J.Ch. Fontes

Geochemistry and origin of formation brines from the Paris Basin, France

Abstract Small amounts of brines associated with Triassic (Keuper) layered halite from the eastern Paris Basin have been collected during salt mining operations. Critical consideration of the possible reactions and interactions involving Br− indicates that Cl − Br − ratio values are conservative except in two cases: (1) precipitation or dissolution of solid chlorides; and (2) convective or diffusive mixing of saline solutions. Calculations suggest that halite recrystallization could be a source of aqueous Br−, but this process is probably not significant under natural conditions as Triassic salt has retained a Br− content higher than calculated for equilibrium conditions. Bromide-chloride data for the brines associated with Triassic salts are presented on a log-log diagram of Cl − Br − vs. Cl−, including the evolution of present-day marine solutions and literature values for the chloride salts between halite and tachyhydrite ((CaCl2, 2MgCl2)·12H2O) deposition. This diagram allows discernment of primary or secondary origins of brines. Chemical variations of major and trace elements during the evaporative concentration of seawater are estimated from selected literature and from additional measurements of salt ponds on the Mediterranean shore. Low values of the Cl − Br − ratios in brines from Triassic salt can be reconciled with high Na+ contents if the sampled solutions are mixtures of a secondary brine (halite dissolution) and a highly evolved primary brine. Mass-balance considerations indicate that the primary brine component was more concentrated than the stage of bischofite (MgCl2·6H2O) deposition. Contents of Br−, Cl−, Na+ and K+ can be accounted for by a conservative behaviour of these ions, possibly slightly affected by some precipitation of NaCl induced by the mixing of the two types of solutions. The secondary brine component would have contributed all Na+ and K+ through halite and sylvite (KCl) dissolution. Several hypotheses are discussed for the origin of the high contents in Li+: diagenesis, halite dissolution, global or local enrichment of Triassic seawater, and contribution from the primary brine. The high Li+ concentration is attributed to the highly concentrated brine component. Rubidium would also behave conservatively after all K+ has been removed by sylvite and carnallite ((KCl,MgCl2)·6H2O) precipitation. Boron, Ca2+, Sr2+, Mg2+ and SO2−4 are not conservative. Mass-balance calculations indicate that the high content in Ca2+ and in Sr2+ imposes a diagenetic origin. The values of the 87 Sr 86 Sr isotope ratio are clearly higher than the Keuper marine values of the literature, which could be explained by a major contribution of reworked Permian evaporites with high 87Sr content. Such hypothesis would be in agreement with the low heavy-isotope content of aqueous SO2−4. Dolomitization, either by the primary brine or by the mixing of primary and secondary brines, would have implied Ca-sulphate precipitation, in agreement with the observed additional depletion in heavy isotopes of aqueous SO2−4, relative to anhydrite deposits from Triassic layers of the Paris Basin. Stable isotope contents (2H and 18O) of the brines can be accounted for by mixings of a secondary brine of continental origin with a primary brine enriched in 18O by evaporation. In conclusion, emphasis is put on the conservation of Li+, the near-conservative behaviour of the elements Cl−, Br−, Na+, K+ and Rb+, whereas diagenetic reactions control B3+, Mg2+, Ca2+, Sr2+ and SO2−4.

Geochemistry and origin of formation brines from the Paris Basin, France: 2. Saline solutions associated with oil fields☆

