Coral biomineralization, climate proxies and the sensitivity of coral reefs to CO2-driven climate change
Citable URI
https://hdl.handle.net/1912/8550DOI
10.1575/1912/8550Keyword
Corals; Coral reef ecologyAbstract
Scleractinian corals extract calcium (Ca2+) and carbonate (CO2−3) ions from seawater
to construct their calcium carbonate (CaCO3) skeletons. Key to the coral biomineralization
process is the active elevation of the CO2−3 concentration of the calcifying fluid
to achieve rapid nucleation and growth of CaCO3 crystals. Coral skeletons contain
valuable records of past climate variability and contribute to the formation of coral
reefs. However, limitations in our understanding of coral biomineralization hinder
the accuracy of (1) coral-based reconstructions of past climate, and (2) predictions
of coral reef futures as anthropogenic CO2 emissions drive declines in seawater CO2−3
concentration.
In this thesis, I investigate the mechanism of coral biomineralization and evaluate
the sensitivity of coral reef CaCO3 production to seawater carbonate chemistry. First,
I conducted abiogenic CaCO3 precipitation experiments that identified the U/Ca ratio
as a proxy for fluid CO2−3 concentration. Based on these experimental results,
I developed a quantitative coral biomineralization model that predicts temperature
can be reconstructed from coral skeletons by combining Sr/Ca - which is sensitive to
both temperature and CO2−3 - with U/Ca into a new proxy called “Sr-U”. I tested
this prediction with 14 corals from the Pacific Ocean and the Red Sea spanning mean
annual temperatures of 25.7-30.1°C and found that Sr-U has uncertainty of only 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,
and the sensitivity of ecosystem-scale calcification to these changes. On Dongsha
Atoll in the northern South China Sea, metabolic activity of resident organisms elevates
reef-water CO2−3 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 ecosystem calcification. Overall,
my thesis highlights how the modulation of CO2−3, by benthic communities on the
reef and individual coral polyps in the colony, controls the sensitivity of coral reefs to
future ocean acidification and influences the climate records contained in the skeleton.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2017
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Suggested Citation
Thesis: DeCarlo, Thomas M., "Coral biomineralization, climate proxies and the sensitivity of coral reefs to CO2-driven climate change", 2017-02, DOI:10.1575/1912/8550, https://hdl.handle.net/1912/8550Related items
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