Interpreting the effect of environmental conditions on elemental partitioning in biogenic carbonates within the framework of Rayleigh fractionation
The elemental composition of calcium carbonate (CaCO3) precipitated by marine invertebrates varies with the physico-chemical properties of seawater, and is thus used for reconstructing past oceanic conditions, including temperature, pH, and seawater composition. Previous studies have used a Rayleigh fractionation model to interpret elemental partitioning within a range of calcitic and aragonitic organisms. This thesis contains analyses of elemental ratios for multiple marine invertebrates that were cultured from seawater under variable conditions using both new unpublished results and data from the literature. The partitioning of strontium, magnesium, barium, manganese, boron, lithium and uranium within these biogenic carbonates, and data from the literature for foraminifera and coccolithophores, are examined within a Rayleigh fractionation framework.
Application of a Rayleigh model to these systems requires constraints on elemental partitioning into inorganic CaCO3. A survey of the literature shows that for some elements (e.g., Mn), there are very little data, while for others (e.g., Sr, Mg), there are a wide range of reported values. Given this uncertainty, I examine the impact that varying the nominal inorganic partition coefficient has on Rayleigh-derived estimates of calcium-utilization during calcification. The elements examined in this thesis have different chemical and ionic properties (atomic mass, ionic radius, charge), and these properties impact how readily the element can be incorporated into the CaCO3 lattice by replacing its constituents. A Rayleigh model allows one to examine how open the calcification system of an organism is, as well as how much calcium is remaining in the calcification reservoir after biomineralization is complete. I show that organisms that are evolutionarily similar to each other behave similarly in this model, and therefore are likely to use similar mechanisms for biomineralization; for example, organisms from the Crustacea phylum calcify from a relatively closed reservoir with a slow flushing rate, while organisms of the Mollusca phylum calcify from an open calcification pool and have a very fast flushing rate. There are still a few problems with the use of a Rayleigh model in the context of biological calcification; these issues are discussed in-depth throughout the study. An overview of the effects of changing pCO2 and temperature on element partitioning in these invertebrates is examined. I used a combined understanding of organism physiology and Rayleigh fractionation geochemistry to elucidate how certain groups of marine calcifiers control their calcification, and how this control can influence elemental ratios in calcium carbonate that are sometimes used as paleo-proxies.