Geochemical and Physical Controls on Vadose Zone Hydrology of Holocene Carbonate Sands,Grand Bahama Island

This paper explores the relationship between vadose zone hydrology and geochemical changes in mixed mineralogy carbonate sands from a Bahamian coastal dune of Holocene age. Cores were taken from two sites: at site A, a shallow humic Entisol is developed beneath open scrub vegetation, while at site B a deeper, more organic-rich Inceptisol has formed beneath a mature hardwood coppice. X-ray diffraction analysis reveals significant contrasts in mineralogy both within and between the two sites, with partial stabilization of high-Mg calcite and aragonite, to low-Mg calcite. Stabilization is greater at site B, and is accompanied by a significant increase in total porosity. Diagenetic changes in pore-size distribution have implications for residence times of percolating water, as determined using measurements of moisture retention characteristics using pressure plate apparatus, and hydrological models of unsaturated zone moisture flux. The diagenetically more mature sands from site B have a 50–100 per cent higher moisture retention, although unsaturated hydraulic conductivity is also higher, particularly at greater suctions. The increase in water retention is likely to enhance further rates of mineral-controlled reactions, while development of an organic-rich soil also enhances the geochemical drive for dissolution. Carbonate diagenesis thus appears to be strongly linked to vadose zone hydrology, and the interactions identified here have important consequences for the nature and long-term rates of mineral stabilization. © 1997 by John Wiley & Sons, Ltd.     

Mechanism for calcite dissolution and its contribution to development of reservoir porosity and permeability in the Kela 2 gas field,Tarim Basin,China

This study is undertaken to understand how calcite precipitation and dissolution contributes to depth-related changes in porosity and permeability of gas-bearing sandstone reservoirs in the Kela 2 gas field of the Tarim Basin, Northwestern China. Sandstone samples and pore water samples are col-lected from well KL201 in the Tarim Basin. Vertical profiles of porosity, permeability, pore water chem-istry, and the relative volume abundance of calcite/dolomite are constructed from 3600 to 4000 m below the ground surface within major oil and gas reservoir rocks. Porosity and permeability values are in-versely correlated with the calcite abundance, indicating that calcite dissolution and precipitation may be controlling porosity and permeability of the reservoir rocks. Pore water chemistry exhibits a sys-tematic variation from the Na2SO4 type at the shallow depth (3600-3630 m), to the NaHCO3 type at the intermediate depth (3630―3695 m),and to the CaCl2 type at the greater depth (3728―3938 m). The geochemical factors that control the calcite solubility include pH, temperature, pressure, Ca2 concen-tration, the total inorganic carbon concentration (ΣCO2), and the type of pore water. Thermodynamic phase equilibrium and mass conservation laws are applied to calculate the calcite saturation state as a function of a few key parameters. The model calculation illustrates that the calcite solubility is strongly dependent on the chemical composition of pore water, mainly the concentration difference between the total dissolved inorganic carbon and dissolved calcium concentration (i.e., [ΣCO2] -[Ca2 ]). In the Na2SO4 water at the shallow depth, this index is close to 0, pore water is near the calcite solubility. Calcite does not dissolve or precipitate in significant quantities. In the NaHCO3 water at the intermedi-ate depth, this index is greater than 0, and pore water is supersaturated with respect to calcite. Massive calcite precipitation was observed at this depth interval and this intensive cementation is responsible for decreased porosity and permeability. In the CaCl2 water at the greater depth, pore water is un-der-saturated with respect to calcite, resulting in dissolution of calcite cements, as consistent with microscopic dissolution features of the samples from this depth interval. Calcite dissolution results in formation of high secondary porosity and permeability, and is responsible for the superior quality of the reservoir rocks at this depth interval. These results illustrate the importance of pore water chemis-try in controlling carbonate precipitation/dissolution, which in turn controls porosity and permeability of oil and gas reservoir rocks in major sedimentary basins.     

