| Summary: | <p>The subsumed work of this dissertation is comprised of three independent but interrelated studies which seek to further the understanding of processes which govern the coprecipitation of trace elements with calcite and aragonite minerals. These studies investigate the effects of: 1) pressure on crystal morphology and trace element incorporation to aragonite; 2) growth rate on uranium partitioning between calcite and fluid; 3) aqueous Mg/Ca on the magnesium partitioning to low-magnesium calcite. The importance of this work is to determine how the environment of formation and growth rate influences the geochemistry of CaCO<sub>3</sub> in order to improve existing paleoproxies and develop new ones.</p>
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In the first study a series of experiments were conducted at 1, 25, 75, 100, and 345 bars of nitrogen this range covers pressures at the oceanic floor. Aragonite precipitation was induced by the one-time addition of a Na<sub>2</sub>CO<sub>3</sub> solution to an artificial seawater. Results suggest that oceanic floor pressures could affect the crystallization of CaCO<sub>3</sub> by altering mineralogical composition and aragonite crystal size.</p>
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In the second study calcite crystallized from NH<sub>4</sub>Cl-CaCl<sub>2</sub>-U solution by diffusion of CO<sub>2</sub>. The calcite growth rate was monitored by sequential spiking of the calcite-precipitating fluids with REE dopants. The resulting crystals were analyzed using Secondary Ion Mass Spectrometry (SIMS). Results showed that the partitioning of uranium increases with increasing growth rate. Growth entrapment model (GEM) and unified uptake kinetics model (UUKM) explain the obtained data. </p>
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In the third study CaCO<sub>3</sub> precipitated in NaCl solution by continuous addition of CaCl<sub>2</sub>, MgCl<sub>2</sub>, and either Na<sub>2</sub>CO<sub>3</sub> or NaHCO<sub>3</sub>. The Mg/Ca of the fluid was adjusted in an attempt to produce calcite where Mg/Ca would match Mg/Ca in foraminifera shells. It was observed that multiple CaCO<sub>3</sub> polymorphs precipitated from fluids at high pH (Na<sub>2</sub>CO<sub>3</sub> doping experiments). This result underscores the potential control of pH and/or supersaturation state on CaCO<sub>3</sub> polymorph precipitated from low Mg/Ca solutions. Calcite was the only mineral crystallized at low pH (NaHCO<sub>3</sub> doping experiments). It was determined that Mg partition coefficient between calcite and fluid (K<sup>Mg</sup>) negatively correlates with Mg/Ca(<sub>Fluid</sub>) when it exceeds 0.5 mol/mol; no systematic correlation was observed when 0.05< Mg/Ca(<sub>Fluid</sub>)<0.5 mol/mol. </p>
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