Understanding Geochemical Tracers in Deep-Sea Corals from a Biomineralization Perspective

Deep-sea corals have been developed as a useful archive of the chemistry and circulation of intermediate and deep waters in past oceans over the last three decades. However, applications of traditional paleoceanographic tracers in deep-sea corals remain a challenge due to our incomplete understanding of the biomineralization mechanisms underlying the incorporation of these tracers and their variabilities in the coral skeletons (a.k.a. the "vital effects"). In this thesis, an effort was made to understand the vital effects associated with the stable isotope as well as minor and trace element compositions of the aragonitic skeletons of the deep-sea coral species Desmophyllum dianthus, through a combination of empirical observations and a numerical model of coral calcification. Observations of the chemical and isotopic compositions of the coral skeletons were performed on four different spatial scales in a suite of modern D. dianthus specimens: bulk samples, micromilled samples, SIMS and nanoSIMS. These observations reveal tracer correlations in deep-sea corals that are coherent over different spatial scales and point toward a universal mechanism of the incorporation of these tracers through the biomineralization process. A few tracers emerge as promising proxies for the temperature (Li/Mg, Sr/Ca) and carbonate chemistry (U/Ca, B/Ca, Ba/Ca) of the oceans. The numerical model for coral calcification explains the strong δ18O and δ13C vital effects in individual deep-sea corals with an updated physicochemical basis, and carbonic anhydrase is found to play a key role in setting the slopes of the strong δ18O-δ13C correlations in different biogenic carbonates. The model also constrains the key physical parameters in the biomineralization process and is extended to explain the observed minor and trace element variabilities and correlations in deep-sea corals. The model can qualitatively explain the observed correlation patterns between Mg/Ca, Li/Ca, B/Ca and Sr/Ca in the coral skeletons, but quantitative data-model comparison is limited by both deficiencies in high-quality data and a lack of a well-constrained inorganic reference frame for aragonite. Future improvements in the geochemical tracers in biogenic carbonates will benefit from more extended empirical calibrations as well as a more complete mechanistic understanding of the key physicochemical and biological processes underlying the incorporation of tracers.