How do corals record climate and ocean conditions?
A colony of stony coral is made up of thousands of colorful little animals called polyps. Each polyp, which is a clone of all the others, builds a hard cup-shaped scaffold of calcium carbonate. It absorbs the calcium it needs from the surrounding seawater.
“It starts as a one-story house," said Terry Quinn, a research scientist at the University of Texas at Austin's Jackson School of Geosciences. "The polyp lives in it for a year, vacates it and then starts building a second story on top.” Slowly, layer by layer, a hard “coral head” forms beneath. All the living coral is on the outer surface.
As the seasons change, sea temperatures rise and fall, as does the amount of sunlight filtering through the water. The layers of a coral head laid down in winter have a different density than those formed in summer. Over time, a coral head develops growth bands similar to those in the trunk of a tree. These appear as light and dark bands in x-ray images.
When a coral core is brought back to the lab, Quinn and his team cut it in half down the middle. A thin slab is cut off of one of the halves. It gets x-rayed to reveal light and dark density bands. Darker, denser bands indicate periods of slow growth. Lighter bands indicate faster growth.
Next, the slab is secured on a bench and a computer-guided dental drill runs along its length, grinding a small trench in it. This produces a fine powder which is sampled 12 times for each year of the core (as determined by the location of the bands). Thousands of tiny vials are loaded up with the powder. Then each sample is analyzed to determine the oxygen and carbon isotopic ratios of the sample. A subset of the sample is then analyzed for the elemental ratio of strontium to calcium. After many careful calculations, a month-by-month history of sea surface temperature and salinity is produced.
One colony of coral can grow for several hundred years. By carefully combining records from several coral colonies, Quinn and others reconstruct ocean conditions at time scales ranging from seasons to multiple centuries. These "climate windows" provide valuable information on how the tropical climate system operated in the most recent past, but also in the geologic past. That’s especially useful information for scientists who build computer models to forecast future climate.