Coral Biomineralization, Climate Proxies and the Sensitivity of Coral Reefs to CO2-Driven Climate Change

Thomas DeCarlo, Ph.D., 2017
Anne Cohen, Advisor

In this thesis, I investigate the mechanism of coral biomineralization and the sensitivity of coral reef calcium carbonate (CaCO3) production to seawater carbonate chemistry. First, I conducted abiogenic CaCO3 precipitation experiments that identified the U/Ca ratio as a proxy for fluid carbonate ion (CO32-) concentration, and based on these results I predicted that coral skeleton Sr/Ca and U/Ca can be combined into a new temperature proxy called “Sr‑U”. Using 14 Indo-Pacific corals spanning mean annual temperatures of 25.7-30.1 °C, I found that Sr‑U has uncertainty of 0.5 °C, twice as accurate as conventional coral-based thermometers. Second, I investigated the processes that differentiate reef-water and open-ocean carbonate chemistry. On Dongsha Atoll in the South China Sea, metabolic activity of resident organisms elevates reef-water CO32- twice as high as the surrounding open ocean, driving rates of ecosystem calcification higher than any other coral reef studied to date. When high temperatures stressed the resident coral community, metabolic activity slowed, with dramatic effects on reef-water chemistry and calcification. My thesis highlights how the modulation of CO32-, by benthic reef communities and by individual coral polyps, controls the sensitivity of coral reefs to future ocean acidification and influences the climate records contained in the skeleton.