Search

The trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Ba and U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation. This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients (where the partitioning (D) of element X (D-X) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in the parent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However, dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, we investigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave pool in Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of 'modern' subaqueous calcite from a drill core taken through CD3 to derive D-Mg, D-Sr, D-Ba and D-U then compare these with published cave, cave-analogue and seawater-analogue studies. The D-Mg for CD3 is anomalously high (0.042 +/- 0.002) compared to previous estimates at similar temperatures (similar to 8 degrees C). The D-Sr (0.100 +/- 0.007) is similar to previously reported values, but data from this study as well as those from Tremaine and Froelich (2013) and Day and Henderson (2013) suggest that [ Na/Sr] might play an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. DBa in CD3 (0.086 +/- 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst DU (0.013 +/- 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates (

In this study, we performed leaching experiments for timescales of hours-to-months in deionized water on fresh volcanic ash from Mt. Etna (Italy) and Popocatépetl (Mexico) volcanos to monitor Fe release as a function of ash mineral chemistry and size, with the aim of clarifying Fe release mechanisms and eventually evaluating the impact of volcanic ash on marine and lacustrine environments. To define sample mineralogy and Fe speciation, inclusive characterization was obtained by means of XRF, SEM, XRPD, EELS and Mössbauer spectroscopies. For Etna and Popocatépetl samples, glass proportions were quantified at 73 and 40%, Fe2O3 total contents at 11.6-13.2 and 5.8 wt%, and Fe3+/FeTot ratios at 0.33 and 0.23, respectively. Leaching experiments showed that significant amounts of iron, ~ 30 to 150 and ~ 750 nmol g-1 l-1 for pristine Etna and Popocatépetl samples, respectively, are released within the first 30 min as a function of decreasing particle size (from 1 to 0.125 mm). The Popocatépetl sample showed a very sustained Fe release (up to 10 times Etna samples) all along the first week, with lowest values never below 400 nmol g-1 l-1 and a maximum of 1672 nmol g-1 l-1 recorded after 5 days. This sample, being composed of very small particles (average particle size 0.125 mm) with large surface area, likely accumulated large quantities of Fe-bearing sublimates that quickly dissolved during leaching tests, determining high Fe release and local pH decrease (that contributed to release more Fe from the glass) at short timescale (hours-to-days). The fractional Fe solubility (FeS) was 0.004-0.011 and 0.23% for Etna and Popocatépetl samples, respectively, but no correlation was found between Fe released in solution and either ash Fe content, glass/mineral ratio or mineral assemblage. Results obtained suggest that volcanic ash chemistry, mineralogy and particle size assume a relevant role on Fe release mostly in the medium-to-long timescale, while Fe release in the short timescale is dominated by dissolution of surface sublimates (formed by physicochemical processes occurring within the eruption plume and volcanic cloud) and the effects of such a dissolution on the local pH conditions. For all samples, a moderate to sustained Fe release occurred for leaching times comparable with their residence time within the euphotic zone of marine and lacustrine environments (variable from few minutes to few hours), revealing their possible contribution to increase Fe bioavailability.

Lake Albano was stratified at the time of our survey, in December 1997, in agreement with previous observations [Martini et al., Geochem. J. 28 (1994) 173–184; Cioni et al., Report for the Civil Protection Department (1995); Pedreschi, Accad. Lucch. Sci. Lett. Arti (1995) 39]. In the absence of phenomena induced by seismic activity, either local or regional, lake stratification may be perturbed by cooling of shallow waters below ∼8.5°C. Circulation is expected to homogenize lake waters and eventually to trigger gas exsolution when total gas pressure exceeds hydrostatic pressure. In December 1997, total gas pressure in lake water was very close to atmospheric pressure (0.9–1.3 bar) at all depths, possibly due to the occurrence of a recent episode of circulation and presumed gas exsolution. The state of saturation of Lake Albano waters and the similarity of the relative concentrations of Na, K, Mg, and Ca in lake waters, local groundwaters, and local volcanic rocks indicate that Na, K, Mg, and Ca concentrations in Lake Albano waters are mainly governed by incongruent dissolution of local volcanic rocks, coupled with minor calcite precipitation at shallow depths.

