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St. Petersburg Coastal and Marine Science Center

Coral Reef Ecosystem Studies (CREST)

Research: Reef History and Climate Change

Corals as Paleo-environmental Archives

A diver uses an underwater drill to take a core sample from a massive brain coral.   A diver uses an underwater drill to take a core sample from a massive brain coral (Diploria strigosa) in Dry Tortugas National Park. [larger version]

In order to better understand how the morphology and ecology of our coastal reefs has changed during the Holocene (past 10,000 years), we are analyzing coral skeletons to document past environmental changes and determine the relationships between the changing environment and coral growth. This information is critical to our understanding of the natural history of coral reefs and will provide clues to the future of reef accretion processes and climate change.

Scleractinian or "hard" corals deposit a skeleton of calcium carbonate (CaCO3), and serve as important geochemical archives for the reconstruction of paleo-environmental conditions on the reef. Mound corals are long-lived (100-300 years) and their skeletons have alternating light and dark layers that result from seasonal changes in growth rates. The couplets of light and dark layers represent annual bands, similar to tree rings, that can be used to determine the age of the coral skeleton. Variations in the chemical composition of the skeleton can be calibrated to environmental parameters such as seawater temperature, salinity, and pH as the coral grows.

Coral Geochemistry

In order to generate a time series of environmental variability, we drill a path of continuous samples along the growth axis of a coral using a computer driven triaxial micro-milling machine. This results in sub-annual sampling with 10-20 samples/year, depending on the coral linear extension rate. Using this approach we can reconstruct centuries-long records of seasonally resolved δ18O and Sr/Ca from long-lived Atlantic coral species such as Siderastraea siderea and Montastraea faveolata.

Inductively Coupled Plasma-Optical Emission Spectrometer
We use a Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) to determine the Sr/Ca in coral aragonite samples in our laboratory at the USGS St. Petersburg Coastal and Marine Science Center. [larger version]
     Using our computer driven triaxial micro-milling machine to subsample a coral slab.
Using our computer driven triaxial micro-milling machine to subsample a coral slab. [larger version]

The Strontium to Calcium ratio (Sr/Ca) of the coral aragonite is related to the water temperature at which a coral calcifies. This is because Sr2+ and Ca2+ are both divalent cations with similar properties (they occupy the same column in the periodic table), so Sr2+ can substitute for Ca2+ in the CaCO3 of coral aragonite. As temperature increases, there is less Sr2+ substitution, and the Sr/Ca ratio decreases. The Sr/Ca to temperature relationship can differ between different species and different colonies of the same species of coral. We have empirically determined the Sr/Ca-temperature calibration equations for both M. faveolata and S. siderea in Dry Tortugas National Park, and applied those calibrations to time series covering the past 100-300 years.

X-ray images are used to better illustrate the annual density bands in corals.
X-ray images are used to better illustrate the annual density bands in corals. Time series of Sr/Ca data are overlain on the x-radiograph to verify that annual bands have been counted correctly. Each Sr/Ca maximum (winter) to maximum (winter) represents an annual temperature cycle. [larger version]
     coral Sr/Ca record
This is a coral Sr/Ca record generated from a Montastraea faveolata colony from Dry Tortugas National Park. The Sr/Ca is calibrated to local instrumental sea surface temperature (SST) to determine a Sr/Ca-SST relationship for this species in the Gulf of Mexico. More information about these data can be found in Flannery et al., 2013. [larger version]

The oxygen isotopic composition (δ18O) of coral aragonite is related to the temperature and the salinity of seawater. We are pairing Sr/Ca and δ18O measurements in corals to make inferences about past changes in temperature and salinity.

We are also exploring additional geochemical proxies in corals (e.g., B/Ca, Ba/Ca, δ11B, Li/Ca), which may tell us more about water nutrients, pH, salinity and temperature.

Publications

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