Abstract Oil fields from the Paris Basin are located in three geological levels: Dogger (Middle Jurassic), Rhaetian and Keuper (Upper Triassic). The origin of water and dissolved salts in saline solutions associated with oil is investigated, using the hypothesis that Cl− and Br− are conservative in solution. In a diagram of Cl − Br − ratios vs. Cl−, the points of the Dogger and the Keuper aquifers lie in the zone of primary brines whereas those of the Rhaetian are located in the zone of secondary brines. The Na + Cl − ratio is very high in all of the saline solutions, suggesting either a relatively poorly evolved brine (at the beginning of halite deposition) or some halite dissolution. All of the saline solutions, especially those from the Dogger aquifer, are clearly diluted. Dilution by meteoric waters would not alter the Cl − Br − ratio. High concentrations of lithium, rubidium and boron are attributed to the presence of an extremely evolved brine of marine origin in mixtures of secondary brines formed by dissolution of halite and small amounts of sylvite and Ca-sulphate. Mass-balance calculations of the excess of Cl− with respect to Na+ + K+ allows the fraction x of water derived from the primary brine to be calculated. Assumptions are that all Na+ and K+ is derived from the secondary brine and that the Cl− content of the primary brine is constrained by bischofite saturation at ∼ 11,000 mmol kg−1. The typical proportion of water from the primary brine waters is ∼ 0.7% in the Dogger formation and ∼ 2.4% in the Keuper. Values of the concentration factor CF, expressed as the ratio of the molal concentration of a conservative ion in the evolved brine to its concentration in seawater have been calculated for lithium. Solutions from the Dogger and Keuper aquifers have CFLi+-values of 1.8 · 103 and of 7.7 · 103, respectively. In the Rhaetian aquifer, the linear relationship between heavy-isotope contents of the brine (2H and 18O) and the Cl− content indicates a mixture of two water sources. The first one is meteoric water as defined by the intersection of the formation water line with the global meteoric water line ( δ 2 H = −65‰ , δ 18 O = −9.3‰ ). The other end-member is enriched in heavy isotopes and in Cl−, and is attributed to seawater that has dissolved evaporites (mainly halite). However, the presence of small amounts of an extremely evolved brine is required to account for the bromide, lithium, rubidium and boron contents of the formation brine from the Rhaetian aquifer. As this contribution is very limited in amount, it cannot affect the heavy-isotope content of the formation water. Mass-balance equations are solved for ternary mixtures of meteoric water, seawater and brine. The same constraints as above (no Na+ and K+ supplied by the primary brine and ClBrine ≈ 11,000 mmol kg−1) are applied. It is also assumed that the seawater component has the same ionic contents as modern seawater and that the salt content of the meteoric component is negligible. The fraction of primary brines is x ≈ 1.4% according to the Cl− balance. This estimate agrees with that obtained from the lithium balance assuming that the primary brine component was the same as that of the Keuper formation water. Mass-balance calculations also indicate that the SO42− content of the Rhaetian formation water is exclusively due to the secondary brine component. In the three saline solutions, the very high Ca2+ contents and high Ca 2+ Mg 2+ ratios, may be attributed to reaction of the very evolved brine (free of SO2−4), with gypsum or anhydrite. The result is an exchange of Mg2+ for Ca2+, which causes a release of Ca2+ to the solution and a precipitation of secondary Mg-sulphate. This process can also account for the high strontium contents of the analysed samples. Values of the ratio 87 Sr 86 Sr are very high in the Keuper formation water due to close contact of the solutions with the granitic basement or with granite-derived detrital deposits. Values in the Rhaetian and Dogger aquifers equal those of Upper Triassic marine deposits. The stable isotope (34S and 18O) content of the dissolved aqueous SO2−4 indicates a Triassic origin for the bulk of sulphate involved in the evolution of formation waters from the Rhaetian and from the Dogger. All geochemical indicators thus show a vertical migration of Triassic brines. However, aqueous SO2−4 from the Keuper has a Permian isotopic signature suggesting a reworking in solution of salts from this period.

Essai de détermination expérimentale du fractionnement des isotopes 13C et 14C du carbone au cours de processus naturels

Theoretical calculations by Bigeleisen and Mayer (1947) and Craig (1954) have shown that the relative isotopic enrichment 14C/12C in the thermodynamic processes in approximately twice that of the ratio 13C/12C (i.e. in terms of isotope enrichment factor: 2 ϵ13C ⋟ ϵ14C). This relationship is used expecially for the normalization of 14C dates. We calculated this theoretical coefficient with the isotope enrichment factor partition functions of various isotopic species of CO2, using the homogeneous set of spectroscopic data of Jobard and Chedin (1975). It appears that the ratio of ϵ14C/ϵ13C should be slightly higher than 2. We experimentally evaluated this relationship in a natural environment. We investigated the isotope fractionation which occurs during photosynthesis between atmospheric CO2 and plants with different (C3 and C4) photosynthetical cycles. Measured values: 2.3 ± 0.2 (C3/C4 plants) and 2.65 ± 0.2 (atmospheric CO2/C3 plant) can be reconciliated at 2.3 assuming a slight (<1%) 14C depletion of CO2 absorbed by the plant.

