Seawater calcium isotopes and the Cenozoic carbonate depositional history of the oceans

Carbonate deposition and dissolution significantly affect oceanic alkalinity, atmospheric CO2 and, ultimately, earth's climate. Determining fluctuations in calcium carbonate (CaCO3) deposition over time, especially over climate transitions, provides important information about how the coupled calcium-carbon biogeochemical system behaves and reveals feedbacks between processes that control it. Such fluctuations may be reflected in the marine calcium (Ca2+) cycle resulting in changes in seawater Ca 2+ concentrations and Ca-isotopic composition. The biological precipitation of CaCO3, the major sink of Ca2+ in the ocean, largely controls the Ca-isotopic ratio in seawater due to discrimination against heavy isotopes during calcification. Reconstructing the seawater Ca-isotopic ratio over time can therefore help quantify the fluctuations in the amount of CaCO 3 deposited in the oceans relative to the input of Ca2+ to the ocean at any given time (assuming some knowledge of the isotopic composition of the sources and sinks).;Work presented in this dissertation focuses primarily on measuring Ca-isotopic ratios in two minerals: calcite (CaCO3) and barite (BaSO4 ). Ca-isotope analyses done on foraminiferal calcite tests contributes to our understanding of the complexity of biomineralization in foraminifera, one of the most commonly used carriers of paleoclimatic and paleoceanographic data. Ca-isotopic data was collected on specimens from sediment trap and coretop samples, and combined with a previously published model of foraminiferal biomineralization to constrain the isotopic composition of the initial biomineralization reservoir. These data are imperative for application of foraminiferal calcite as a recorder of seawater Ca-isotopic composition or paleo-temperature and interpretation of downcore Ca-isotopic records.;The remainder of the dissertation focusses on marine barite (BaSO 4), a minor component of marine sediments, which has been useful as a recorder of seawater chemistry through time. It has advantages over carbonate minerals for studying the seawater Ca-isotopic ratio because of its resistance to diagenesis in oxic pelagic sediments and its uninterrupted record over important climate intervals associated with carbonate dissolution. The Ca-isotopic composition of pristine marine barite was shown to be a recorder of seawater Ca-isotopic composition through time with a constant offset from seawater of -2.01 +/- 0.15‰ (at least for when temporal changes in environmental parameters fluctuated within the range of present day global ocean values).;Variations in reconstructed seawater Ca-isotopes over the past 28 million years from marine barite were then determined at finer than 1 million year resolution (the residence time of Ca2+ in the modern ocean). Results indicate that the marine Ca2+ cycle appears to be more dynamic than previously assumed, revealing previously unrecognized transient features in seawater Ca2+ concentration. The detailed isotope record shows an increase in the concentration of Ca2+ at ∼15 million years which corresponds to a major climatic transition and global change as seen in marine oxygen and carbon isotopes.;Finally, two periods of extreme change in the global calcite compensation depth (CCD) and climate during the Cenozoic, the Eocene-Oligocene Transition (EOT) and the Paleocene-Eocene Thermal Maximum (PETM), were examined at high resolution to gain insight into potential causes of these fluctuations, contributing to our knowledge of the Ca2+ biogeochemical cycle during rapid climate change events.