Paleoclimate implication of oxygen and carbon isotopic composition from diagenesis coral

Oxygen and carbon isotopic compositions of diagenesis? coral samples from NY-1 core at the depth of 22 to 50 m, together with mineral compositions of transitive coral samples of 17 to 18 m at the depth, were meas ured. The data exhibited that when aragonite changed to calcite, its oxygen and carbon isotopic compositions dropped roughly on a linear trend. The linear trend implies that the oscillation pattern of the origin oxygen and carbon isotopic ratios of the aragonite could probably be retained after they diageneticly changed into calcite. Oxygen isotopic stratigraphy for the NY-1 core at the depth between 22 and 50 m was determined according to the δ⁸O ratios of the calcite coral. The oxygen stratigraphy provided an age of about 289 ka for 45 m of the NY-1 core, which agreed with that by paleomagnetic stratigraphy.     

Uranium loss and aragonite–calcite age discordance in a calcitized aragonite stalagmite

We analyzed uranium-series concentrations and isotopic ratios in a mixed aragonite and calcite stalagmite from Juxtlahuaca Cave, from the Sierra Madre del Sur of Mexico. The U-series data for the aragonite layers return highly precise and stratigraphically correct ages over the past ca. 4300 years. In contrast, age determinations from calcite layers are too old by several hundred years relative to the precise aragonite ages, have analytical uncertainties an order of magnitude larger than aragonite ages, and yield ages that do not overlap the aragonite ages within analytical uncertainties. Based on geochemical and petrographic observations, we interpret the calcite layers to have formed from recrystallization of aragonite soon after primary aragonite deposition. Calcite occurs as discontinuous lenses on and off the growth axis, and laminae can be traced between aragonite and calcite layers, demonstrating that visible growth banding is not effaced in the recrystallization process. Paired aragonite-calcite U-series data from coeval stratigraphic layers demonstrate that uranium concentrations decrease by two orders of magnitude during calcitization, and result in decreased (²³⁴U/²³⁸U). Uranium loss during diagenesis mimics a need for an age correction using an initial ²³⁰Th/²³²Th ratio one to two orders of magnitude larger than the bulk Earth ratio of 4.4 ± 2.2 × 10⁻⁶. A need for apparent high initial ²³⁰Th/²³²Th ratios results from ingrowth of ²³⁰Th during ²³⁴U decay.     

Shock-induced devolatilization of calcite

The amounts of CO₂ and CO evolved upon shock compression and decompression of calcite to 18 GPa (180 kbar) have been determined using a new gas phase shock recovery technique and gas source mass spectrometry. The data demonstrate that from ～0.03 to 0.3 mole percent of calcite is devolatilized at shock pressures significantly lower than those predicted (30 GPa) for the onset of volatilization by continuum thermodynamic theory and are in qualitative agreement with release adiabat data for calcite and aragonite. Carbon and oxygen isotope ratios in the shock-released CO₂ are the same as those in the unshocked (hydrothermal) calcite, demonstrating that the CO₂ comes from the calcite rather than other potential sources.     

Carbon isotopic compositions of Recent planktonic foraminifera of the Indian Ocean

Carbon and oxygen isotopic determinations have been made of 29 species of Recent Indian Ocean planktonic foraminifera. Fourteen core-top samples were used and as many as 18 species were chosen from a single core-top sample. The δ¹³C of the foraminifera was compared with that of total dissolved CO₂ (ΣCO₂) and of calcite precipitated in isotopic equilibrium with ΣCO₂. The foraminiferal calcite is always at least 1.2‰ less than the value estimated for equilibrium calcite. This carbon isotopic disequilibrium suggests the partial utilization of¹³C-depleted metabolic CO₂. The calcite tests of several species, however, have δ¹³C values which are similar to the δ¹³C of ΣCO₂ in seawater. This relationship suggests that important paleohydrographic information may be obtained from carbon isotope records based on analyses of several foraminiferal species from single deep-sea sediment samples.     

