Coral Cores Yield Climate Clues

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UW oceanography professor Glen Shen has pioneered the use of corals to document climate changes in the past. The work is shedding light on the history of past El-Niño-like weather events, and may lead to more accurate forecasts of weather changes in the future.

Donning scuba gear and armed with a hydraulic drill, Shen goes hunting for coral samples in places like the Galapagos Islands in the Eastern Pacific. The three-inch diameter cores he brings back yield a climate record spanning hundreds of years.

Corals are colonies of tiny organisms that produce stony skeletons of calcium carbonate. As they grow, they incorporate chemical substances from the surrounding waters. Shen "reads" the climate record by measuring concentrations of key chemical species in the coral. Oxygen atoms, for instance, may tell about variations in temperature or rainfall. The elements cadmium and barium, which follow the flow of deep, nutrient-carrying colder water, tell when upwelling of ocean water was occurring and when it was stopped. Those are key parameters in El-Niño events (see Understanding El-Niño).

A curious observation led Shen to figure out a way to track changes in a third important feature of El-Niño: changes in wind. In the analysis of coral cores from the island of Tarawa in the western Pacific, he noticed that manganese levels were sometimes high. At first he couldn't explain it, and wondered if the higher manganese levels were associated with higher rainfall. But he realized there was not enough manganese in the rain water to account for this observation. Shen then realized that during El Niño, the trade winds, which usually blow across the Pacific from west to east, reverse direction. The wind change would churn up the waters in normally protected lagoons, releasing manganese from stirred-up sediments.

The coral cores can be dated by measuring their uranium and thorium contents, or by hand-counting rings on x-ray images of the cores. A sample 500 years old can be dated to within five years in this way, notes Shen.

  1. "Coral's Climate Clues," Jeff Ried, The Oregonian, June 30, 1993, p. B1.
  2. "Recent Variability in the Southern Oscillation: Isotopic Results from a Tarawa Atoll Coral," J. E. Cole, R. G. Fairbanks, and G. T. Shen, Science, 260, 1790 (1993).

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