Publication Date:
2022-05-25
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
Description:
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:
This research was funded by a National Science Foundation (NSF) Graduate Research
Fellowship, NSF grants OCE 1041106, OCE 1338320, and OCE 1220529, by a
thematic project at Academia Sinica, Taiwan, the WHOI Ocean Ventures Fund, and
by the WHOI Coastal Ocean Institute.
Keywords:
Corals
;
Coral reef ecology
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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