Lead isotopes and age of Hawaiian lherzolite nodules

The reaction between enstatite (En_95.3Fs_4.7) and CaCO₃ has been studied at pressures between 23 and 77 kbars and at temperatures between 800° and 1400°C. At 1000°C enstatite and CaCO₃ react to form dolomite and diopside solid solutions at pressures below approximately 45 kbars and magnesite and diopside solid solutions at higher pressures. The curve for the reaction dolomite_ss + enstatite_ss ? magnesite_ss + diopside_ss lies between 40 to 45 kbars at 1000°C and between 45 and 50 kbars at 1200°C. It is very close to the graphite-diamond transition curve. These experimental results indicate that calcite (or aragonite) is unstable in the presence of enstatite, and that dolomite and magnesite are the stable carbonates at high pressures. The forsterite + aragonite assemblage is, however, stable to at least 80 kbars at 800°C. It is suggested that in the upper mantle where enstatite is present, dolomite is stable to depths of about 150 km and magnesite is stable at greater depths in the continental regions, assuming that the partial pressure of CO₂ is equal or close to the total pressure. It is also suggested that carbonate inclusions in pyroxene can be used as an indicator of the depth of their equilibration; dolomite inclusions in enstatite would be formed at depths shallower than 150 km and magnesite inclusions in diopside at greater depths. Eclogite and peridotite inclusions in kimberlite may be classified on this basis.     

The role of gypsum and/or dolomite dissolution in tufa precipitation: lessons from the hydrochemistry of a carbonate–sulphate karst system

The precipitation of freshwater carbonates (tufa) along karstic rivers is enhanced by degassing of carbon dioxide (CO₂) downstream of karstic springs. However, in most karstic springs CO₂ degassing is not enough to force the precipitation of tufa sediments. Little is known about the role of dissolution of gypsum or dolomite in the hydrochemistry of these systems and how this affects the formation of tufa deposits. Here we present a monitoring study conducted over a year in Trabaque River (Spain). The river has typical karst hydrological dynamics with water sinking upstream and re‐emerging downstream of the canyon. Mixing of calcium–magnesium bicarbonate and calcium sulphate waters downstream of the sink enhances the dissolution of carbonates and potentially plays a positive role in the formation of tufa sediments. However, due to the common‐ion effect, dissolution of dolomite and/or gypsum causes precipitation of underground calcite cements as part of the incongruent dissolution of dolomite/dedolomitization process, which limits the precipitation of tufa sediments. Current precipitation of tufa is scant compared to previous Holocene tufa deposits, which likely precipitated from solutions with higher saturation indexes of calcite (SIcc values) than nowadays. Limited incongruent dissolution of dolomite/dedolomitization favours higher SIcc values. This circumstance occurs when waters with relatively high supersaturation of dolomite and low SO₄^2? composition sink in the upper sector of the canyon. In such a scenario, the process of mixing waters enhances the exclusive dissolution of limestones, preventing the precipitation of calcite within the aquifer and favouring the increase of SIcc values downstream of the springs. Such conditions were recorded during periods of high water level of the aquifers and during floods. This research shows that the common‐ion effect caused by the dissolution of gypsum and/or dolomite rocks can limit [or favour] the precipitation of tufa sediments depending on the occurrence [or not] of incongruent dissolution of dolomite/dedolomitization. Copyright © 2016 John Wiley & Sons, Ltd.     

Assessment and modelling of groundwater quality data and evaluation of their corrosiveness and scaling potential using environmetric methods in Bangalore South Taluk, Karnataka state, India

Study Area is located in the southwestern part of Bangalore South Taluk, Bangalore district, Karnataka state between 12°48??24.52?? to 12°53??59.85?? North latitude and 77°24??59.95?? to 77°30??6.72?? East Longitude. The major hydro-chemical facies that predominates in the study area is Ca²⁺-Mg²⁺-HCO ₃ ^? type during both pre- and post-monsoon seasons of the year 2007, could be as a result of dissolution of carbonate minerals like calcite and dolomite prevailing in the study area. However, cation-exchange processes could be responsible for the formation of the Ca²⁺-Mg²⁺-Cl^?-SO ₄ ^2? water type (??32%) from the CaSO₄, MgCO₃ and NaCl type that are formed due to the dissolution of anhydrite, gypsum, magnesite and halite. Besides, suitability of water for irrigation is evaluated based on sodium adsorption ratio, residual sodium carbonate, sodium percent, salinity hazard and USSL diagram. Hydrogeochemical speciation model calculations carried out using WATEQ4F program showed similar seasonal variation in the concentration of saturation indices of specific mineral phases, majority of the samples kinetically saturated with carbonate minerals (viz., aragonite, calcite and dolomite) indicating the influence of carbonate mineral phases on the chemistry of groundwater. On one hand, the samples were significantly oversaturated with Florapatite while on the other, they were undersaturated with respect to with anhydrite, gypsum and fluorite with halite being highly undersaturated. The Gibbs plots also gave an indication that there exists an interaction between rock and the percolating water into the subsurface by means of mineral dissolution. Factor analysis determined two factors mainly responsible for water quality during pre- and post-monsoon seasons, accounting to 52.84% and 51.09% of total variance respectively. Q-mode HCA Cluster analysis grouped the sampling stations into three clusters based on the similarity of water quality while R-mode HCA grouped analyzed parameters into two groups based on the effects of factors in the hydrochemistry.     

