromatolites formed not far from isotopic equilibrium, as did the recent stromatolite. The Pleistocene d 18 O calcite biannual oscillation is wider in amplitude than in the recent stromatolite, which suggests larger differences in Tw through the year in the Pleistocene than at present. The Pleistocene d 13 C calcite does not show any pattern; and the values are slightly higher than the recent ones. The co-evolution of d 18 O and d 13 C is parallel in the Pleistocene stromatolites, matching the recent stromatolite behavior. These results and their comparison with other ancient examples prove that textural and isotopic features in ancient stromatolites are useful tools to infer past depositional, climatic and hydrological conditions. Moreover, interpretations from recent fluvial stromatolites can be extrapolated to past environments to help decipher patterns of past processes, in cases where both recent and ancient stromatolites can be compared within one environmental setting. Such comparisons may be used to help interpretations of ancient stromatolites where the modern ones are not available to study

Geochemical characterisation and modelling of the Luchon hydrothermal system (Central Pyrenees, France) and lessons learnt for the use of geochemical modelling techniques in granite-hosted alkaline thermal waters

Alkaline hydrothermal systems hosted in granitic rocks have been extensively investigated as natural analogues for radioactive waste disposal and for the geological storage of CO2. Thereby, their geochemical characterisation provides useful information for the long-term performance assessment predictions. The geochemical modelling of one of these alkaline hydrothermal systems has been performed in Luchon (France), together with the application of different geothermometrical techniques to determine the temperature, pH and mineral equilibrium conditions at depth. The modelling results show that the main processes controlling the hydrogeochemical evolution of this system are: (1) the mixing between deep thermal and cold surface waters, (2) the conductive cooling and (3) the external input of CO2. Other important results are that the most alkaline thermal waters are characterised by a high pH-buffering capacity during the mixing processes, and that the high pH values that characterise these thermal waters are not only inherited from the deep reservoir but strongly enhanced by conductive cooling. The reservoir temperature predicted by the geothermometrical modelling is in the range of 117 ± 8 °C, in good agreement with the temperatures ranging from 108 to 133 °C predicted by the classical geothermometers. The results of these calculations indicate as well that the thermal solutions have reached equilibrium with quartz, albite, potassium feldspar, zoisite and prehnite, and that a re-equilibirum with kaolinite and calcite is reached during their ascent to the surface. Another important outcome is that the precipitation of calcite in the deep reservoir could take place as a CO2 mineral trapping mechanism in similar systems. Further, the sharp influence of the surface waters on the deep thermal waters reveals a high susceptibility of the system to potential contamination processes

Intrinsic and extrinsic controls of spatial and temporal variations in modern fluvial tufa sedimentation: A thirteen-year record from a semi-arid environment

Sedimentological and hydrochemical parameters of the River Piedra (north-east Spain) were monitored every six months (from 1999 to 2012) at 24 sites, at which tablets were installed all along the river. The river water is of HCO3–SO4–Ca type and is notably influenced by inputs from upstream karstic springs. Tufa deposition was first detected 8 km downstream of these springs and greatly increased from there, primarily along the steeper stretch (i.e. within the Monasterio de Piedra Natural Park); then, deposition decreased through the most downstream stretch, with smaller ground water inputs. The spatial evolution of the tufa thickness, with parallel variations of PWP (Plummer, Wigley, Parkhurst) rates, was thus determined by the river water pCO2 which was controlled by ground water inputs and by the river bed slope. Five fluvial subenvironments and seven sedimentary facies were characterized. The water flow conditions are the primary factor responsible for the distinct deposition rates of facies, mainly through CO2-outgassing. Stromatolites and moss-tufa and alga-tufa had the highest rates, whereas loose tufa formed in slow-flowing water and tufa of spray areas had thinner deposition. A six-month pattern in the deposition rate was detected through thickness measurements. That pattern was parallel to the seasonal PWP rates. The increased deposition during warm periods (spring and summer; mean: 5·08 mm) compared with cool periods (autumn and winter; mean: 2·77 mm) is linked chiefly to temperature, which controlled the seasonal changes in the physico-chemical and biological processes; this finding is supported by a principal components analysis. Seasonal variations of insolation and day duration also contributed to such a deposition pattern. Large discharge events, which provoked erosion of tufa deposits and dilution of water, caused the reversal of the seasonal deposition rate pattern. Stromatolites are likely to preserve the most complete sedimentary record. Although tufas are a potentially sensitive record of climate-related parameters, erosion is an intrinsic process that may overwhelm the effects of such parameters. This issue should be considered in palaeoclimatic studies based on the tufa record, particularly in semi-arid conditions

Tufa sedimentation in changing hydrological conditions: the River Mesa (Spain)

