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Historical Records of Tropical Surface Ocean pH, Aragonite Saturation, 
and Sea Surface Temperature Recorded in Coral Skeletons: Ryan P. Moseman


Project Title: Historical Records of Tropical Surface Ocean pH, Aragonite Saturation, and Sea Surface Temperature Recorded in Coral Skeletons
Mendenhall Fellow: Ryan P. Moyer, (727) 803-8747, x 3030, rmoyer@usgs.gov
Duty Station: St. Petersburg, FL
Start Date: January 5, 2009
Education: Ph.D. The Ohio State University, School of Earth Sciences, 2008–Earth Science (major); Marine Biogeochemistry, Carbon Cycling (areas of specialization)
Research Advisors: Kim Yates, (727) 803-8747, kyates@usgs.gov

Project Description: The ocean is the ultimate sink for excess anthropogenic CO2 in the atmosphere, and this has had drastic impacts on seawater chemistry. As the amount of CO2 dissolved in seawater increases, oceanic pH decreases. It is estimated that the surface oceans may have already experienced a reduction of 0.1 pH units since pre-industrial times (Caldeira and Wickett, 2003).

Corals are excellent recorders of such changes because they deposit calcium carbonate skeletons in annual bands and can grow for several centuries. Cyclic variations in coral skeletal density (fig. 1) within the growth record of corals are evident on X-radiographs (Buddemeier and others,1974), which can be combined with isotope and (or) trace metal geochemistry of the coral skeleton to serve as proxies for a host of paleo-environmental events and conditions (Druffel, 1997; Grottoli, 2001; Grottoli and Eakin, 2007). Of these proxies, several have direct relevance to research addressing the question of coral response to increased anthropogenic CO2 and decreasing ocean pH. Stable isotopes of Boron (δ11B) record changes in seawater pH (Hönisch and others, 2004) and coral skeletal density as inferred from X-radiographs record relative changes in growth and calcification (Chalker and others,1985; Barnes and Lough, 1996) over the lifespan of the coral.

Coral-based paleo-pH records have been successfully produced using a coral from the Great Barrier Reef (Pelejero and others, 2005), and the authors of that study concluded that “Additional paleo-pH records are required from a range of coral reef ecosystems to improve our understanding of the physical and biological controls on reef-water pH, and the long-term impact of future ocean acidification.” My research will use a multi-proxy approach in modern coral cores (fig. 1) taken from Florida and the Caribbean Sea region in order to determine the relationship between paleo-variations in seawater pH, temperature, and coral growth and calcification. This study will not only address the problem of ocean acidification in a geological context but will also provide data that can be used in models to help better predict the response of coral reef ecosystems to increased atmospheric CO2.

Broader Impacts

Coral reefs are uniquely complex ecosystems in that they are defined by geological structures (“reefs”) that are built primarily by calcifying biological organisms (primarily coral and algae). Coral reefs are one of the most important ecosystems on Earth. Along with exhibiting the highest biodiversity of any known marine ecosystem, they also provide critical habitat for many fish and invertebrate species that are of great commercial importance worldwide. However, a recent increase in a combination of anthropogenic and climatic stresses has resulted in degradation and near collapse of many coral reef ecosystems worldwide. Focused and coordinated science efforts are needed to understand the complex physical, chemical, and biological processes and interactions that are impacting coral reefs and their ability to respond to changing conditions, including ocean acidification. Such information will effectively guide policies and best management practices in order to preserve coral reef resources for future generations.

  Diver collecting a coral core (left) and X-radiograph positive of the coral skeleton with information on skeletal density overlain on the x-radiograph (right)

 

Figure 1. Diver collecting a coral core (left) and X-radiograph positive of the coral skeleton with information on skeletal density overlain on the x-radiograph (right).


References

Barnes, D.J., and Lough, J.M., 1996, Coral skeletons: Storage and recovery of environmental information: Global Change Biology, v. 2, p. 569–582.

Buddemeier, R.W., Maragos, J.E., and Knutson, D.W., 1974, Radiographic studies of reef coral exoskeletons: Rates and patterns of coral growth: Journal of Experimental Marine Biology and Ecology, v. 14, p. 179–200.

Caldeira, K., and Wickett, M.E., 2003, Anthropogenic carbon and ocean pH: Nature, v. 425, p. 365.

Chalker, B., Barnes, D., and Isdale, P., 1985, Calibration of X-ray densitometry for the measurement of coral skeletal density: Coral Reefs, v. 4, p. 95–100.

Druffel, E.R.M., 1997, Geochemistry of corals: Proxies of past ocean chemistry, ocean circulation, and climate: Proceedings of the National Academy of Science USA, v. 94, p. 8354–8361.

Grottoli, A.G., 2001, Climate: Past climate from corals, in Steele, J., Thorpe, S., and Turekian, K., eds., Encyclopedia of ocean sciences: London, Academic Press, p. 2098–2107.

Grottoli, A.G., and Eakin, C.M., 2007, A review of modern coral δ18O and Δ14C proxy records: Earth Science Reviews, v. 81, p. 67–91, doi: 10.1016/j.earscirev.2006.10.001

Hönisch, B., Hemming, N.G., Grottoli, A.G., Amat, A., Hanson, G.N., and Bijma, J., 2004, Assessing scleractinian corals as recorders for paleo-pH: Empirical calibration and vital effects: Geochimica et Cosmochimica Acta, v. 68, p. 3675–3685.

Pelejero, C., Calvo, E., McCulloch, M.T., Marshall, J.F., Gagan, M.K., Lough, J.M., and Opdyke, B.N., 2005, Preindustrial to modern interdecadal variability in coral reef pH: Science, v. 309, p. 2204–2207.

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