Quarterdeck 3.3
Quarterdeck 3.3
by Niall C. Slowey and Thomas
J. Crowley
. . .Continued from Part 2
[104K] An x-ray of Montastrea
annularis coral skeletal material from the Flower Garden Banks shows distinct
couplets of high- and low-density annual bands. High-density bands form
during the summer. To find the age of each couplet we count back from the
recently deposited colony surface toward older skeletal material. The distance
between density bands reveals annual growth rates. Stress bands, marked
by the green arrows, are high-density bands that form when corals are subject
to environmental stress.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.
Oceanography, Texas A&M
University
rshatto@ocean.tamu.edu
URL=http://oceanography.tamu.edu/Quarterdeck/QD3.3/Slowey/slowey-c.html
Updated December 20, 1995