Mg isotopic composition of carbonate: insight from speleothem formation

Simultaneous high-precision measurement of ²⁴Mg, ²⁵Mg and ²⁶Mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry (MC-ICP-MS) relative to the international standard SRM980. Data are presented on low-Mg calcite speleothems and their associated host rocks and waters from four caves, one in the French Alps and three in Israel, covering various climate conditions. In addition, data are presented on three dolostones and three limestones from the Himalaya. The overall variation is 4.13‰ and 2.14‰ in δ²⁶Mg and δ²⁵Mg, respectively. This is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of Mg is not unique. Each speleothem shows a characteristic range of δ²⁶Mg values that are attributed to the isotopic composition of the local water. Differences between the isotopic composition of Mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to Mg isotopic fractionation during carbonate precipitation in the temperature range of 4-18°C. The low-Mg calcite is enriched in light isotopes by 1.35‰/AMU and the dependence on temperature has been found to be less than 0.02‰/AMU/°C. Despite various geological settings, the δ²⁶Mg of the studied dolostones is 2.0±1.2‰ higher than the δ²⁶Mg of the limestones. All together, these results suggest a strong mineralogical control and a weak temperature effect on the Mg isotopic composition of carbonate.     

Vadose CO2 gas drives dissolution at water tables in eogenetic karst aquifers more than mixing dissolution

Most models of cave formation in limestone that remains near its depositional environment and has not been deeply buried (i.e. eogenetic limestone) invoke dissolution from mixing of waters that have different ionic strengths or have equilibrated with calcite at different _pCO₂ values. In eogenetic karst aquifers lacking saline water, mixing of vadose and phreatic waters is thought to form caves. We show here calcite dissolution in a cave in eogenetic limestone occurred due to increases in vadose CO₂ gas concentrations and subsequent dissolution of CO₂ into groundwater, not by mixing dissolution. We collected high‐resolution time series measurements (1 year) of specific conductivity (SpC), temperature, meteorological data, and synoptic water chemical composition from a water table cave in central Florida (Briar Cave). We found SpC, _pCO₂ and calcite undersaturation increased through late summer, when Briar Cave experienced little ventilation by outside air, and decreased through winter, when increased ventilation lowered cave CO_2(g) concentrations. We hypothesize dissolution occurred when water flowed from aquifer regions with low _pCO₂ into the cave, which had elevated _pCO₂. Elevated _pCO₂ would be promoted by fractures connecting the soil to the water table. Simple geochemical models demonstrate that changes in _pCO₂ of less than 1% along flow paths are an order of magnitude more efficient at dissolving limestone than mixing of vadose and phreatic water. We conclude that spatially or temporally variable vadose CO_2(g) concentrations are responsible for cave formation because mixing is too slow to generate observed cave sizes in the time available for formation. While this study emphasized dissolution, gas exchange between the atmosphere and karst aquifer vadose zones that is facilitated by conduits likely exerts important controls on other geochemical processes in limestone critical zones by transporting oxygen deep into vadose zones, creating redox boundaries that would not exist in the absence of caves. Copyright © 2014 John Wiley & Sons, Ltd.     

