Ca cycling in seawater: insights from isotopic studies of elasmobranch teeth and shallow-water carbonates

Abstract

The global calcium (Ca) cycle is intimately linked to that of carbon (C) through carbonate weathering and precipitation of calcium carbonates, processes that impact oceanic alkalinity budget and atmospheric CO$_{2}$, thus influencing global climate on million year time-scales. This dissertation expands our understanding of marine calcium cycling via reconstructions of the Ca isotopic composition of seawater ($\delta^{44}$Ca$_{seawater}$) over 100 million years of Earth history using biogenic apatite found in elasmobranch tooth enameloid. A $\sim$0.5$\permil$ increase in inferred $\delta^{44}$Ca$_{seawater}$ is observed over the Cenozoic, and attributed to the proliferation of pelagic calcifiers starting in the Mesozoic. This shift in the locus of carbonate deposition away from shallow (neritic) environments impacts the dominant carbonate diagenetic regime, reducing the flux of isotopically light Ca to oceans.

The elasmobranch enameloid archive is calibrated by constraining the fractionation associated with the formation of apatitic minerals. By synthesizing abiotic calcium phosphates under a series of temperature and pH conditions, I calculate fractionation factors between 0.3$\permil$~ and 1.0$\permil$~ associated with precipitation of different mineral phases (e.g., hydroxyapatite and brushite). I also present a preliminary evaluation of the controls on internal Ca cycling in elasmobranchs, including the role of diet and trophic level, and homeostatic ionic- and osmo- regulation, and their associated isotopic effects.

Additionally, I offer insight into the behavior of $\delta^{44}$Ca during meteoric diagenesis of shallow-water carbonates by presenting a paired Ca and Mg ($\delta^{26}$Mg) isotope study of Holocene platform carbonates and Pleistocene reef deposits. Coupled with a numerical model of carbonate diagenesis, I show that meteoric diagenesis results in a progressive lowering of bulk sediment $\delta^{44}$Ca (and $\delta^{26}$Mg). This result suggests that Ca isotopes may serve as a robust indicator of meteoric diagenesis in bulk carbonates, and cautions the use of carbonate-based archives in reconstructions of $\delta^{44}$Ca$_{seawater}$.