The processes controlling tufa deposition along the River Mesa (NE Spain) were studied from April 2003 to September 2009, based on six-monthly monitoring of physical and chemical parameters of the river water and sedimentological characteristics, including deposition rates on tablets. With a mean annual discharge around 1.5m3 /s, the sedimentation rate (mean 2mm/yr) recorded important spatial, seasonal and interannual variations. The river waters are of the calcium bicarbonate type. In this study, three distinct river stretches were distinguished based on the steady groundwater inputs, some of low-thermal nature. Groundwater discharges controlled the water chemical composition, and some sedimentation features too. At each stretch, an increase in pCO2 and conductivity was measured around the spring sites. Decreasing trends in conductivity or alkalinity with high enough saturation values with respect to calcite were only clearly observed in the intermediate stretch, which had higher tufa deposition rates than the other two. Tufa deposition rates were higher in cool (autumn+winter) than in warm (spring+summer) periods. In some low-rainfall warm periods, tufa deposition was inhibited or limited due to the low flow –mainly from groundwater inputs– and to the dryness of some river sites, which indeed favoured erosion during flooding. A decrease in yearly deposition rates from April 2006 onwards paralleled an important reduction in the river discharge. Groundwater inputs, drought periods and flood events should therefore be considered to understand fluvial tufa sedimentation in semi-arid conditions

Characters of the Stephanian-Permian Bronchales basin and the tourmalinization associated to calc-alkaline rhyolites (Teruel Province)

[Resumen] La turmalinización que afecta a riolitas calco-alcalinas presentes en la fosa transtensional de Bronchales, con edad Autuniense, destaca por el elevado contenido en B (0.87 O/o de promedio). Esta mineralización muestra un enriqueciIl\iento de HREE y un empobrecimiento en LREE debido a que el fluido mineralizante (rico en F) pudo ejercer una función acomplejante para estos elementos y, también, con carácter selectivo, para As, Sn, Sb y Bi.[Abstract] Calc-alkaline rhyolites of Bronchales transtensional basin (Autunian age) are affected by a tourmalinization process. Tourmalinized rhyolites with 0.87 O/o ofboron as average value show As, Sb, Sn and Bi increments over nontourmalinized ones, as well as important enrichments of HREE over LREE possibly because tourmalinization fluids were enriched in complexing agents (i.e .F)

Benthic diatoms on fluvial tufas of the Mesa River, Iberian Range, Spain

Background. The Mesa River (MR) in the Iberian Range (Spain) displays prominent, Pleistocene to present-day fluvial tufa deposits. Little of their associated microbiota has been studied to date despite the regional and historical relevance of these calcareous buildups. Goals. This paper is a preliminary exploration of the diatom (Bacillariophyta) genera associated with actively-growing tufa from 10 benthic environments along 24 km of the Mesa River. Methods. Bright- field microscopy, as well as consultation with specialists and specialized literature was used for taxonomic classification of diatoms. Results. We identified 25 diatom genera in three different types of sedimentary facies (porous and moss-algae rich, dense-laminated, and tufa-free gravel). Most diatoms were raphid pennate (class Bacillariophyceae), while few were centric (class Coscinodiscophyceae) or araphid pennate (class Fragilariophyceae). They appeared as integral components of the tufa structure along with cyanobacteria and other algae and mosses. Conclusions. Together with previous studies on the hydrochemistry and sedimentology of the MR, our interpretations suggest that HCO3-, pCO2, Ca2+, and TDIC negatively affect diatom richness and that their abundance is positively related to the presence of mosses and algae

Simulación de la alteración de Microclina. L. Comparación de códigos de modelización geoquímica e incidencia de los datos termodinámicos

Simulations of microcline weathering reactions (in closed system and reacting with a HCl solution of pH =4, at 25° C and 50° C) are formulated with the more recent versions of the computer codes EQ3/6, CHILLER, PHREEQE YHARPHRQ. The calculated reaction paths are strongly dependent of the thermodynamic constants for mineral equilibria but almost insensitive to variations in the speciation reactions enclosed in those codes for the selected conditions of simulation. Changes in the thermodynamic values for the mineral equilibria lead to important numerical differences in the predicted mass transfers, solution compositional parameters and also affect the number of secondary minerals involved in the simulation. So, differences in the thermodynamic data can induce variations in reaction path calculations as important as those produced by changes in the initial conditions of simulation. In spite of efforts on database updating, model sensitivity to mineral equilibrium constants is important and it must be taken into account in reaction path modeling of microcline weathering process.En este trabajo se comparan los resultados de las versiones más recientes de los códigos de modelización termodinámica EQ3/6, CHILLER, PHREEQE YHARPHRQ en la simulación de un proceso de alteración de microlina (con HCl diluido en agua pura a pH =4, en condiciones de sistema cerrado y a temperaturas de 25 y 50° C). Los resultados de la simulación presentan una marcada dependencia de las constantes de equilibrio mineral, siendo apenas afectados por las diferencias en las constantes de las reacciones de especiación consideradas por cada código. Los distintos valores de las constantes de equilibrio mineral que intervienen en el proceso provocan apreciables diferencias en las transferencias de masa y en los caracteres composicionales de la solución calculados por los códigos; e incluso llegan a afectar al número de minerales secundarios en los que se resuelve la simulación. Las diferencias entre los conjuntos de valores termodinámicos utilizados pueden provocar, por tanto, variaciones tan importantes como las producidas por cambios en las condiciones iniciales de simulación. A pesar de los esfuerzos realizados en el refinamiento y mejora de las bases de datos de los códigos, existen todavía importantes diferencias en los valores de las constantes de equilibrio mineral consideradas; circunstancia que se traduce en un importante grado de dependencia de los resultados respecto a esos parámetros en la simulación del proceso mediante cálculos de pautas de reacción