Geochemistry, origin and recharge mechanisms of groundwaters from the Garoua Sandstone aquifer, northen Cameroon

This paper presents results of the geochemical and isotopic studies on groundwater samples from the Garoua Sandstone Basin, in northern Cameroon. Chemical and environmental isotope data are presented and discussed in terms of the origin of dissolved species and of groundwater. All of the investigated groundwaters are categorized into two chemical types: low mineralized and acidic waters of CaNaHCO3 type, and relatively high mineralized waters of NaHCO3 type, with relatively high pH, between 7.40 and 8.22. Interpretation of chemical data, based upon both thermodynamic calculations and stability diagrams, suggests that the chemical evolution of groundwater is primarily controlled by water-rock interactions, involving (1) acidic weathering of aluminosilicates, (2) dissolution of secondary carbonate minerals, and (3) cation exchange of Na+ for Ca2+. However, the original composition of groundwater may have been modified by further secondary processes such as mixing of chemically different water masses, and anthropogenic NO−3 pollution. Tritium contents in the considered groundwaters are very heterogeneous, suggesting differences in the water turnover time or in the recharge conditions, or both. On the basis of the geographical distribution of 3H contents, a hydrogeological relation between river and the groundwater systems has been proposed for the study site. Interpretation of both 18O and 2H, as well as 3H, suggests that the recharge of the investigated groundwaters may result from three major mechanisms: (1) direct infiltration of local precipitation; (2) lateral inflow of river waters; (3) probably upward leakage of old groundwaters.

Isotopic approach to calcite dissolution and precipitation in soils under semi-arid conditions

Abstract Two phases of crystallization of secondary calcite are identified in shallow soil horizons from northeastern Brazil where present-day climatic conditions are semi-arid. The 14C and 13C contents of the total dissolved inorganic carbon (TDIC) from soil solutions are compatible with open-system conditions. In the deeper part of the profile (1.5–3-m depth) the precipitation of secondary calcite is attributed to the degassing of solutions under wet climatic conditions. Interpretation of the radiocarbon activities leads to ages between 22,000 and 17,000 yr. for the pure secondary calcite. In the upper part (0.3–1.5-m depth) of the profile, active formation of calcrete through dissolution-precipitation cycles is occurring. In this zone the precipitation is controlled mainly by the evaporation of soil solution as shown by the progressive enrichment in 18O of secondary carbonates. The accretion of calcrete occurs at the surface and at 1.5-m depth due to changes in the position of the evaporation front.

Geochimie et teneurs isotopiques des systèmes saisonniers de dissolution de la calcite dans un sol sur craie

In a soil developed on the Cretaceous chalk of the Eastern Paris basin, calcite dissolution begins at the surface. The soil water is rapidly saturated in calcite. Calcite dissolution follows two different pathways according to seasonal pedoclimatic conditions. During winter: the soil is only partly saturated in water and the CO2 partial pressure is low (Ca 10−3 atm.). As a consequence total inorganic dissolved carbon (TIDC) is a hundred times the carbon content of the gaseous phase. Equilibrium is usually observed between the two phases. It is a closed system. The measured carbon 14 activity (87,5%) and 13C content (δtidc13C = −12,2%0) of the drainage water are very close to theoretical values calculated for an ideal mixing system between gaseous and mineral phases (respectively characterized by the following isotopic values: δG13C = −21,5%0; AG14C = 118%; δM13C = +2,9%0; AM14C = 28%). During spring and summer: the soil moisture decreases, the input of biogenic CO2 induces an increase of the soil CO2 partial pressure (Ca from 3.10−3 atm to 7.10−3 atm). The carbon content of the gaseous phase is higher by an order of magnitude compared to winter conditions. Therefore the aqueous phase is undersaturated in CO2 with respect to the latter. This disequilibrium occurs as a result of unbalanced rates of CO2 dissolution and CO2 effusion toward atmosphere. It is an open system. The carbon isotopic ratio of the aqueous phase is regulated by that of the gaseous phase, as demonstrated by the agreement between measured and calculated isotopic compositions (respectively δL mes = from −9,4%0 to −11,5%0, δl calc = from −9,8%0 to −13,9%0 AL mes = 119%, AL calc = from 119% to 125%). The solutions originating from both systems (open and closed) move downwards without significant mixing together. It has also been observed that no significant variation of the TIDC isotopic composition occurs during precipitation of secondary calcite.

