This thesis is concerned with the measurement and interpretation of isotope shifts. The first part describes an experimental determination of isotope shifts in the 4s^{2 1}S<subo</sub> - 4s5s¹ S_o transition of neutral calcium. This measurement was performed using the method of Doppler-free two-photon absorption spectroscopy in an atomic beam of the natural mixture of calcium isotopes. The laser was stabilized and scanned by means of an external, pressure-swept etalon, and its scan was calibrated by recording the transmission fringes of the laser through a confocal etalon, 2m in length. This calibration etalon was stabilized with reference to an I₂-stabilized Helium- Neon laser. The isotope shifts of all the stable isotopes of calcium were measured; the results are believed to be free from systematic error and so the standard deviations are derived from the scatter of the results. A Isotope Shift ^δv^A-44/MHz 40 -1169.99(28) 42 -563.66(7) 43 -263.67(6) 46 534.34(70) 48 1023.50(7) These results are compared with other accurate measurements of isotope shifts in calcium by laser spectroscopy, by means of the King plot. All the measurements are consistent except for the only odd isotope ⁴³Ca, which, in this transition, unlike those in the other experiments, is free from hyperfine structure. The best values of the isotope shifts are thus determined by a least squares procedure. The mass and field shifts are separated using the values of δ<r²> determined from muonic isotope shifts, and this leads to more accurate values of δ<r²>. The second part of the thesis concerns the interpretation of anomalous isotope shifts in samarium. Pairs of spectral lines had been found for which the shifts of even-even isotopes did not lie on a straight line on a King plot. These deviations are interpreted as evidence for the insufficiency of first-order perturbation theory for the treatment of isotope shifts in a number of levels of the samarium spectrum. By comparing shifts in transitions connecting these levels with shifts in transitions connecting levels for which first-order perturbation theory appears to be a good approximation, values of off-diagonal matrix elements are found. These are not required to be unreasonably large to explain the observed deviations.

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Over the course of my PhD, I developed the analytical capability to measure calcium isotopes at the University of Cambridge and analyzed calcium isotopes in a range of fluids and minerals in modern and past environments. After discussing my methodological development, I report my work in the glacial precursor to the Dead Sea. Here, the calcium isotope composition of aragonite varies synchronously with lake level fluctuations over the last 70,000 years. My numerically reconstructed lake calcium budget led me to conclude that 20,000 years ago, the Dead Sea was wetter than modern conditions, rather than colder and drier, as had previously been proposed. The primary focus of my PhD research was understanding the sedimentary sink for carbon. The formation of authigenic carbonate in marine sediments is caused by the microbial degradation of organic matter. I used pore fluid measurements and a numerical model to identify zones of authigenic carbonate precipitation. This knowledge was then combined with an understanding of the microbial processes that occur beneath the seafloor to determine the link between microbial activity and authigenic carbonate formation. Two processes, sulfate reduction and the anaerobic oxidation of methane were determined to be the main drivers of authigenic carbonate precipitation. In order to assess the importance of the carbon isotope signal imparted by each of the two identified processes, I created Artificial Neural Networks to predict the areal extent of authigenic carbonate precipitation and the dominant microbial process driving the precipitation. My ANNs identified that 37% of the modern seafloor is precipitating authigenic carbonate, which leads to a flux of 1.2*10^12 moles of carbon per year, of which 88% is due to the anaerobic oxidation of methane, and 12% is due to sulfate reduction. This represents 2-3% of the modern global carbon deposition, however I was able to show that this would be significantly higher in the geological past when ocean conditions were vastly different to how they are today. Finally, I conclude by proposing that some variations in the global carbon cycle in the past can be explored by linking marine calcium concentrations to authigenic carbonate formation and the flux of alkalinity from the seafloor.

The applications of calcium-isotope measurements in marine carbonates are explored in several different contexts within this thesis. As a record of global ion fluxes, seawater calcium-isotope ratios can be used as tracers for large weathering imbalances, which develop as a feedback system in response to intervals of climate change. This approach provides valuable constraints on the complex climatic and oceanographic phenomena known as the Oceanic Anoxic Events. Over much longer timescales, the calcium-isotope ratio of seawater is influenced by steady-state processes that reflect the evolution of seawater chemistry. To understand these influences, the modern calcium-isotope budget is assessed quantitatively using a compilation of marine carbonate samples, revealing several distinctive components of the carbonate burial sink that can affect the steady-state balance of the calcium cycle. Changes in the major ion composition of seawater and in the organisms that contribute to sedimentary carbonate burial are shown to contribute significantly to the geological record of seawater calcium-isotope ratios. The importance of skeletal carbonate in the calcium cycle leads to another application of calcium isotopes towards understanding biomineralization. This large and complex topic is approached with calcium-isotope data from two unique growth experiments that constrain some of the mechanisms by which biogenic aragonite acquires its geochemical signatures. This range of topics presents a diverse, but by no means exclusive, sample of the topics that are accessible for investigation through calcium-isotope analysis. The potential of this isotopic tool is demonstrated by the breadth of environments and timescales represented in this work.

