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The Mineralogy and Chemistry of Modern Shallow-Water and Deep-Sea Corals

Gabriela Farfan, Ph.D., 2019
Colleen Hansel, Advisor

Under lower seawater aragonite saturation states (Ωsw), coral skeletal health is expected to deteriorate via lower calcification rates, decreased skeletal density, and increased skeletal dissolution. By characterizing the crystallography and chemistry of coral skeletons, we observe similarities between coral aragonite and synthetic aragonites precipitated from seawater. This suggests that coral aragonite formation is likely controlled by geochemical, rather than biological processes. Geochemical controls over crystal formation likely translates into higher sensitivity of coral skeleton formation to surrounding seawater chemistry. By tracing the crystallography, mineralogy and element trace-incorporation chemistry of deep-sea corals growing under a natural Ωsw gradient (1.15–1.44) in the Gulf of Mexico and shallow-water corals growing across a natural Ωsw (2.3–3.7) gradient in Palau, we observe minor shifts in aragonite crystal structure, likely governed by skeletal calcite contents, density, and Ω of the coral calcifying fluid modified from seawater. Yet, overall, coral aragonite in these natural systems remain mineralogically resilient across these modest Ωsw gradients. Further work is required to determine whether the modest crystallographic shifts I observe are representative on a global scale and whether they could influence bulk skeletal material properties.