A geochemical survey carried out in November 1993 revealed that Lake Quilotoa was composed by a thin (,14 m) oxic epilimnion overlying a ,200 m-thick anoxic hypolimnion. Dissolved CO2 concentrations reached 1000 mg/kg in the lower stratum. Loss of CO2 from epilimnetic waters, followed by calcite precipitation and a consequent lowering in density, was the apparent cause of the stratification. The Cl, SO4 and HCO3 contents of Lake Quilotoa are intermediate between those of acid‐SO4‐Cl Crater lakes and those of neutral-HCO3 Crater lakes, indicating that Lake Quilotoa has a ‘memory’ of the inflow and absorption of HC1- and S-bearing volcanic (magmatic) gases. The Mg/Ca ratios of the lake waters are governed by dissolution of local volcanic rocks or magmas, but K/Na ratios were likely modified by precipitation of alunite, a typical mineral in acid‐SO 4‐Cl Crater lakes. The constant concentrations of several conservative chemical species from lake surface to lake bottom suggest that physical, chemical and biological processes did not have enough time, after the last overturn, to cause significant changes in the contents of these chemical species. This lapse of time might be relatively large, but it cannot be established on the basis of available data. Besides, the lake may not be close to steady state. Mixing of Lake Quilotoa waters could presently be triggered by either cooling epilimnetic waters by ,48C or providing heat to hypolimnetic waters or by seismic activity. Although Quilotoa lake contains a huge amount of dissolved CO2O,3 £ 10 11 gU; at present the risk of a dangerous limnic eruption seems to be nil even though some gas exsolution might occur if deep lake waters were brought to the surface. Carbon dioxide could build up to higher levels in deep waters than at present without any volcanic re-awakening, due to either a large inflow of relatively cool CO 2-rich gases, or possibly a long interval between overturns. Periodical geochemical surveys of Lake Quilotoa are, therefore, recommended. q 2000 Elsevier Science B.V. All rights reserved.

The accumulation chamber methodology allows one to obtain reliable values of the soil CO2 flux, ϕsoil CO2, in the range 0.2 to over 10 000 g m−2 d−1, as proven by both laboratory tests and field surveys in geothermal and volcanic areas. A strong negative correlation is observed between Δϕsoil CO2/Δt and ΔPatm/Δt. Maps of classes of log ϕsoil CO2 for the northern sector of Vulcano Island, Solfatara of Pozzuoli, Nea Kameni Islet and Yanbajain geothermal field evidence that active faults and fractures act as uprising channels of deep, CO2-rich geothermal or magmatic gases. The total diffuse CO2 output was evaluated for each surveyed area.

Two well-equilibrated NaCl geothermal liquids are recognizable in the San Marcos area. Both have the same Cl concentration (540 mg/kg) and the same isotopic composition (δD of −66.5‰ and δ18O of −9‰) under reservoir conditions, but they come from two distinct aquifers with different temperatures, i.e. 240°C below La Cimarrona and 185°C below La Castalia. The numerous thermal NaCl to NaClHCO3 springs located in the San Marcos area originate through dilution and boiling of these two geothermal liquids and different degrees of re-equilibration at lower temperatures. Silica and K contents are useful in discriminating between dilution, boiling and re-equilibration phenomena. Thermal NaHCO3 waters, generated through conductive heat transfer or input of geothermal vapor or gases from below, delineate the extent of the geothermal reservoir(s) at depth.

This paper describes a method for computing: (1) the chemical composition of the first drop of condensate which forms at dew-point temperature through expansion of superheated steam, and (2) the saturation index of the drop with respect to relevant solid phases, such as halite, amorphous silica, boric acid, borax and sal ammoniac. Boiling-point elevation is taken into account in these calculations. Preliminary application to some wells in the Larderello geothermal field indicate that: (1) the high concentration of HCl in the steam causes both the low pH and very high TDS of the first drop; (2) the lower the dew-point temperature, the higher the TDS of the first drop; (3) for a given chemical composition, the lower the steam pressure, the higher the risk of corrosion and scaling in the steam path.

The Monte Corchia Cave is one of the most promising sites for studying the paleoclimate of the Mediterranean basin, but its hydrology and hydrogeochemistry are still poorly known. In this paper, we report some meteoclimatic and hydrochemical data for different parts of the cave. Conductivity and water level data from La Gronda channel show that this system reacts rapidly to external meteoric events, indicating the presence of a conductive epikarst. Data on two different drips indicate that the physicochemical parameters, suchas conductivity, pH, δ 13 C DIC and drip rate depend on the local structural setting and water pathlength. The data presented show that Galleria delle Stalattiti (the focus of the paleoclimate research) has the most stable conditions in terms of temperature, and the dripwaters show constant pH, electrical conductivity, alkalinity, calcium and magnesium content and δ 18 O. Drip rate is not affected by rain events and displays long-term trends that require a longer period of monitoring for elucidating their nature. The preliminary data presented here corroborate the hypotheses suggesting Galleria delle Stalattiti as a good example of a “deep” hypogean system of Fairchild et al. (2007).