Dissolution-filling mechanism of atmospheric precipitation controlled by both thermodynamics and kinetics

Affected by structural uplift,the Ordovician carbonate rockbed in the Tarim Basin,China,was exposed to dissolution and reformation of atmospheric precipitation many times,and formed a large quantity of karst caves serving as hydrocarbon reservoir.However,drilling in Tahe area showed that many large karst caves,small pores and fractures are filled by calcite,resulting in decrease in their reservoir ability.Calcite filled in the karst caves has very light oxygen isotopic composition and87Sr/86Sr ratio.Its 18OPDB ranges from 21.2‰to 13.3‰with the average of 16.3‰and its87Sr/86Sr ratio ranges from0.709561 to 0.710070 with the average of 0.709843.The isotope composition showed that calcite is related to atmospheric precipitation.Theoretic analyses indicated that the dissolving and filling actions of the precipitation on carbonate rocks are controlled by both thermodynamic and kinetic mechanisms.Among them,the thermodynamic factor determines that the precipitation during its flow from the earth surface downward plays important roles on carbonate rocks from dissolution to saturation,further sedimentation,and finally filling.In other words,the depth of the karstification development is not unrestricted,but limited by the precipitation beneath the earth surface.On the other hand,the kinetic factor controls the intensity,depth,and breadth of the karstification development,that is,the karstification is also affected by topographic,geomorphologic,climatic factors,the degree of fracture or fault,etc.Therefore,subject to their joint effects,the karstification of the precipitation on the Ordovician carbonate rocks occurs only within a certain depth(most about 200 m)under the unconformity surface,deeper than which carbonate minerals begin to sedimentate and fill the karst caves that were formed previously.     

Pseudo-anticlines and other structures in some calcretes of Botswana and South Africa

Deformation structures, within some Quaternary calcretes of Botswana and South Africa, have been classified into five types. Type 1 folds are small-scale (< 2 m wavelength) anticlines in shale or sheet calcrete, separated by wedges of calcrete, polygonal in plan. The anticlines have resulted from horizontal expansion and buckling caused by the disruptive growth of the calcrete wedges. Type 2(a) folds are medium-scale, strongly convoluted features in sheet calcrete and bedrock and are attributed to the introduction of calcrete along major vertical joint planes. Type 2(b) folds are large wavelength (> 20 m), but low amplitude (< 1 m), anticlines in hardpan calcrete; structural evidence suggests dominantly horizontal compression within the hardpan due either to an overall increase in volume of the calcrete, or to the disruptive formation of calcrete in joints in the synclinal areas. The mineralogy of the calcretes is dominated by low-Mg calcite which may have inverted from a high-Mg form; the deformation in type 1 and 2 folds was probably caused by the disruptive and displacive growth of calcite during calcretization. Type 3 folds are saucer-shaped depressions in hardpan calcrete and may be due to the removal of soluble salts below the folded layer during or after calcretization. Finally, type 4 folds are small, diapiric anticlines resulting from the upward injection of swelling clays into calcrete or calcretized shale.     

Patterns of Entrapped Air Dissolution in a Two‐Dimensional Pilot‐Scale Synthetic Aquifer

Past studies of entrapped air dissolution have focused on one‐dimensional laboratory columns. Here the multidimensional nature of entrapped air dissolution was investigated using an indoor tank (180 × 240 × 600 cm³) simulating an unconfined sand aquifer with horizontal flow. Time domain reflectometry (TDR) probes directly measured entrapped air contents, while dissolved gas conditions were monitored with total dissolved gas pressure (P_TDG) probes. Dissolution occurred as a diffuse wedge‐shaped front from the inlet downgradient, with preferential dissolution at depth. This pattern was mainly attributed to increased gas solubility, as shown by P_TDG measurements. However, compression of entrapped air at greater depths, captured by TDR and leading to lower quasi‐saturated hydraulic conductivities and thus greater velocities, also played a small role. Linear propagation of the dissolution front downgradient was observed at each depth, with both TDR and P_TDG, with increasing rates with depth (e.g, 4.1 to 5.7× slower at 15 cm vs. 165 cm depth). P_TDG values revealed equilibrium with the entrapped gas initially, being higher at greater depth and fluctuating with the barometric pressure, before declining concurrently with entrapped air contents to the lower P_TDG of the source water. The observed dissolution pattern has long‐term implications for a wide variety of groundwater management issues, from recharge to contaminant transport and remediation strategies, due to the persistence of entrapped air near the water table (potential timescale of years). This study also demonstrated the utility of P_TDG probes for simple in situ measurements to detect entrapped air and monitor its dissolution.     