Los sistemas geotermales del Pirineo central. I. Caracteres geoquímicos y fisicoquímicos de los manantiales termales

The compositional characters of selected hot springs from the Panticosa, Benasque, Arties, Caldas de Bohí, Luchon and Cauterets geothermal systems in the Central Pyrenees have been studied. All the springs have the same global geochemical features (high pH, low total dissolved solids, very low Mg concentrations, Na as the dominant cation, and absence of a dominant anion) and belong to the same granite-related alkaline thermal waters group. Chemical composition of the springs is similar to each other, and to that found in other Pyrenean geothermal systems. A more detailed comparison shows, however, that Panticosa and Benasque systems have the lowest sodium and potassium concentrations found in any water belonging to the group of alkaline waters. Differences in pH values among the springs are related to emergence temperature as it occurs in the Pyrenean geothermal systems. According to the high pH of these waters, speciation calculations at emergence temperature show a very low CO2 partial pressure, dissolved silica partially ionized as H3Si04-, and Al(OH)4- as the dominant aluminium species. Saturation indexes show that almost al1 the waters are highly oversaturated with quartz (close to equilibrium with chalcedony), potassium feldspar, and albite, which are the minerals equilibrated at depth with the solutions. Furthermore, all waters are in equilibrium with kaolinite and ca1cite at emergence conditions, showing the existence of a reequilibrium process during their ascent to the surface. This process happens to be one of the most important in the geochemical evo1ution of these alkaline systems.Las aguas de los sistemas geotermales de Panticosa, Benasque, Arties, Caldas de Bohí, Luchon y Cauterets en el Pirineo Central presentan rasgos geoquímicos similares (valores de pH elevados, bajo contenido de sólidos disueltos, muy bajas concentraciones de Mg, sodio como catión dominante y sin un anión en proporciones preponderantes sobre los demás), comunes a los existentes en otros sistemas pirenaicos y característicos de la tipología de aguas termales alcalinas asociadas a materiales graníticos. Sin embargo y más específicamente, las concentraciones de Na y K en las aguas de Panticosa y Benasque son las más bajas de todas las definidas hasta el momento en este grupo de aguas termales alcalinas. Los valores de pH de los manantiales termales se encuentran relacionados con la temperatura de surgencia y, como ocurre en el resto de sistemas geotermales pirenaicos, dependen de la intensidad del proceso de enfriamiento durante el ascenso de la solución. Los cálculos de especiación en condiciones de surgencia indican que: los valores de presión parcial de CO2 de las soluciones son muy bajos; la sílice disuelta se encuentra parcialmente ionizada; y la especie Al(OH)4- es la forma dominante de aluminio en solución. Los índices de saturación obtenidos demuestran que casi todas las aguas consideradas se encuentran sobresaturadas respecto a cuarzo (próximas al equilibrio con calcedonia), feldespato potásico y albita, minerales con los que factiblemente las soluciones establecen relaciones de equilibrio en el reservorio. Pero, además, todas las aguas se encuentran en situaciones de equilibrio respecto a calcita y caolinita en condiciones de surgencia, lo que demuestra la existencia de procesos de reequilibrio durante su ascenso hasta los manantiales y evidencia uno de los caracteres más importantes en la evolución geoquímica de este tipo de sistemas alcalinos

Variación de los índices de saturación en función del calculo de coeficientes de actividad. su aplicación a las fases mineralogicas de las lagunas de Monegros (Zaragoza, España)

