Scripps Institution of Oceanography

This dissertation uses calcium isotopes to examine two major geochemical processes: sequestration of Ca in the late Miocene Mediterranean evaporites and Ca cycling in the late Cenozoic oceans. Each chapter focuses on a different part of this work. Chapter I contains information regarding calcium isotope systematics and analytical methods, reproducibility of standard data, double spiking theory, and methods for sample extraction and loading of Ca.

Chapter II explores the idea of large evaporite deposits (Saline Giants) as potential sinks for Ca in the oceans. These may record significant isotopic fractionation effects, which could expand the limited range of Ca isotopes on Earth. No past Ca isotope studies have addressed the range of isotopic variation in Ca-bearing phases of marine, hydrothermal, or continental evaporites. Ca isotope data was obtained for various gypsum and anhydrite samples and an experiment was performed to determine the fractionation factor of inorganically precipitated anhydrite.

In Chapter III, evaporites from ODP Site 654 in the Tyrrhenian Basin were analyzed for Ca and Sr isotopes. This saline giant did not sequester enough Ca to affect global calcium cycling in the late Miocene, and the average isotopic composition overlaps with the average carbonate value. Ca data exhibit a total range of 1.43 per mil, and little correlation between the Ca and Sr isotope systems is apparent. Several samples show significant disagreement, suggesting that Ca isotopes in evaporite deposits may be overprinted by a reservoir effect at times of rapid deposition, whereas Sr isotopes record true brine composition.

Calcium isotopes could shed light on the balance between continental weathering and carbonate sedimentation, and few high resolution records of seawater Ca isotopic variation exist for the late Neogene. Chapter IV reports Ca isotope data for bulk carbonates from ODP Site 850 for the past 12 Ma. Additionally, Ca concentrations were calculated from the isotopic data. The data show no consistent trend towards increasing or decreasing 44Ca for the past 12 Ma, in contrast to other isotope systems such as Os and Sr. Short term fluctuations on the order of 1-2 Ma occur, but these excursions are not uniquely correlated to known paleoceanographic events.