LABORATORY RESULTS IN RESISTIVITY LOGGING*

This paper is an experimental extension of the theoretical investigations by Roy (1975) on the relative performances of the Laterolog 7, normal and some other sondes in logging of resistive formations. Only infinitely resistive formations have been simulated and placed in a tank containing tap water (true resistivity 27 Ωm) as electrolyte—representing both the mud column and the adjacent formations. Two sets of laboratory results (Doll 1951, NN 1958, 1969), have been repeated and we find that, for both these sets, the performance of the normal device is by far the superior of the two. In addition, we have studied the effect of varying the spacings A₁A₂, O₁O₂ and AM of Laterolog 7, normal, and two new sondes—Laterolog 4 and modified unipole—for two bore hole diameters in each case. For formation thicknesses less than A₁A₂ or AM, the Laterolog 7 is unsuitable because its response is flat and close to the base-line value. The normal device is more diagnostic, although, in such a case, it registers a trough or a resistivity low even against a resistive formation. For bed thicknesses clearly greater than A₁A₂ or AM, the normal sonde is decidedly superior to Laterolog 7, since its anomalies are sharper and larger. When the formation thickness is equal to or only slightly larger than A₁A₂ or AM, Laterolog 7 is somewhat better as it records a readable positive deflection while the normal does not. However, one must remember that a single run of the conventional resistivity log includes two normals and a lateral at different spacings. Laterolog 4 and modified unipole can in many instances produce better logs than normal, other considerations apart. The results are consistent with our own theoretical predictions and experience in surface resistivity profiling. They do not, however, agree with the prevalent concepts on Laterolog 7 vis-a-vis normal sonde.     

南极洲万达盐湖水中方解石饱和指数的垂直变化及其控制因素

南极洲万达盐湖为饱和方解石湖水。计算出的方解石饱和指数随水深增加而变化。在Ca²⁺及HCO₃^-活度值均一的上部氧化环境湖水中,pH及溶解氧(DO)是控制该变化的主要因素,在下部还原环境湖水中,Ca²⁺及HCO₃^-的活度对方解石的饱和指数SI的变化起主要作用。这充分揭示了湖底部近代沉积物中分布较多方解石矿物的原因。     

Diamond-bearing and diamond-free metacarbonate rocks from Kumdy–Kol in the Kokchetav Massif, northern Kazakhstan

Abstract Dolomite marble from the Kumdy–Kol area of the Kokchetav Massif contains abundant microdiamond, mainly in garnet and a few in diopside. The mineral assemblage at peak metamorphic condition consists of dolomite + diopside + garnet (+ aragonite) ± diamond. Inclusions of very low MgCO₃ calcite and almost pure calcite occur in diopside and are interpreted as aragonite and/or aragonite + dolomite. Single-phase Mg–calcite in diopside with a very high MgCO₃ component (up to 21.7 mol%) was also found in diamond-free dolomitic marble, and is interpreted as a retrograde product from aragonite + dolomite to Mg–calcite. The dolomite stability constrains the maximum pressure (P) at < 7 GPa using previous experimental data, whereas the occurrence of diamond yields the minimum peak pressure–temperature (P–T) condition at 4.2 GPa and 980 °C at X co ₂ = 0.1. The highest MgCO₃ in Mg–calcite constrains the minimum P–T condition higher than 2.5 GPa and 800 °C for the exhumation stage. As these marbles were subjected to nearly identical P–T metamorphic conditions, the appearance of diamond in some carbonate rocks was explained by high X co ₂. A low X co ₂ condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in X co ₂ for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh-pressure metamorphism.     

