Thesis (M.S. in Chemistry)--University of California, Berkeley, Jan. 1961. / "UCRL-9494." TID-4500 (15th ed.). Includes bibliographical references (leaves 32-33).

No description available.

No description available.

No description available.

Sedimentary basins can contain close to 20% by volume pore fluids that are commonly classified as brines. These fluids can become undersaturated with respect to calcite as a result of processes such as migration, dispersive mixing, or anthropogenic injection of CO2. This study measured calcite solubility and dissolution rates in geologically relevant Na-Ca-Mg-Cl synthetic brines (35 to 200 g L-1 TDS). In brines < 50 g L-1 TDS, the EQPITZER calculated calcium carbonate ion activity product (IAP) at steady-state was in reasonable agreement (±10%) with the thermodynamic solubility constant for calcite (Kc). However, the IAP systematically exceeded Kc in more concentrated brines. The deviation was strongly correlated with calcium concentration and also was observed in magnesium-free solutions. This is interpreted as an uncertainty in the carbonate ion activity coefficient, and minor adjustment in stoichiometric association constants (K*M2+CO30) for the CaCO30 or MgCo30 ion pairs would correct for the error. The dissolution rate dependency on brine composition, pCO2 (0.1 to 1 bar), and temperature (25.0 to 82.5 °C) was modeled using the empirical rate equation ()nkRΩ−=1 where R is the rate, k and n are empirical fitting terms, and Ω the degree of disequilibrium with respect to calcite. When Ω was defined relative to an apparent kinetic solubility, n could be assumed first-order over the range of Ω investigated (Ω = 0.2 to 1.0). Rates increased with increasing pCO2 as did the sensitivity to brine concentration. At 0.1 bar, rates were nearly independent of concentration (k = 13.0 ±2.0 x 10-3 moles m-1 hr-1). However, at higher CO2 partial pressures rates became composition dependent and the rate constant, k, was shown to be a function of temperature, pCO2, ionic strength, and calcium and magnesium activity. The rate constant (k) can be estimated from a multiple regression (MR) model of the form k = B0 + B1(T) + B2(pCo2) + B4(aCa2+) + B5(aMg2+). A relatively high activation energy (Ea = 20 kJ mol-1) was measured, along with a stirring rate independence suggesting the dissolution is dominated by surface controlled processes at saturation states Ω > 0.2 in these calcium-rich brines. These findings offer important implications to reaction-transport models in carbonate-bearing saline reservoirs.