Application of WATEQ4F and SOLMINEQ.88 speciation-solubility codes to brines fmm La Playa in Los Monegros (Zaragoza) has been used to contrast different approaches in activity calculations and to determinate saturation indexo The evaporation process increases ionic force and activities of dissolved species but with notorious singularities; the salt precipitation is theoretically reached only with ionic forces greater than 3. Pitzer's formulation included in SOLMINEQ.88 options is the best one for approximation to natural observations, and WATEQ4F also shows good concordance with evolutive trendLa aplicación de los códigos de especiación WATEQ4F y SOLMINEQ.88 a los datos del sistema natural de la laguna de La Playa en Los Monegros (Zaragoza) proporcionados por autores previos, ha servido para contrastar distintas opciones de cálculo. Como era de esperar, el aumento de la Fuerza Iónica con el avance de la evaporación proporciona un incremento de la actividad de las especies disueltas (y una disminución de la del agua) sea cual sea la opción escogida, aunque existen notorias diferencias y singularidades. Los Indices de Saturación de las especies minerales presentes en el sistema (calcita, dolomita, yeso, anhidrita, thenardita y halita) muestran que la espectacular precipitación de sales observada en el fondo de las lagunas sólo se consigue, de acuerdo con la serie de desmuestre tratada, cuando la evaporación está ya bastante avanzada y con Fuerzas Iónicas mayores de 3. De todas las opciones de cálculo analizadas, la que proporciona unos resultados más acordes con lo observado en el medio natural es la correspondiente al SOLMINEQ.88 con el desarrollo de ecuaciones de Pitzer, aunque también el WATEQ4F muestra concordancias en cuanto a las tendencias

Pautas de distribución de especies de las Tierras Raras en soluciones ácidas naturales

A speciation analysis of Rare Earth elements (REE) in acidic waters of four natural systems in Brasil, Spain, Canada and USA is carried out. The geochemical diversity of the samples span a pH range of 3.3-5.2, a fairly broad range of SO=4 F- and P03-4 concentrations and a diference in measured REE concentrations of four orders of magnitude. The shale-normalized REE patterns show a systematic, almost universal, IREE enrichment in all the acidic waters studied.From REE speciation calculations using the PHREEQE code, it can be concluded that the dominant forms of REEs are sulfate complexes (mainly LnSO+4) with minor free ionic species (Ln3+). These free ionic species only became dominant when sulfate concentrations in waters are very low.Other ligands present in solution (P03-4 and F- ) do not compete with sulfate in REE speciation in the concentration range spanned by the samples. At least two reasons can be put forward to explain this result: (i) P03-4 concentrations are too low to affect REE speciation and (ii) F- which is present in concentrations high enough to interfere with lanthanide speciation, actually associates with aluminum. Aluminum, usually in high concentrations in acidic waters, is then the responsible of inhibiting REE fluoride complexation and hence, of inhibiting the important effect that this ligand has in REE speciation. Therefore, analytical determination of aluminum must be performed in order to properly calculate REE speciation in acidic waters.En este trabajo se analizan las pautas generales de especiación de las Tierras Raras (TR) en aguas ácidas mediante la utilización de muestras procedentes de cuatro sistemas naturales distintos (en Brasil, España, Canadá y USA). Los caracteres geoquímicos del conjunto de muestras seleccionado permiten considerar variaciones de pH de 3,3 a 5,2, un rango relativamente amplio de concentraciones de SO=4 F- y P03-4 y diferencias en los contenidos de TR de hasta cuatro órdenes de magnitud. Las pautas de distribución de TR normalizadas frente a NASC en casi todas las aguas estudiadas muestran un marcado enriquecimiento en las Tierras Raras Intermedias (TRI).Los cálculos de especiación realizados con el código PHREEQE muestran que la distribución de especies de las TR se encuentra controlada por los complejos sulfatados (LnSO+4 principalmente) y, en menor proporción, por las formas iónicas libres (Ln3+). Sólo cuando las concentraciones de sulfato son considerablemente bajas, las formas libres se encuentran en proporciones preponderantes sobre las de los complejos sulfatados.Los otros ligando s presentes en las soluciones ácidas (P03-4 y F-) no entran en competición con el sulfato en la especiación de las Tierras Raras (al menos para el rango de concentración abarcado por las muestras manejadas) por distintos motivos. Las concentraciones de P03-4 no son lo suficientemente elevadas como para incidir en la especiación de esos elementos. El F- presenta, en casi todas las muestras, valores de concentración suficientes como para influir, a priori, en la especiación de los lantánidos; sin embargo este ligando forma complejos con el aluminio en solución de manera preferente. Por ello, las normalmente altas concentraciones de este último elemento en soluciones ácidas evitan que el F- compita con el SO=4 en la distribución de especies de las Tierras Raras, impidiendo los notables efectos de aquel ligando en la especiación de esos elementos.Desde un punto de vista metodológico, los resultados obtenidos señalan la necesidad de determinar las concentraciones de aluminio para una correcta realización de los cálculos de especiación de Tierras Raras en aguas ácidas