Mineralogical and Sedimentological Characterization of the Clay-Rich Sediments from Ases Cave (Cova Dets Ases, Mallorca, Spain): Origin and Classification

The Mallorca coastal caves present large amounts of speleothems that have been studied for decades. However, the sedimentary deposits also present in these cases have not been given the same attention. This work is the first study entirely focused on these deposits, specifically the ones found in the Ases cave. These deposits are formed by clay minerals (illitic phases, kaolinite, smectite, and chlorite), calcite and quartz, and minor proportions of dolomite, albite, orthoclase, hematite, and goethite. The grain size and the electron microscopy studies suggested the presence of different sedimentation processes (bedrock degradation, creep or saltation, and suspension) and different origins (authigenic and detrital origins) for the different sediments. Based on these differences, two types of deposits were characterized: autochthonous and allochthonous deposits. The first ones are located on the floor of chambers and corridors in subaqueous zones, indicating the stability of the mixing zone (and therefore the sea level) over time. The second ones appear filling voids on the walls and the ceiling in the terrestrial zone, evidencing the filling of the cavity in the presence of water (during a wet period). These results are very important to complete the understanding of the caves and their evolution and support the relevance of these materials in paleoenvironmental studies

Estudio geotermométrico de las aguas termales ricas en CO2 del acuífero de La Ermita del Saladillo (Murcia)

Las aguas del acuífero carbonatado de La Ermita del Saladillo representan una de las manifestaciones termales de aguas ricas en CO2 del sureste de la Península Ibérica. En este tipo de sistemas, la aplicación de las técnicas geotermométricas puede presentar problemas y por ello, en este trabajo se han verificado los resultados obtenidos mediante distintos geotermómetros en dos muestras de agua tomadas en este acuífero termal a partir de sondeos. Los geotermómetros catiónicos sobreestiman las temperaturas del reservorio ya que las aguas no parecen haber alcanzado los equilibrios en los que se basan esas técnicas. Sin embargo, los geotermómetros Ca-Mg y anhidrita – calcedonia, de uso en sistemas carbonatado – evaporíticos no carbónicos proporcionan resultados coherentes en las aguas estudiadas ricas en CO2. También el geotermómetro SiO2-calcedonia ha proporcionado resultados satisfactorios

Hydrogeochemical characterisation of the groundwater in the crystalline basement of Forsmark, the selected area for the geological nuclear repositories in Sweden

Numerous groundwater analyses from the crystalline bedrock in the Forsmark area have been performed between 2002 and 2019, together with thorough geological, geophysical, and hydrogeological studies, within the site investigations carried out by the Swedish Nuclear Fuel and Waste Management Company. The groundwater samples have been taken from boreholes down to ≈ 1000 m and the analysis include major- and trace-elements, stable and radiogenic isotopes, gases and microbes. The chemical and isotopic composition of these groundwaters evidences the presence of non-marine brackish to saline groundwaters with very long residence times (many hundreds of thousands of years) and a series of complex mixing events resulting from the recharge of different waters over time: glacial meltwaters, probably from different glaciations of which the latest culminated some 20,000 years ago, and marine waters from the Baltic starting some 7000 years ago. Later, meteoric water and present Baltic Sea water have recharged in different parts of the upper 100 m. These mixing events have also triggered chemical and microbial reactions that have conditioned some of the important groundwater parameters and, together with the structural complexity of the area, they have promoted a heterogeneous distribution of groundwater compositions in the bedrock. Due to these evident differences in chemistry, residence time and origin of the groundwater, several groundwater types were defined in order to facilitate the visualisation and communication. The differentiation (linked to the paleohydrological history of the area) was based on Cl concentration, Cl/Mg ratio (marine component), and δ18O value (glacial component). The work presented in this paper increases the understanding of the groundwater evolution in fractured and compartmentalised aquifers where mixing processes are the most important mechanisms. The model proposed to characterise the present groundwater system of the Forsmark area will also help to predict the future hydrogeochemical behaviour of the groundwater system after the construction of the repositories for the nuclear wastes

Mineral equilibria and thermodynamic uncertainties in the geothermometrical characterisation of carbonate geothermal systems of low temperature. The case of the Alhama-Jaraba system (Spain)