De nombreux organismes marins précipitent des squelettes en calcite magnésienne. Depuis près d’un siècle, il est connu que les concentrations en magnésium de ces squelettes sont influencées par les conditions environnementales, telle la température, régnant au moment de leur dépôt. Dans le contexte actuel de changement climatique, cette propriété a promu l’usage de plusieurs taxons en tant qu’archive naturelle des conditions environnementales du passé. Cependant, les squelettes d’espèces sympatriques, voire d’individus de la même espèce, peuvent présenter des concentrations en magnésium très différentes, attestant de l’influence de facteurs biologiques sur la détermination de la concentration squelettique en cet élément. Une parfaite compréhension des mécanismes d’incorporation du magnésium dans les squelettes est donc requise pour valider l’usage de ce paléotraceur. De plus, la solubilité des calcites augmentant avec leur concentration en magnésium, l’incorporation de cet élément conditionne en partie la stabilité des squelettes calcitiques dans un océan en cours d’acidification. Le présent travail contribue à l’étude des différents facteurs, tant environnementaux que physiologiques et minéralogiques, susceptibles d’affecter l’incorporation du magnésium dans les squelettes en calcite de trois taxons présentant des concentrations en cet élément particulièrement élevées, une éponge hypercalcifiée, Petrobiona massiliana, et deux échinodermes, Paracentrotus lividus et Asterias rubens. Dans une première partie, les effets de plusieurs facteurs environnementaux ont été étudiés, en milieu naturel dans le cas de l’éponge, étant donné son incapacité à survivre en aquarium, et en conditions contrôlées d’aquarium dans le cas des deux échinodermes. Une influence environnementale prépondérante de la température sur la concentration en magnésium squelettique a été mise en évidence dans les 3 modèles biologiques étudiés. Une fois les facteurs génétiques (espèce) et structurels (élément squelettique) fixés, une relation positive liant la température à la concentration en magnésium squelettique a été caractérisée en milieu naturel chez l’éponge hypercalcifiée P. massiliana et en conditions contrôlées chez l’oursin P. lividus. Chez ce dernier, cette relation, non linéaire, se stabilise aux plus hautes températures envisagées, probablement suite à la saturation d’un processus biologique intervenant dans l’incorporation de cet élément. La salinité, un autre facteur environnemental majeur en milieu marin, influence elle aussi positivement la concentration en magnésium dans le squelette de l’étoile de mer A. rubens. A nouveau, il est proposé que cette influence de l’environnement soit modulée par un processus biologique: chez les échinodermes, la concentration en magnésium, contrairement à celle du calcium, n’est pas régulée dans le liquide coelomique. Elle est donc directement influencée par la salinité, et affecte probablement la concentration en cet élément dans le squelette formé. La diffusion depuis l’eau de mer jusqu’au site de calcification par l’intermédiaire des fluides internes a en effet été suggérée sur base du fait que le rapport Mg/Ca de l’eau de mer influence celui des squelettes calcaires Une fois l’influence, directe ou indirecte, des facteurs environnementaux exclue, 44% de la variabilité du rapport Mg/Ca du squelette des échinodermes restent à expliquer. Les expériences de croissance d’échinodermes réalisées en conditions contrôlées indiquent que ce rapport est indépendant de la vitesse de croissance dans ce groupe, contrairement aux hypothèses émises dans la littérature. Dans la seconde partie, la modulation des facteurs minéralogiques par les facteurs biologiques a été investiguée. Pour ce faire, d’une part, les interactions entre rapport Mg/Ca en solution et matrice organique de minéralisation ont été étudiées dans un modèle in vitro. D’autre part, les relations entre soufre et magnésium dans le squelette ont été décryptées. Le rapport Mg/Ca de la solution de précipitation a une influence prépondérante sur la concentration en magnésium du carbonate de calcium précipité in vitro, attestant de l’importance de la régulation de la composition du fluide de calcification et des mécanismes de transport la contrôlant. Deux mécanismes biologiques complémentaires permettent de favoriser l’incorporation, dans les calcites biogéniques, de quantités de magnésium largement supérieures à celles observées dans les calcites inorganiques, et ce, malgré la forte hydratation de ce cation : l’intervention d’agents chélateurs du magnésium et le passage par une phase de carbonate de calcium amorphe (CCA). Les molécules de la matrice organique de minéralisation jouent entre autres le rôle de chélateur du magnésium, réduisant son état d’hydratation et facilitant ainsi son incorporation dans le minéral. Un rôle similaire a été suggéré pour les sulfates en solution, au vu de la corrélation observée dans ce travail entre les rapports Mg/Ca et S/Ca dans la phase minérale des calcites biogéniques étudiées. La matrice organique affecte elle aussi la concentration en magnésium dans le cristal, probablement via la stabilisation de la phase de CCA nécessaire à l’incorporation de concentrations élevées de cet élément: ainsi, les macromolécules de la matrice organique du test d’oursin induisent in vitro la formation de calcites plus riches en magnésium que celles formées en présence de matrice de piquant, un résultat concordant avec le fait que, in vivo, le test contient des concentrations en magnésium plus élevées que les piquants. Cette thèse de doctorat a donc soulevé l’importance des effets biologiques dans la détermination du rapport Mg/Ca dans les calcites biogéniques. Les résultats obtenus montrent que le décryptage des mécanismes impliqués dans l’incorporation du magnésium se doit de considérer la phase amorphe transitoire qui précède la cristallisation. Des effets environnementaux affectent eux aussi la concentration squelettique en magnésium, mais nos résultats suggèrent qu’ils agissent au travers d’une modulation des effets biologiques, et non par une influence thermodynamique directe. Cette hypothèse, si elle est confirmée, impose la plus grande prudence lors de l’utilisation des squelettes en calcite en tant que paléotraceurs. SUMMARY The magnesium concentration in calcite skeletons produced by marine invertebrates is known to be dependent on several environmental parameters, including temperature, salinity and seawater Mg/Ca ratio. This property prompted the use of this concentration as a proxy of the considered parameters. However, skeletal magnesium contents in sympatric species and even in individuals of the same species may be rather different. These inter and intra-individual variabilities indicate that biological factors also affect magnesium incorporation into biogenic calcites. Magnesium incorporation mechanisms are still unknown in calcifying invertebrates, a fact that questions the validity of this element as a paleoproxy. Moreover, higher magnesium contents increase calcite solubility and could therefore worsen the case of calcifying organisms facing ocean acidification linked to global change. The present thesis is a contribution to the study of the environmental, biological and mineralogical factors affecting magnesium incorporation into the calcitic skeletons of 3 taxa, i.e. one hypercalcified sponge, Petrobiona massiliana, and two echinoderms, Paracentrotus lividus and Asterias rubens. The first part of this work was dedicated to the study of several environmental factors affecting the magnesium concentration in the calcite skeleton of the 3 studied organisms. Consequently to its low survival in aquarium, the sponge was studied using field specimens collected along an environmental gradient. Echinoderms were grown in controlled conditions in aquarium. Once the genetic (species) and structural (skeletal element) factors were fixed, skeletal magnesium concentration was positively related to temperature in the 3 studied species. The Mg/Ca ratio of the test of aquarium-grown P. lividus increased with temperature until a plateau which was probably due to the saturation of a biological process involved in magnesium incorporation. A positive effect of salinity, an other major environmental parameter, on skeletal Mg/Ca was demonstrated in aquarium-grown A. rubens. This influence can also be linked to a biological process: contrary to magnesium, calcium concentration is controlled in the coelomic fluid, from which ions probably diffuse through the living tissues to the calcification site. Thus, the observed positive relation can be explained by the fact that a salinity increase raises the coelomic Mg/Ca ratio, which, according to previous studies, affected the Mg/Ca ratio of the precipitated skeleton. In addition to the reported environmental influences, 44% of the skeletal Mg/Ca ratio variation remained unexplained in echinoderms. The absence of growth rate effect on magnesium incorporation into the echinoderm skeleton was demonstrated in aquarium experiments, contrary to previous literature statements. Other biological factors must therefore affect the incorporation of this element. In the second part of this work, the modulation of mineralogical factors by biological factors was investigated. The interaction between Mg/Ca ratio in the precipitation solution and organic matrix was studied in an in vitro precipitation experiment. In addition, the relation between skeletal Mg/Ca and S/Ca ratios was investigated. A major influence of the precipitation solution Mg/Ca ratio on the magnesium concentration of in vitro precipitated minerals was evidenced, highlighting the importance of transport mechanisms which determine the composition of the calcifying solution. The higher magnesium concentrations presented in some biogenic calcites in comparison to inorganic calcites can be attributed to the action of chelating molecules and to the transition trough an amorphous phase. The strong tendency of magnesium towards hydration can be overcome by the involvement of molecules that can function as magnesium chelators and, therefore, favour the formation of calcite with a high magnesium content. Organic matrix macromolecules have been suggested to proceed as magnesium chelators, reducing the hydration of this ion and facilitating its incorporation into calcite. A similar function was suggested for sulphates that were measured in the echinoderm skeleton. This would explain the positive correlation between skeletal Mg/Ca and S/Ca ratios observed in the studied species. Organic matrix macromolecules also increased the magnesium concentration of minerals precipitated in vitro, probably stabilizing the transient phase of amorphous calcium carbonate, which can incorporate high quantities of magnesium in its structure. The enhancement of magnesium incorporation was more pronounced with the organic matrix extracted from the test of sea urchin than with that extracted from their spines. This result was in agreement with the in vivo skeletal Mg/Ca ratios in P. lividus skeleton that were higher in the test than in the spines. This study demonstrated the importance of the biological effects in the determination of Mg/Ca ratios in biogenic calcites. According to the suggested hypotheses, the understanding of mechanisms involved in magnesium incorporation should take the transient amorphous phase into account. Magnesium concentration in biogenic calcite was also affected by environmental parameters, but these influences could proceed through the indirect modulation of biological rather than a direct thermodynamic control. This hypothesis, if proved correct, would have deep implications for the use of magnesium in calcite skeletons as a paleoproxy.

