Abstracts Category : Other

Coral biomineralization, climate proxies and the sensitivity of coral reefs to CO2-driven climate change

by Thomas M DeCarlo

Scleractinian corals extract calcium (Ca2+) and carbonate (CO23) ions from seawaterto construct their calcium carbonate (CaCO3) skeletons. Key to the coral biomineralizationprocess is the active elevation of the CO23 concentration of the calcifying fluidto achieve rapid nucleation and growth of CaCO3 crystals. Coral skeletons containvaluable records of past climate variability and contribute to the formation of coralreefs. However, limitations in our understanding of coral biomineralization hinderthe accuracy of (1) coral-based reconstructions of past climate, and (2) predictionsof coral reef futures as anthropogenic CO2 emissions drive declines in seawater CO23concentration.In this thesis, I investigate the mechanism of coral biomineralization and evaluatethe sensitivity of coral reef CaCO3 production to seawater carbonate chemistry. First,I conducted abiogenic CaCO3 precipitation experiments that identified the U/Ca ratioas a proxy for fluid CO23 concentration. Based on these experimental results,I developed a quantitative coral biomineralization model that predicts temperaturecan be reconstructed from coral skeletons by combining Sr/Ca - which is sensitive toboth temperature and CO23 - with U/Ca into a new proxy called Sr-U. I testedthis prediction with 14 corals from the Pacific Ocean and the Red Sea spanning meanannual temperatures of 25.7-30.1C and found that Sr-U has uncertainty of only 0.5C, twice as accurate as conventional coral-based thermometers. Second, I investigatedthe processes that differentiate reef-water and open-ocean carbonate chemistry,and the sensitivity of ecosystem-scale calcification to these changes. On DongshaAtoll in the northern South China Sea, metabolic activity of resident organisms elevatesreef-water CO23 twice as high as the surrounding open ocean, driving ratesof ecosystem calcification higher than any other coral reef studied to date. Whenhigh temperatures stressed the resident coral community, metabolic activity slowed,with dramatic effects on reef-water chemistry and ecosystem calcification. Overall,my thesis highlights how the modulation of CO23, by benthic communities on thereef and individual coral polyps in the colony, controls the sensitivity of coral reefs tofuture ocean acidification and influences the climate records contained in the skeleton.