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CORAL CORES OFFER NEW RECORD OF CLIMATE
Coral reefs have long been known for their unparalleled beauty and for serving as home to a wide variety of fishes and plants. Now scientists at UCSD's Scripps Institution of Oceanography are discovering that the reefs also hold a precise record of past climate.
Christopher Charles, a professor in the Scripps Geosciences Research Division, reports in the Aug. 15 issue of the journal Science that a coral core sample taken from the Indian Ocean contains a detailed record of sea surface temperatures in the region over the past 150 years. While researchers have taken instrument temperature readings in the Indian Ocean over the past few decades, the core represents the first long-term detailed record of regional sea surface temperatures.
"Climatologists have not taken advantage of the wealth of information that coral has to offer," Charles said. "Some of the climate trends in instrumental records that are now the source of heated arguments -- for example, whether the tropics have experienced greenhouse warming or whether they undergo climate fluctuations over periods of decades -- could be resolved by analyzing the coral records."
Charles' research team, which includes Michael Moore and graduate student David
Hunter, recently retrieved a 10-foot core from the Seychelles, a group of islands in the southwestern equatorial Indian Ocean. When Charles compared swings in the temperature record with those obtained in Pacific coral by researchers at Columbia University's Lamont-Doherty Earth Observatory, they revealed similar interannual changes in temperature, suggesting a link in climate cycles between the two ocean basins.
"What we were looking for was a link between monsoon events in the equatorial Indian Ocean and El NiÃ’o/Southern Oscillation events," Charles said. "While a general connection between the monsoon and El NiÃ’o has been recognized for many years, the nature of the connection has remained something of a mystery. Our coral captures a long record of both phenomena, and that offers a way to investigate the climate processes involved."
El NiÃ’o/Southern Oscillation events are a periodic climatic condition that warms Pacific Ocean surface temperatures along a broad band of the equator and shifts wind patterns that cover a quarter of the globe.This can change worldwide weather patterns, sometimes dramatically. For example, broad areas of Southeast Asia experience drought, and monsoons tend to be weak during El NiÃ’o periods.
The coral indicated that these interannual climate fluctuations have appeared in tandem between the Pacific and Indian Oceans for over a century, but the Seychelles core also revealed that sea surface temperatures in the equatorial Indian Ocean have been warming and cooling on a ten-year cycle for the past 150 years.
"In terms of societal importance, these decadal cycles are probably among the most important issues that we have to deal with," Charles said. "People can generally cope with fluctuations in climate on a year-to-year basis, but events such as prolonged droughts are much more difficult to respond to."
The data indicate that the decadal cycles in the tropical Indian Ocean are linked to the behavior of the Asian monsoon. The simple recognition that such cycles exist and that they are transmitted around the Earth should help improve the accuracy of computer climate models, Charles said.
"Researchers have long realized that if you could predict on the basis of the El NiÃ’o cycle what would happen to the Asian monsoon, then you could go a long way toward predicting vitally important climate change," he said. "However, our coral record suggests that the opposite may be true -- that the monsoon may stamp its signature on the tropical ocean."
Charles and his research team of paleoclimatologists are among a small group of oceanographers who in the 1980s began using corals to study past climates. They extract long cores of coral ranging between 1 and 4 inches in diameter in order to retrieve details of past tropical ocean conditions in much the same way that researchers study tree rings in order to glean information about ancient climate over continents.
Because corals grow close to the sea surface, they provide excellent records of tropical air and sea conditions. Such data is of extreme importance to climatologists because the tropical oceans play a central role in the global climate machine as regulators of heat and moisture exchange between massive areas of the sea surface and atmosphere. This interaction creates and fuels great ocean circulations and wind patterns that generate and push weather systems around the world.
Charles' team scuba dives into shallow reef waters and use a hydraulic drill to take a core sample, most of which is made up of deep fossilized coral that is no longer living (living tissue is confined to the uppermost half inch of the coral's structure). The researchers are careful not to damage the surface of the coral colony during drilling. They plug the hole and the small drill site is soon overgrown by the surrounding coral.
The coral cores clearly display annual growth bands similar to those found in trees that can be counted from the present surface layer to earlier times. The banded layers alternate in density with more porous layers being formed by warmer water and denser layers formed by cooler water.
Back in the laboratory, the cores are X-rayed to image the growth bands, which can then be divided into seasons and even months. While measuring the relative thickness of layers within the bands distinguishes favorable and unfavorable ocean conditions for growth,the composition of individual layers holds even more quantitative data about past climates.
"Instead of dealing with complex laws of biology and ecology that dictate whether or not a coral is growing well, we are looking at very precise chemical measurements that depend entirely on the conditions of the seawater," Charles said.
Using a mass spectrometer, the researchers measure the concentrations of stable isotopes and other geochemical tracers found in coral skeletons. The most significant is the ratio of heavy oxygen (18O) to light oxygen (16O) in a sample, which is established by the conditions of the seawater when the coral was growing. The ratio depends mainly on water temperature -- the warmer the water, the less 18O in the skeleton. This measurement also can be used to determine past precipitation in regions of the tropics with heavy rainfall.
One of Charles' long-term goals is to use coral fossils to track changes in climates over the past few thousand years.
"What I am most interested in is using corals to track long-term trends in climate, for example, over ice age cycles," Charles said. "We are only beginning to tap into the potential of what these fossilized records can tell us."
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