The effect of ionic strength (I), pCO2, and temperature on the reaction kinetics of calcite was investigated in magnesium-free, phosphate-free, low calcium (mCa^2 ≈ 0.01 – 0.02 molal) simple KCl and NaCl solutions from both undersaturated and oversaturated conditions. First order kinetics were found sufficient to describe the dissolution rate data. Dissolution rates decreased with increasing I and were faster in KCl than NaCl solutions at the same I indicating that Na^ interacts more strongly with the calcite surface than K^ or that water is less available in NaCl solutions. Rates increased with increasing pCO2 and temperature, and their influence diminished at high I. Arrhenius plots yielded a relatively high activation energy (Ea ≈ 20 ± 2 kJ mol-1) which indicated that dissolution was dominated by surface controlled processes. These results are consistent with the hypothesis that the mole fraction of "free" solvent plays a significant role in the dissolution kinetics of calcite with a minimum value of ~45-50 percent required for dissolution to proceed in undersaturated solutions at 25-55 degrees C and pCO2 = 0.1 – 1 atm. Precipitation rates were modeled using the general and Davies and Jones rate equations yielding similar results. Reaction orders were found to typically range between 0.8 and 2.5 for both rate equations regardless of electrolyte. For both solutions, rate constants were found to range between 100.8 and 101.7 mmole m-2 hr-1 (general rate equation) and 101.5 and 102.2 mmole m-2 hr-1 (Davies and Jones rate equation). Under the experimental conditions employed and the resultant precision (~20-25 percent), I and pCO2 do not have a significant influence on the precipitation rate of calcite. Precipitation rates increased with temperature although Arrhenius plots yield a broad range of activation energies (Ea ≈ 15 – 28 kJ mol-1, R2 = 0.72). The relatively low calculated activation energies coupled with the precision of the results suggest the possibility of surface nucleation in the present results. Overall, these findings may be useful in understanding and predicting the interaction and reactivity of the host carbonate minerals in subsurface reservoirs to the injection of CO2 although much work needs to be completed at elevated temperatures and pressures.