Geothermometrical characterisation of low-temperature, carbonate-evaporitic geothermal systems is usually hampered by the lack of appropriate mineral equilibria to successfully use most of the classical geothermometers and/or by the thermodynamic uncertainties affecting some of the most probable mineral equilibria in low temperature conditions. This situation is further hindered if the thermal waters are additionally affected by secondary processes (e.g., CO2 loss) during their ascent to surface. All these problems cluster together in the low-temperature Alhama-Jaraba thermal system, hosted in carbonate rocks, with spring temperatures about 30 °C and waters of Ca-Mg−HCO3/SO4 type. This system, one of the largest naturally flowing (600 L/s) low temperature thermal systems in Europe, is used in this paper as a suitable frame to assess the problems in the application of chemical geothermometrical techniques (classical geothermometers and geothermometrical modelling) and to provide a methodology that could be used in this type of geothermal system or in potential CO2 storage sites in similar aquifers. The results obtained have shown that the effects of the secondary processes can be avoided by selecting the samples unaffected by such processes and, therefore, representative of the conditions at depth, or by applying existing methodologies to reconstruct the original composition, as is usually done for medium to high temperature systems. The effective mineral equilibria at depth depend on the temperature, the residence time and the specific lithological/mineralogical characteristics of the system studied. In the present case, the mineral equilibria on which classical cation geothermometers are based have not been attained. The low proportion of evaporitic minerals in the hosting aquifer prevents the system from reaching anhydrite equilibrium, otherwise common in carbonate-evaporitic systems and necessary for the specific SO4-F geothermometer or the specially reliable quartz (or chalcedony) – anhydrite equilibrium in the geothermometrical modelling of these geothermal systems. Under these circumstances, the temperature estimation must rely on quartz (or chalcedony), clay minerals and, especially, calcite and dolomite. However, clay minerals and dolomite present important thermodynamic uncertainties related to possible variations in composition or crystallinity degree for clays and order/disorder degree for dolomite. To deal with these problems, a sensitivity analysis to the thermodynamic data for clay minerals has been carried out, comparing the results obtained when considering different solubility data. The uncertainties associated with dolomite have been addressed by reviewing the solubility data available for dolomites with different order degrees and performing specific calculations for the order degree of the dolomite in the aquifer. This approach can be used to find the most adequate dolomite thermodynamic data for the system under consideration, including medium-high temperature geothermal systems. Finally, the temperature estimation of the Alhama-Jaraba waters in the deep reservoir has been obtained from simultaneous equilibria of quartz, calcite, partially disordered dolomite and some aluminosilicate phases. The obtained value of 51 ± 14 °C is within the uncertainty range normally affecting this type of estimations and is coherent with independent estimations from geophysical data

Geothermometrical modelling applied to the CO2-rich thermal waters of the Lanjarón hydrothermal system (Granada, Spain)

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Travertines associated with the Alhama-Jaraba thermal waters (NE, Spain): Genesis and geochemistry

Freshwater carbonates are interesting archives in palaeoenvironmental reconstructions. However, more studies of those systems are needed to fully understand past environments. In this work the actively-forming travertines of the Alhama-Jaraba thermal system were studied for the first time in order to evaluate the relationship between the geochemical and mineralogical composition and the environmental conditions during their formation. With that aim, a combination of petrographical, mineralogical, geochemical and stable isotope analyses were carried out. These carbonates provide a natural laboratory for the study of the effect of different variables (natural and anthropogenic) on carbonate precipitation. The results showed that there is a close relationship between the mineralogy of the solid precipitates and the formation temperature, and only the samples formed from overheated waters (40–60 °C) show significant concentrations of aragonite. Aragonite-bearing samples show higher concentrations in Sr, Ba and U while calcitic solids are enriched in Mg. These differences could be attributed to mineralogy, temperature or different precipitation rates. The geochemical evaluation of the chemistry of both the solids and their parental waters suggests that differences in the rate of CO2-degassing and, in some cases, evaporation are the primary environmental controls on isotopic compositions. In addition, the results show that, if strong evaporation and CO2-degassing are involved, calcite precipitation occurs under conditions of isotopic disequilibrium with its parental water. The results of our study are useful to interpret old depositional environments and palaeotemperatures

Primeros resultados de monitorización de la Cueva de El Recuenco (Ejulve, Teruel).