International audience; The South Poroto-Rungwe geothermal field, in the northern part of the Malawi rift, Tanzania divides in two main areas. The relatively high altitude northern area around the main Ngozi, Rungwe, Tukuyu and Kyejo volcanoes, is characterised by cold and gas-rich springs. In contrast, hot springs occur in the southern and low-altitude area between the Kyela and Livingstone faults. The isotopic signature of the almost stagnant, cold springs of the Northern district is clearly influenced by H2O-CO2(g) exchange as evidenced from negative oxygen-shifts in the order of few deltas permil. In contrast, the isotopic signature of waters discharged from the hot springs of the Southern district is markedly less affected by the H2O-CO2(g) interaction. This evidence is interpreted as an effect of the large, permanent outflow of these springs, which supports the hypothesis of a regional-scale recharge of the major thermal springs. Measurements of carbon isotope variations of the dissolved inorganic carbon of waters and CO2(g) from the Northern and Southern springs support a model of CO2(g)-driven reactivity all over the investigated area. Our combined chemical and isotopic results show that the composition of hot springs is consistent with a mixing between (i) cold surface fresh (SFW) and (ii) Deep Hot Mineralised (DHMW) Water, indicating that the deep-originated fluids also supply most of the aqueous species dissolved in the surface waters used as local potable water. Based on geothermometric approaches, the temperature of the deep hydrothermal system has been estimated to be higher than 110 degrees C up to 185 degrees C, in agreement with the geological and thermal setting of the Malawi rift basin. Geochemical data point to (i) a major upflow zone of geothermal fluids mixed with shallow meteoric waters in the Southern part of the province, and (ii) gas absorption phenomena in the small, perched aquifers of the Northern volcanic highlands. (C) 2010 Elsevier B.V. All rights reserved.

Many boiling springs and fumaroles are present along the shores of Lake Bogoria, which is a closed-basin alkaline saline lake typical of African Rifts. Two different geothermal waters, both of Na-HCO3 type have been recognized. The first, discharged by the boiling springs located along the western shores of the lake, comes from a shallow steam-heated thermal aquifer. Its temperature is close to 100°C, as indicated by chalcedony solubility, while the chloride content of these waters is slightly higher than 200 mg L−1. The second, recognizable in the southernmost boiling springs, is representative of a deeper and hotter geothermal reservoir. The occurrence of mixing and boiling processes complicates the interpretation of geochemical data. Nevertheless, a chloride content of about 660 mg L−1, an equilibrium temperature close to 170°C and a high carbon dioxide partial pressure, at least 10–20 bar, have been estimated for the deep geothermal reservoir.

The theoretical compositions of an aqueous solution in equilibrium with a mineral assemblage made up of low-albite, K-feldspar, either a Ca-Al-silicate or calcite, clinochlore, muscovite, quartz, anhydrite, and fluorite, under varying T-PCO2-mcl conditions of geothermal interest, indicate that 1. 1) the total SO4 content as well as the Na K , K 2 Mg , and SO 4 F 2 ratios are potential geothermometers; 2. 2) the total HCO3 content as well as the K 2 Ca , Ca Mg , HCO 3 F , and (HCO 3 ) 2 SO 4 ratios are potential PCO2 indicators; 3. 3) the Na, K, Ca, and Mg total contents as well as the Na 2 Mg and Na 2 Ca ratios are mainly controlled by the total ionic salinity and are therefore hardly suitable as geoindicators. A preliminary test of the equations involving total HCO3 content as well as K 2 Ca and HCO 3 F ratios as PCO2 indicators have provided satisfactory results.

