Coral Layers Good Proxy for Atlantic Climate Cycles

San Francisco, Calif. – Tree rings may tell how old a tree is, but the rings or annual bands in some skeletal coral may tell not only the age of the animal, but also something of the dynamics of the ocean in which it grew, according to Penn State and University of Miami researchers.

“Some coral grows like a tree; each year a complete layer with both a high and low-density skeletal calcium carbonate band is formed by the coral animal,” says Dr. Lisa Greer, assistant professor of geosciences. “Not all corals create rings, but the massive corals like boulder star coral or pin cushion coral do.”

Greer, and Dr. Peter Swart, professor at the Rosenstiel School of Marine and Atmospheric Science, University of Miami, looked at both modern and fossil corals to see if the calcium carbonate of the coral skeletons could shed light on the temperature and salinity of the tropical Atlantic Ocean.

“In contrast to research in the tropical Pacific, there have been few definitive studies utilizing proxy records within Atlantic coral skeletons to provide information on the climate dynamics in this region,” Greer told attendees at the fall meeting of the American Geophysical Union today (Dec. 7) in San Francisco.

Information on the climate dynamics of the Atlantic is necessary as input to global and regional climate models. Not only are the current dynamic patterns important, but patterns from the past, which may differ from today, are important as well.

Greer uses a dental x-ray to locate the coral layers in a very thin slice taken from the core. In modern coral, she then counts down from the top to associate the layers with actual years. The sample she reported on was taken in 1994 and went back to 1935. Once the layers are located, Greer takes very small samples of the coral in a line running up the coral section. She averages about 22 samples per year covering the 59-year span.

Greer and Swart were investigating the relationship between non-radioactive or stable isotopes of oxygen in the calcium carbonate of the coral. The ratio of normal to heavier oxygen depends on the temperature of the water the coral grew in and its salinity. In warmer water, the coral incorporates more of the normal oxygen into its structure, but if the water is cooler, the coral will incorporate a higher percentage of the heavier oxygen isotope.

The salinity effect has more to do with what is available. Fresh water has relatively less of the heavier isotope of oxygen. The saltier the water, the more of the heavier isotope is in the water and the more that is available for incorporation into the coral skeleton.

“When we look at the averaged annual data from 1935 to about 1994, we see it has the shape of a sine wave,” says Greer. “It is periodic and has a significant pattern of oxygen isotope composition that has a peak at about every 12 to 15 years.”

Many people have measured the sea surface temperature in the subtropical Atlantic. They find that peak warming comes about every 12.5 years.

“The recorded temperature data shows an important node at 12 to 15 years, but there is no way to pick it up except for the last 50 years when sea surface temperatures were recorded.”

Because temperature data only exists for the last 50 years, Greer is also looking at fossil corals taken from the western portion of the Dominican Republic. She can date these corals to an accuracy of up to plus or minus 44 years using an uranium/thorium method. The fossil corals she is looking at are the same species, Montastraea annularis, as the modern samples she studies and these fossil corals also show a layered pattern that indicates a cycle, but this cycle is 16 to 20 years long.

“This longer cycle could simply show that conditions have changed slightly over the past 7,000 years,” says Greer. “However, not all corals show these cycles and one has to pick corals that are in the right place at the right time.”

Greer’s modern samples are taken from the Parque National del Este off the coast of the Dominican Republic. She and Swart use a method of collection that ensure no harm comes to the living coral. They remove a 2-inch diameter core from the coral and then fill the hole to ensure nothing can get inside and erode the skeleton. The surface is finished so that within approximately six years, new coral begins to grow over the surface. They even attach a platform to the coral before they drill so that their hands do not damage the animals.

“The only part of the coral that is living is a few millimeters on the top of the core,” says Greer. “This is quickly replaced once the hole is filled.”