El Recuenco Cave (Ejulve, Teruel, NE Spain) is developed with a complex topography in Upper Cretaceous dolomitic limestones. A monitoring survey is in progress in the cave (covering now the 1st semester of 2013) in order to know the isotopic path from rainfall to dripwater and in-situ farmed calcite. Rainfall results show that d18O values reveal a clear seasonality (-12,73‰V-SMOW in winter, -8,94‰V-SMOW in spring). Dripwater values are mainly influenced by homogenization processes in the epikarst and also reveals seasonality in d18O values (-10,9‰ V-SMOW in winter, -9,8‰ V-SMOW in spring). Finally, isotopic values of in-situ farmed calcite confirm this seasonality pattern (in d18O values, -8,17‰ PDB in winter, -8,11‰ PDB in spring; also in d13C values, -9,84‰ PDB in winter, -9,68‰ PDB in spring). Nonetheless, new monitoring tasks are needed to confirm this preliminary pattern. These conclusions will help to better interpret of speleothem records in this cave for paleoclimatic reconstructions. La cueva de El Recuenco (Ejulve, Teruel) presenta una topografía compleja desarrollada en calizas dolomíticas del Cretácico superior. En ella se ha efectuado un seguimiento instrumental del desarrollo espeleotémico actual (primer semestre de 2013) de cara a evaluar el recorrido isotópico desde el agua de lluvia, pasando por el goteo interno hasta el carbonato precipitado. Los resultados del agua de lluvia evidencian que el d18O muestra una clara estacionalidad (-12,73‰ V-SMOW en invierno, -8,94‰ V-SMOW en primavera). Los valores isotópicos del goteo se encuentran controlados por la homogeneización producida en el epikarst,mostrando de nuevo cierta estacionalidad en d18O (-10,9‰V-SMOW en invierno, -9,8‰V-SMOW en primavera). Finalmente, la señal isotópica del carbonato resultante confirma este patrón de estacionalidad (en d18O, -8,17‰ PDB en invierno, -8,11‰ PDB en primavera; en d13C, -9,84‰ PDB en invierno, -9,68‰ PDB en primavera). No obstante, es necesario ampliar el seguimiento temporal para confirmar este patrón estacional preliminar. Estas consideraciones ayudarán a una mejor interpretación de los registros espeleotémicos en esta cueva de cara a realizar reconstrucciones paleoclimáticas

Seasonal and decadal stable isotope evolution recorded by recent tufa deposited on artificial substrates in the Monasterio de Piedra Natural Park (NE Spain).

Identification of six-month intervals in carbonate deposits formed on tablets installed in several fluvial subenvironments of the Monasterio de Piedra Natural Park, from 1999 to 2009, allowed six-monthly stable-isotope analysis of such records. Slight differences in d13C and d18O exist between stromatolites (fast-flowing water areas) and moss-bearing deposits (cascades). Sediment d13C values did not show clear regular variations through time. A chiefly cyclic pattern of sediment d18O values reflected the seasonal variations in temperature. The calculated water temperature values were consistent with measured air and water temperature values. The increasing tendency of air temperature is closely reflected by the estimated temperature tendencies. The isotopic results stress the validity of the seasonal variation pattern detected through thickness measures, and underscore the environmental significance of tufas, which accounts for the use of this type of analysis in climate interpretation from ancient tufa records. La identificación de intervalos semestrales en depósitos carbonatados formados sobre losetas instaladas en diversos subambientes fluviales del Parque Natural del Monasterio de Piedra, desde 1999 hasta 2009, ha permitido el análisis semestral de isótopos estables de dichos registros. Existen pequeñas diferencias en d13C y d18O entre las facies estromatolíticas (áreas de flujo rápido) y las ricas en musgos (cascadas). Los valores de d13C no muestran variaciones temporales regulares. Los valores de d18O presentan una pauta cíclica que refleja las variaciones estacionales de temperatura. Los valores calculados de la temperatura estacional del agua son acordes con las temperaturas medidas de aire y agua. La tendencia creciente de la temperatura del aire se refleja en las tendencias de temperatura estimadas. Estos resultados refuerzan la validez del patrón estacional detectado mediante la medida de espesores de los depósitos, y confirman la utilidad de este tipo de análisis en la interpretación climática de tobas antiguas

Controls on Mg/Ca Ratios in Recent Stromatolites: Insights from Fluvial Systems in the Iberian Range (Spain)

The utility of the Mg/Ca elemental ratio of calcite ((Mg/Ca)calcite) as a temperature indicator in continental carbonate deposits is a matter of debate due to the different results obtained by diverse authors. In this study, we aimed to test the reliability of the (Mg/Ca)calcite in fluvial carbonates. We selected the recent tufa stromatolite records of four rivers on the Iberian Peninsula for the trace element analysis based on six-monthly sampling. Previous sedimentary and hydrological studies on these fluvial basins provided the information for this work. The water temperature estimates for the stromatolite (Mg/Ca)calcite substantially differed from the measured water temperatures in most of the studied cases. We thus assessed other factors that participate in the control of the Mg partitioning between water and calcite. The correction of the detrital Mg content yielded water temperatures that matched the measured ones in one of the rivers. The (Mg/Ca)water, water discharge and calcite precipitation rates may also occasionally influence the (Mg/Ca)calcite. The six-month behaviour of some of these parameters could interfere with the relationship between the (Mg/Ca)calcite and water temperature. According to these results, and their comparison with other non-marine carbonates, the wide variety of parameters that are involved in the (Mg/Ca)calcite limit it as a geochemical thermometer in continental sedimentary environments