Sedimentary basins can contain close to 20% by volume pore fluids that are commonly classified as brines. These fluids can become undersaturated with respect to calcite as a result of processes such as migration, dispersive mixing, or anthropogenic injection of CO2. This study measured calcite solubility and dissolution rates in geologically relevant Na-Ca-Mg-Cl synthetic brines (35 to 200 g L-1 TDS). In brines < 50 g L-1 TDS, the EQPITZER calculated calcium carbonate ion activity product (IAP) at steady-state was in reasonable agreement (±10%) with the thermodynamic solubility constant for calcite (Kc). However, the IAP systematically exceeded Kc in more concentrated brines. The deviation was strongly correlated with calcium concentration and also was observed in magnesium-free solutions. This is interpreted as an uncertainty in the carbonate ion activity coefficient, and minor adjustment in stoichiometric association constants (K*M2+CO30) for the CaCO30 or MgCo30 ion pairs would correct for the error. The dissolution rate dependency on brine composition, pCO2 (0.1 to 1 bar), and temperature (25.0 to 82.5 °C) was modeled using the empirical rate equation ()nkR&#8486;&#8722;=1 where R is the rate, k and n are empirical fitting terms, and &#8486; the degree of disequilibrium with respect to calcite. When &#8486; was defined relative to an apparent kinetic solubility, n could be assumed first-order over the range of &#8486; investigated (&#8486; = 0.2 to 1.0). Rates increased with increasing pCO2 as did the sensitivity to brine concentration. At 0.1 bar, rates were nearly independent of concentration (k = 13.0 ±2.0 x 10-3 moles m-1 hr-1). However, at higher CO2 partial pressures rates became composition dependent and the rate constant, k, was shown to be a function of temperature, pCO2, ionic strength, and calcium and magnesium activity. The rate constant (k) can be estimated from a multiple regression (MR) model of the form k = B0 + B1(T) + B2(pCo2) + B4(aCa2+) + B5(aMg2+). A relatively high activation energy (Ea = 20 kJ mol-1) was measured, along with a stirring rate independence suggesting the dissolution is dominated by surface controlled processes at saturation states &#8486; > 0.2 in these calcium-rich brines. These findings offer important implications to reaction-transport models in carbonate-bearing saline reservoirs.

Thesis (M.S.)--Miami University, Dept. of Geology, 2002. / Title from first page of PDF document. Document formatted into pages; contains vi, 78 p. : ill. Includes bibliographical references (p. 37-40).

No description available.