The Sr/Ca ratios in calcium carbonate fossils are used by the paleooceanographic community to infer past environmental conditions, such as sea surface temperature and ocean chemistry. The processes of biogenic calcification that produce these chemical signatures are complex and not fully understood, however, and vital effects are known to affect the trace element composition of the CaCO₃ biomineral products. The recent discovery that calcifying organisms produce amorphous calcium carbonate (ACC) as an intermediate phase during the crystallization process calls into question whether this pathway to mineral formation affects trace element distributions in the final product. This non-classical mineralization process raises the question of whether the Sr/Ca ratios of the final products are dependent upon temperature. That is, what is the temperature dependence of Sr/Ca ratios in calcite produced via ACC compared to the measurements obtained from calcite grown by the classical process in laboratory experiments and from biogenic settings.
The goal of this study is to determine the effects of solution chemistry and temperature on the Sr composition of ACC and resultant crystalline CaCO₃. Two types of experiments were designed: First, experiments were conducted to synthesize inorganic ACC in a batch reactor for a suite of selected chemical compositions and allowing this intermediate phase to transform into calcite in the reactant solution. In a second series of experiments, ACC was precipitated by a flow-through method to compare results to the batch reactor experiments. The experimental design focused on determining the Sr/Ca ratio and Sr distribution coefficients (KD, Sr) of the amorphous and final crystalline products. Mg/Ca ratios of 5/1 were found to suppress Sr uptake into ACC by a factor of 25% when the initial Sr solution had concentration of one millimolar. ICP-AES data collected across the 18° to 30°C range showed that the Sr/Ca ratio in both ACC and the resultant calcite was independent of temperature. Upon transformation, the Sr/Ca ratios of both the ACC and calcite product were found to be similar, showing that Sr/Ca ratios were independent of the transformation process. Analysis of the data determined KD, Sr values of 0.564(±0.006) for ACC and 0.466(±0.009) for the resultant calcite in the 18-30°C temperature range.
The findings show that the Sr/Ca ratios of ACC and the transformed calcite are independent of temperature. However, the corresponding KD, Sr values exceed those reported for calcite grown by classical processes by an order of magnitude. The findings for the inorganic calcite yield KD, Sr values up to four times higher than those found in biogenic calcites. Because the findings of this study show that Sr/Ca is independent of temperature, this study calls into question whether previously reported Sr/Ca measurements in biogenic calcites should be revisited. It is plausible that biological factors have a significant influence on trace element incorporation into biogenic calcite. Vital effects, such as the influence of macromolecules during the ion uptake process, may regulate the apparent Sr/Ca versus temperature trends observed in marine paleontology. Higher KD, Sr values in marine calcifiers may indicate that organisms use the non-classical mineralization pathway in whole or in part. Future studies of trace element incorporation in calcifying species should consider the pathway to mineralization in tandem with interpretations of environmental controls on distribution coefficients. / Master of Science
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/23714 |
| Date | 15 August 2013 |
| Creators | Angel, Adam M. |
| Contributors | Geosciences, Dove, Patricia M., Rimstidt, J. Donald, Gill, Benjamin C. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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