Calcium isotope ratios (⁴⁴Ca/⁴²Ca) have shown promise as a milk dietary tracer. Previous studies have focused on bone but, due to homeostatic processes, &delta;^44/42Ca values are highly variable. This has greatly complicated the identification of mammal milk consumption through bone analysis, resulting in a search for an alternative. This thesis describes controlled studies to assess the effects of milk consumption on &delta;^44/42Ca values in bulk and sequential samples of dental enamel, using modern samples from Yorkshire, England, the Isle of Hoy, Scotland, and archaeological samples from Abu Hureyra, Syria. The samples from Yorkshire consisted of dental enamel, ewe milk, feed and bone. Dental enamel was sampled from the Hoy sheep. Milk and plants are the greatest contributors of calcium in the mammal diet. Feed and ewe milk samples confirmed that milk has lower &delta;^44/42Ca values than plants in the diet, and with a common diet between ewes, uniform milk &delta;^44/42Ca values are produced. Also, there is a significant difference between bulk molar enamel &delta;^44/42Ca values between males and females, with males having higher values. Additionally, analysis of bulk and sequential samples of Hoy mature sheep molar enamel, although mass fractionation effects were small, produced &delta;^44/42Ca values with a clear isotopic offset between the enamel likely formed during nursing, weaning and the full conversion to a plant diet. The &delta;^44/42Ca values, once associated with dental development chronologies, led to the identification of a clear pattern of nursing and weaning in the enamel of the first and second molars of sheep with known dietary and weaning histories. The modern results were used to identify patterns of ancient nursing and weaning. Although evidence was insufficient to establish early weaning of the Abu Hureyra sheep, this research successfully established the potential of this approach for investigating milk production and consumption in the archaeological record.

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abstract: Applications of non-traditional stable isotope variations are moving beyond geosciences to biomedicine, made possible by advances in multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) technology. Mass-dependent isotope variation can provide information about the sources of elements and the chemical reactions that they undergo. Iron and calcium isotope systematics in biomedicine are relatively unexplored but have great potential scientific interest due to their essential nature in metabolism. Iron, a crucial element in biology, fractionates during biochemically relevant reactions. To test the extent of this fractionation in an important reaction process, equilibrium iron isotope fractionation during organic ligand exchange was determined. The results show that iron fractionates during organic ligand exchange, and that isotope enrichment increases as a function of the difference in binding constants between ligands. Additionally, to create a mass balance model for iron in a whole organism, iron isotope compositions in a whole mouse and in individual mouse organs were measured. The results indicate that fractionation occurs during transfer between individual organs, and that the whole organism was isotopically light compared with food. These two experiments advance our ability to interpret stable iron isotopes in biomedicine. Previous research demonstrated that calcium isotope variations in urine can be used as an indicator of changes in net bone mineral balance. In order to measure calcium isotopes by MC-ICP-MS, a chemical purification method was developed to quantitatively separate calcium from other elements in a biological matrix. Subsequently, this method was used to evaluate if calcium isotopes respond when organisms are subjected to conditions known to induce bone loss: 1) Rhesus monkeys were given an estrogen-suppressing drug; 2) Human patients underwent extended bed rest. In both studies, there were rapid, detectable changes in calcium isotope compositions from baseline - verifying that calcium isotopes can be used to rapidly detect changes in bone mineral balance. By characterizing iron isotope fractionation in biologically relevant processes and by demonstrating that calcium isotopes vary rapidly in response to bone loss, this thesis represents an important step in utilizing these isotope systems as a diagnostic and mechanistic tool to study the metabolism of these elements in vivo. / Dissertation/Thesis / Ph.D. Chemistry 2011

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Ce travail de thèse a appréhendé les mécanismes de stockage/libération, d’un point de vue élémentaire et isotopique, du calcium (Ca) et du strontium (Sr) sur ou dans des phases minérales communes des sols (minéraux primaires, minéraux argileux, oxy-hydroxydes, carbonate pédogénétiques). Une méthode de mesure robuste du δ88Sr a dû être développée et validée par la mesure de matériaux de références internationaux, pour la plupart jamais mesuré dans la littérature. Les résultats montrent que l’adsorption du Ca sur des minéraux phyllosilicatés génère un fractionnement isotopique par prélèvement préférentiel de l’isotope léger (40Ca) dans nos conditions expérimentales lorsque les minéraux possèdent une charge structurale et une surface spécifique importante et/ou un espace interfoliaire ouvert à l’adsorption de cations hydratés. Une étude sur des rhizolithes en milieu naturel a mis en avant que l’utilisation des isotopes du Ca et du Sr permet un traçage de source et de mécanismes efficaces dans les thématiques touchant au stockage de ces deux éléments dans les sols. / This PhD thesis examined the mechanisms of storage and release, from and elementary and isotopically point of view, of calcium (Ca) and strontium (Sr) onto or into mineral phases commonly encountered within soils (primary minerals, clay minerals, oxy-hydroxides, pedogenic carbonate). A robust δ88Sr measurements method had to be developed in the laboratory and validated by the measurement of international reference materials, mostly never measured in the literature. The results of this work show that Ca adsorption onto phyllosilicate minerals generates a quantifiable isotopic fractionation by preferential uptake of the light isotope (40Ca) under our experimental conditions when the minerals have a significant structural charge and specific surface area and/or an interlayer space open to hydrated cations adsorption. A study on rhizoliths in natural environment has also highlighted that the combination of Ca and Sr isotopes allows an effective tracing of sources and mechanisms in the problematic related to the storage of these two elements within soils.