The densely populated metropolitan area of Quito is located on the slopes of the active Guagua Pichincha volcano at only 10 km from the crater. Recently, the Italian Ministry of Foreign Affairs sponsored a project for the mitigation of volcanic hazard in this area. The geochemical study carried out as part of this project was aimed at constructing a geochemical model of the zone for use in volcanic surveillance. According to this geochemical model, a hydrothermal aquifer (T = 200–240°C), fed both by meteoric waters and by fluids released by a magma body, lies at shallow levels beneath Guagua Pichincha crater. The crater fumaroles are essentially fed by steam boiled off from the hydrothermal aquifer. The high flow rate fumaroles located in the dome area show significant SO2 contents, which suggest a relatively high contribution of magmatic fluids in the zone of the aquifer feeding them. The absence of SO2 in the fumarolic discharges near the southern crater wall indicates instead that the magmatic fluids dissolve entirely into the aquifer here. The hot springs located at the western end of the crater represent the lateral discharge of the hydrothermal aquifer. On the basis of this model, it is likely that an increment in the flux of both the magmatic fluids and the heat from a magma body produces an increase, albeit small, of the pressure-temperature conditions of the hydrothermal system and consequent changes in flow rate and fluid chemistry of the fumarolic vents. In particular, total sulphur and possibly hydrochloric acid may increase in all the vents and sulphur dioxide may appear in other fumarolic discharges. The varying thermodynamic conditions in the hydrothermal aquifer can be evaluated on the basis of the equilibria among carbon species and hydrogen. Only minor delayed changes are expected in the physical-chemical characteristics of the springs located at the western end of the crater.

The main geothermal reservoir of Acqui Terme-Visone hosts Na–Cl waters, which are in chemical equilibrium at 120–130°C with typical hydrothermal minerals including quartz, albite, K-feldspar, illite, chlorite (or smectite), anhydrite, calcite and an unspecified Ca-Al-silicate. In the Acqui Terme-Visone area, these geothermal waters ascend along zones of high vertical permeability and discharge at the surface almost undiluted or mixed with cold, shallow waters. To the SW of Acqui Terme, other ascending geothermal waters, either undiluted or mixed with low-salinity waters, enter relatively shallow secondary reservoirs, where they reequilibrate at 65–70°C. Both chemical and isotopic data indicate that bacterial SO4 reduction affects all these waters, especially those discharged by the secondary reservoirs. Therefore, geothermal waters must get in contact with oil, acquiring the relatively oxidized organic substances needed by SO4-reducing bacteria. This oil–water interaction process deserves further investigations, for potential economic implications.

Both geochemical and geophysical evidence indicates that the activity of the fumarolic system of the Fossa crater can be divided into two periods. From 1978 to 1983, such activity has been mainly controlled by two competing processes which affect the permeability of the fumarolic system at deep levels: (1) rock fracturing induced by faulting and (2) mineral deposition-alteration. The latter, which causes a slow decrease of the permeability at deep levels through clogging of fractures and voids, prevailed at the end of this period, determining a fluid pressure increment in the deep parts of the system. From 1984 to 1989, fluid pressure at deep levels remained persistently high, triggering local microseismic swarms. Furthermore, PT conditions much higher than before were attained in the zones where hydrothermal fluids seep into the conduits from lateral aquifers, owing to an uprising of the isotherms, a deepening of these seeping zones, or both.

Two geochemical surveys carried out in March 1991 and September 1992 revealed the existence of a hydrothermal system in the southern portion of Montserrat Island, below Soufriere Hills Volcano. This conclusion is supported by the presence of: (a) the thermal springs of Plymouth which are fed by deep Na–Cl waters (Cl concentration ∼25 000 mg/kg, temperature ca. 250 °C) mixed with shallow steam-heated waters; (b) the four fumarolic fields of Galway's Soufriere, Gages Upper Soufriere, Gages Lower Soufriere, and Tar River Soufriere, where acid to neutral, steam-heated waters are present together with several fumarolic vents, discharging vapors formed through boiling of hydrothermal aqueous solutions. Involvement of magmatic fluids in the recharge of the hydrothermal aquifers is suggested by: (a) the high 3He/4He ratios of fumarolic fluids, i.e., 8.2 RA at Galway's Soufriere and 5.9 RA at Gages Lower Soufriere; (b) the δD and δ18O values of Na–Cl thermal springs and steam condensates, indicating the involvement of arc-type magmatic water in the formation of deep geothermal liquids; and (c) the CH4/CO2 ratios of fumarolic fluids, which are lower than expected for equilibrium with the FeO–FeO1.5 hydrothermal rock buffer, but being shifted towards the SO2–H2S magmatic gas buffer.