Hydrochemistry and source of high fluoride in groundwater of the Nairobi area,Kenya / Hydrochimie et origine des fortes concentrations en fluorure dans l'eau souterraine de la région de Nairobi,au Kenya

Abstract

This study aims to identify the hydrogeochemical processes influencing the high fluoride concentrations in groundwater of the Nairobi area, Kenya. For this purpose 16 groundwater samples were collected and analysed. Fluoride concentrations above the WHO standard are found in the downstream areas. The high F^? concentrations are correlated with high sodium and pH and low Ca²⁺ concentrations. Weathering of sodium-rich alkaline igneous rocks causes a pH increase resulting in an increase in HCO₃ ^? and CO₃ ^2- by dissolution of CO₂. Groundwater becomes oversaturated compared to calcite and calcite precipitation occurs, leading to a decrease in Ca²⁺. This causes a sub-saturation with respect to fluorite and dissolution of fluorite increases the F^? concentration. These reactions were modelled using the PHREEQC model and the results showed a good agreement with the measured groundwater quality, indicating that the proposed reactions are plausible for explaining the observed concentrations in groundwater.     

δCa evolution in a carbonate aquifer and its bearing on the equilibrium isotope fractionation factor for calcite

To improve understanding of Ca isotope transport during water-rock interaction on the continents, we measured dissolved δ⁴⁴Ca values along a 236 km flow path in the Madison aquifer, South Dakota, where fluids have chemically evolved according to dolomite and anhydrite dissolution, calcite precipitation, and Ca-for-Na ion-exchange over a timescale spanning ~ 15 kyr. We used a reactive transport model employing rate data constrained from major ion mass-balances to evaluate the extent to which calcite precipitation and ion-exchange fractionate Ca isotopes. Elevated δ⁴⁴Ca values during the initial and final stages of water transport possibly result from calcite precipitation under supersaturated conditions and Ca-for-Na ion-exchange, respectively. However, for the bulk of the flow path, δ⁴⁴Ca values evolve by mixing between anhydrite and dolomite dissolution, with no fractionation during calcite precipitation under saturated conditions. We attribute the absence of Ca isotope fractionation to the long timescale of water-rock interaction and slow rate of calcite precipitation, which have enabled fluids to chemically and isotopically equilibrate with calcite. We therefore conclude that the equilibrium Ca isotope fractionation factor between calcite and water (Δ_cal–w) is very close to zero. To the extent that the Madison aquifer typifies other groundwater systems where calcite slowly precipitates from solutions at or near chemical equilibrium, this study suggests that groundwater contributions to δ⁴⁴Ca variability on the continents can be modeled according to simple mixing theory without invoking isotope discrimination.     

Dissolution rates of calcium carbonate in the deep ocean; an in-situ experiment in the North Atlantic Ocean

An in-situ water circulator (ISWAC) was developed to allow accurate measurement of dissolution rates of various carbonate particles with minimal stagnation and mechanical weight loss. Three ISWAC packages were deployed at 3600, 4800 and 5518 m for 79 days in the Sargasso Sea (Northwest Atlantic). Weight losses for different particles during 79 days at 5518 m were as follows: pteropods and synthetic aragonite, 72.8%; reagent calcite, 57.5%; foraminifera, 23–36%; coccoliths, 11.3–24%, and diatoms, 12%. These weight losses are 2.5–7.5 times higher than those reported in earlier in-situ experiments. Normalization of weight losses with respect to BET specific surface area for different calcite particles yielded specific dissolution rates that differed by more than 2 orders of magnitude. Bleached biogenic particles dissolve significantly faster than non-bleached although their surface area is identical. We suggest that the BET surface area does not represent the reactive surface area available for dissolution, especially in biogenic calcite particles. Coatings, probably of organic matter, may reduce the reactive surface area and thus retard dissolution rates.The existence of a chemical lysocline in the Northwest Atlantic was confirmed. However, it seems that different kinds of particles have different lysoclines. The origin of the lysocline cannot be attributed to water flow or to the thermodynamic transition from supersaturation to undersaturation (Ω = 1). It seems to be a kinetic phenomenon.A simple model comparing the complete dissolution time and the residence time of a particle on the sediment-water interface suggests that coccoliths can be preserved in the sediments of the deep Northwest Atlantic below the CCD, in good agreement with SEM observations.