Measurement of isotopic equilibrium between water, water vapour and soil CO2 in arid zone soils

Abstract Some preliminary results from a sampling expedition in the Sahara suggest that it may be possible to use the isotope composition of either soil water vapour or soil CO 2 to obtain isotope profiles of soil water. Samples of soil air at atmospheric pressure were taken using a sampling probe which was driven into the soil. However, it is suggested that best results will be obtained if the samples of soil air are taken at reduced pressure. The use of a preparation line specifically designed for treating small samples of water allowed us to analyse both oxygen-18 and deuterium on a single water sample of ∼ 5 μ ml. Laboratory experiments are described which demonstrate that the isotope fractionation between soil water, even under very high matric suctions, and soil water vapour, is the same as that for free water.

Aquifers as archives of paleoclimate

The isotopic and elemental compositions of groundwater are influenced by climatic conditions at the time of recharge and are therefore indicators of climate change. The potential of aquifers as archives of paleoclimate was discussed at a 3-day conference held at Lamont-Doherty Geological Observatory, Palisades, N.Y., from May 18 to 20. About forty scientists from nine countries attended the meeting, which was sponsored by the Northeast Regional Center of the National Institute for Global Environmental Change (NIGEC) and the National Science Foundation. Fifteen invited papers were presented, which formed the background of extensive discussions on applying isotopic methods to deriving information on climate change during the last 30,000 years from the groundwater archive.

Etude des transferts d'eau à l'intérieur d'une formation morainique dans le bassin du Léman. Transferts d'eau dans la zone non saturée

Isotopic and chemical studies were carried out from June 1985 to June 1986 on a small agricultural morainic water basin (0,14 km2) to determine the different flow components. The pedologic-lithologic profile is divided into four main horizons: (1) at the surface, a cultural horizon (0–50 cm) clayey and loamy with granular structure; (2) a weathered layer from 50 to 90 cm, also clayey and loamy, but with a compact structure; (3) a fine sandy horizon between 90 and 120 cm; and (4) at the bottom clayey moraine deposits from 120 cm downwards. Measurements were made of precipitation, soil water content, and discharge at the outlet of the basin. Discharges varied between 6 and 53 l s−1 and reflected the snowy-rainy of the basin which received 1036 mm during the twelve-month study. The soil humidity indicates three periods: (1) in summer, a drying period which affects the depth of 90 cm; (2) wetting during autumn which modifies the water content of only the upper 45 cm; and (3) during the winter and spring, saturation of the soil. Tensiometric measurements show a desaturation with upward water movement during the period from September to November. Then a saturation of the whole profile occurs from November to June. This latter period indicates that the 45–80 cm layer acts as a water reservoir. The data monitoring exhibits two systems of flow movement in the soil: (1) a superficial one, which facilitates runoff or subsurface lateral movement, linked directly to precipitation; and (2) a lower system, practically independent from the previous one, originating outside the basin. The water balance in the basin showed an excess of 346 mm over the total of 1036 mm of precipitation. The knowledge of the isotopic gradient of the region suggest a recharge at a mean altitude of 700 m. The water behaviour in the superficial system is directly linked to precipitations the isotopic signal of which moves downwards in the soil. Hydrograph separation of discharge at the outlet indicates the origin of the different flow components. When the runoff flow starts in the river bed after the dry season it is the sandy layer which provides the major flow. Then when the saturation level is reached one can observe lateral