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Th. univ.--géol.--Rennes I, 1987. / Bibliogr. p. 121-126.

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).

The objective of this project was to test the hypothesis that micritic, microbial, dendritic shrub structures transition into aragonite botryoids by serving as an organic substrate that promotes the initiation of aragonite crystal precipitation. Samples for this study were taken from three sources: 1) a stalactite found in the Lighthouse Reef Blue Hole, Belize; 2) aragonite botryoids in the reef framework of the Permian Capitan Formation and 3) the Lower Permian Laborcita Formation found in the Sacramento Mountains, south-central New Mexico. Samples studied in thin section and with scanning electron microscopy (SEM) showed dendritic micrite within botryoids and spheroidal shapes associated with aragonite. Precipitation experiments were conducted to grow calcite crystals with organic molecules in solution. The textures formed were very similar to those found at the three sample sites. Despite the similarity, all evidence of an organic substrate promoting precipitation remains circumstantial and therefore inconclusive.

Biosignatures have been extensively studied at hot springs sites, such as Yellowstone, because liquid water is fundamental to the existence of life but also owing to the influx of mineral nutrients in these environments. However, some hot springs have upper temperatures exceeding the boundaries capable of sustaining life in all the spring facies, particularly those nearest the vent. Cold springs provide the same nutrient-rich environment with more ambient temperatures potentially capable of sustaining a diverse consortium of microorganisms across the entirety of the system. Ten Mile Graben Cold Springs, located in Southern Utah, is one such site known for its biota and preservation potential. This study aimed to observe the possible effects of the microorganisms on aragonite and calcite precipitation. Scanning electron microscope imagery observed biogenic fabric such as botryoidal aragonite and aragonite microspheres; however, the δ13C enrichment values of +2.80‰ to +7.30‰ imply the springs were dominantly precipitated through CO2 degassing. This discrepancy in the chemical and morphological data has been observed at other astrobiology analog sites such as Yellowstone; therefore, travertine and tufa seemingly do not preserve isotopic chemical biosignatures.

abstract: Natural variations in 238U/235U of marine carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~ 8.5 to study possible U isotope fractionation during incorporation into these minerals. Small but significant U isotope fractionation was observed in aragonite experiments at pH ~ 8.5, with heavier U in the solid phase. 238U/235U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007+0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)3(aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)3 (aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism. These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, PCO2, [Ca], or [Mg] concentrations. In general, these effects are likely to be small (<0.13 ‰), but are nevertheless potentially significant because of the small natural range of variation of 238U/235U. / Dissertation/Thesis / Masters Thesis Chemistry 2015

Calcium carbonate minerals (CaCO3) are important for our understanding of past marine conditions as well as tools for constructing paleoclimate. However, very little experimental work has been done to determine the influence of oxygen depletion on the geochemistry of CaCO3. To determine how oxygen depletion affects elemental incorporation and partitioning, aragonite was grown inorganically in artificial seawater at pressures of 1 atm and 5 bars (0.1%CH4-N2 mixture). Solution of Na2CO3 was used to induce aragonite precipitation. N2 was bubbled through solution in order to minimize oxygen content and iron powder was used to trap remaining O2. Experimental products (aragonite and fluid) were analyzed with ICP-MS, and isotope ratio mass spectrometer. Results suggest that Eh affects incorporation of Mn, S, Cu, and V into aragonite. No methane oxidation was observed.

A new paradigm for the formation of calcified skeletons suggests mineralization proceeds through amorphous calcium carbonate (ACC) precursors. The implications of this strategy in carbonate crystallization are widespread, particularly for understanding factors controlling impurity and isotopic signatures in calcium carbonates. The first chapter is a literature review of the biomineralization processes used by two important model organisms: the sea urchin larva and the foraminifera. Sea urchin larvae provide a thoroughly studied example of mineralization by an ACC pathway that is under biological control through regulation of protein chemistry and the local mineralization environment. A review of how foraminifera produce their test structures is also examined to explore the question of how organisms regulate the Mg content in proportion to the temperature their environments of formation. The second chapter demonstrates that acidic biomolecules regulate the composition of ACC for a suite of model carboxylated molecules. The physical basis for the systematic trend in Mg content is related to the ability of the affinity of the biomolecule for binding Ca versus Mg. The third chapter builds on these findings to explore the transformation of Mg-rich ACC precursors to calcites of exceptionally high Mg-contents that could not be produced by classical step-dominated growth processes. The data indicate that these materials are likely a result of a nucleation-dominated pathway. The final, fourth chapter develops Raman spectroscopy-based calibrations for determining Mg contents in ACC. The calibrations are based upon peak position or peak width of the carbonate υ₁ stretch. / Ph. D.

The rate of the aqueous transformation of aragonite to calcite was measured at 50°, 77°, and 101°C. The observed mole fraction calcite versus time relationship can be fit by the integrated rate model: t = [(3/C₂)(1-X)2/3 + (3/C₁)(X2/3)/[K₂-K₁] The constants C₁ and C₂ combine geometric factors, especially relative surface areas of the solids, K₁ and K₂ are the thermodynamic equilibrium constants for aragonite and calcite respectively. Apparent activation energies (E_A’) and absolute rates were calculated from Arrhenius plots of data from this study and others: E_A’ Conditions Material Time-50% CAL, 25°C Metzger and Barnard, 1968 58 kJ mol⁻¹ wet cm cubes 2.25X10² yr Taft, 1967 67 wet syn. powder 2.0X10⁻¹ This study 55 wet syn. powder 5.7X10⁻² Brown et al.,1962 373 dry 4.7X10³³ The E_A’ for this study is comparable with that of Metzger and Barnard indicating a similar mechanism, but absolute rates differ dramatically because of the different geometries of the run material. The dry transformation rates are so slow at diagenetic temperatures that this mechanism is of no importance geologically. Because the rate of the transformation is dependent on the geometry of the reacting system it is not surprising that most studies of neomorphic calcites find that the calcite textures are related to the original aragonite textures. Three transformation regimes, macroscale (passive dissolution), mesoscale (chalk zone), and microscale (thin film) dissolution-precipitation, are proposed to explain the variability in observed diagenetic calcite textures. These are differentiated by the surface area/solution ratio in the reaction zone. In general the smaller the geometric factor in the rate equation. i.e. the smaller the surface area/solution ratio, the slower the transformation rate and the higher the degree of precursor fabric retention in the neomorphic calcite. / Master of Science

The effect of magnesium, strontium, and other alkaline earths on the formation and persistence of metastable carbonates in the natural environment was investigated to determine the nature of the controlling mechanism. Barium and beryllium were studied to evaluate the effect of ionic radius; magnesium and strontium, in order to determine if the results correlate with the usual order of stability for complexes and adsorbed species. Known weights of aragonite were placed in contact with solutions of beryllium, magnesium, calcium, strontium, and barium. Samples were covered and periodically both pH and percent composition of aragonite determined; supernatant liquids and precipitates were analyzed for cation concentrations by atomic absorption spectroscopy and titrimetric methods. Results indicated that the order of effectiveness of alkaline earth metals in inhibiting recrystallization is : Be > Mg > Sr > Ba. This is the expected order of effectiveness for both surface and solution effects. A solution effect (i.e., sequestration of bicarbonate ions) is strongly suggested by the chemical behavior of each cation.