by Niall C. Slowey and Thomas J. Crowley
. . .Continued from Part 2
Corals are sensitive monitors of the marine environment. Their calcium
carbonate skeletal material preserves a detailed record of past environmental
conditions which can be used to reconstruct the history of climate variability
and understand its impact on the marine environment. We focus upon two aspects
of coral skeletons to learn about the climate of the past-their density
bands and the proportions of the isotopes of oxygen they contain.
Seasonal changes in environmental factors, particularly water temperature, cause variations in skeletal extension, density, and calcification, resulting in the formation of distinct pairs of high- and low-density annual bands in certain coral species. High-density bands may also form on a sub-annual basis in response to winter cold-water stress. Annual bands allow the precise age and growth rate of various portions of the coral to be determined. Changes in growth rate reflect changes in environmental conditions while stress bands indicate times when particularly extreme conditions existed. Fluctuations in the stable oxygen isotopic composition (18O/16O ratio) of a coral skeleton reflect changes in the temperature and isotopic composition of the water in which the coral grew.
Corals grow rapidly enough that many samples can be taken from each annual band for isotopic analysis, allowing a precise record of environmental conditions to be reconstructed for nearly each month in a given year. The record obtained from a single long-lived coral can span several centuries.
Investigations of the growth and isotopic compositions of corals in the equatorial Pacific have demonstrated that they provide an invaluable historical perspective on climatically controlled fluctuations of sea-surface temperature and rainfall there. We use records of past environmental change preserved in the corals living in the Gulf of Mexico to reconstruct the history of past interannual and interdecadal changes in the PNA pattern. Features of the coral reefs at the Flower Garden Banks make them almost ideally suited for this purpose.
The banks are the northernmost tropical reefs on the Atlantic continental shelf. They are located 180 kilometers off the Texas-Louisiana coast at the edge of the shelf (27.9°N, 93.7°W) where sea-surface temperatures range seasonally from about 18° to 30°C. The reef crests, about 20-26 meters deep, are dominated by Montastrea, Diploria, and Porities species of corals. The geographic location of the Flower Gardens is sensitive to PNA pattern-related changes in climate. Moreover, while most coral reefs are located in nearshore waters, the Flower Garden Banks are located at the edge of the Texas-Louisiana continental shelf. They are among the only reefs that grow in and preserve a record of typical open-ocean conditions in the Gulf of Mexico. A coralline isotopic record that is not influenced by local coastal processes is necessary to clearly relate changes in the near-surface hydrographic conditions (predominantly temperature) of the gulf caused by seasonal variation and changes in the PNA pattern. The striking correspondence between interdecadal changes in coral growth rates and the PNA pattern during the past century demonstrates the monitoring potential of Flower-Garden corals.
Updated December 20, 1995