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UCAR Office of Education and Outreach

Dr. Randy M. Russell

Randy Russell

Coral Reef Cores - Paleoclimate Proxy Records

Samples extracted from coral reefs are one type of paleoclimate proxy record - a source of data that tells us about past climates. Coral reef records are especially sensitive to conditions in the world's oceans and seas.

Coral reefs are the massed skeletons formed by colonies of tiny animals called coral polyps. The polyps secrete the solid reef's rock-hard structural material which is composed of the mineral calcite (also called calcium carbonate, CaCO3). Over time, the polyps migrate outward towards the ever-expanding surface of the coral reef, laying down layer upon layer of calcite beneath them. The coral polyp animals actually live in a symbiotic relationship with a type of algae, which requires sunlight for photosynthesis. Coral has a fairly narrow range of tolerance of environmental conditions in which it can grow; it thrives only in warm (but not too warm!) tropical or subtropical oceans, in clear, shallow water that allows enough sunlight to reach the coral to support the algal symbionts. This sensitivity to environmental factors makes coral a good gage of local climate conditions. Since coral are only prevalent in low-latitude oceans, the climate record they provide nicely compliments the data from tree rings and ice cores, which comes from intermediate and high latitude regions and from high-altitude locales.

Map of Ice Core data collection sites
Yellow dots on the map show typical locations from which coral samples have been collected. The paleoclimate proxy data gleaned from coral nicely compliments tree ring and ice core data, since coral is found near the equator, tree ring data is collected in mid-latitude temperate forests, and most ice cores come from high-latitude polar regions.
Credit: National Oceanic and Atmospheric Administration Paleoclimatology Program/Department of Commerce

The layered structures that corals deposit as they grow have annual banding patterns reminiscent of those found in tree rings, ice cores, and varved lake sediments. The texture of the calcite deposits varies seasonally, with lighter-shaded layers appearing during the summer when growth is fast and darker layers forming in the winter when growth slows. Individual corals can live for decades to centuries. As is the case with tree ring records, we can splice together data sets from living corals with those from older, dead corals to establish continuous climate records spanning several hundred years. Sometimes coral growth varies in ways that we can detect on a seasonal basis, so the resolution of the climate records gleaned from corals is annual to seasonal.

Coral X-ray   Coral X-section
This X-ray image (left) of coral samples from the Galapagos Islands clearly shows the banded growth pattern. These coral samples (right), also from the Galapagos, show the appearance of samples before and after they have been X-rayed and the growth bands have been marked. Black lines indicate annual growth bands, while the orange and blue lines indicate seasonal patterns discerned within the annual trends.
Credits: Jerry Wellington, Department of Biology, University of Houston and National Oceanic and Atmospheric Administration Paleoclimatology Program/Department of Commerce (left). Rob Dunbar, Department of Geology and Geophysics, Rice University and National Oceanic and Atmospheric Administration Paleoclimatology Program/Department of Commerce (right).

Unlike tree ring records, it is not the thickness of the annual layers of coral that serves as the primary climate indicator from this source. Instead, the composition of the layers provides scientists with clues about past climates, while the layered structure allows them to precisely date the changes they find. As is the case with water found in ice cores, the relative abundance of the two naturally-occurring isotopes of oxygen (remember that calcite is CaCO3) tells the climate tale encoded within coral skeletons. Varying ratios of oxygen-16 abundance as compared to oxygen-18 signal changes in the temperature of the ocean. As is so often the case with climate proxies, scientists must be careful in their analyses of the implications of 18O/16O ratios, for changes in salinity also can alter this balance.

Other aspects of corals can provide data about different aspects of climate besides ocean temperatures. Since corals only grow in fairly shallow water because of the symbiotic photosynthetic algae, changes in sea level caused by shrinking or expanding ice caps can alter the depth (or height!) at which ancient coral reefs are found. Dead corals can also indicate changes in conditions that made environments unsuitable for corals; water that is too warm kills off corals, as does water that is cloudy or muddy (such as ocean environments clouded by increased river runoff that can indicate higher levels of precipitation). Since many of the coral reefs that scientists have studied grow around the numerous small islands and atolls of the Pacific Ocean, climate data from corals is especially valuable for the study of the El Niño cycles of ocean temperature variation in the Pacific.

Coral reef   Divers drilling a coral core
A coral reef consisting of numerous different species of coral in the Palau Archipelago of Micronesia (left). Scientists in SCUBA gear use a drill to extract a coral sample from Clipperton Atoll (right).
Credits: Jerry Wellington, Department of Biology, University of Houston and National Oceanic and Atmospheric Administration Paleoclimatology Program/Department of Commerce (left). Maris Kazmers, SharkSong Photography, Okemos, Michigan and National Oceanic and Atmospheric Administration Paleoclimatology Program/Department of Commerce (right).

To gather coral reef proxy data, scientists must first don SCUBA gear and dive down to drill core samples from the reefs. Although the annual banding patterns in the coral samples are sometimes evident from visual inspection alone, in many cases X-ray imaging is required to clearly distinguish the bands. After scientists mark the annual layers in the X-ray images, they can extract samples from various layers for chemical and isotopic analysis. As noted earlier, data from living corals can be combined with information from older, dead coral deposits to extend the climate record derived from corals back as far as several centuries!

Although not directly related to paleoclimate studies, data from some very ancient corals provides information about another interesting long-term change in the Earth system. Besides the annual banding patterns clearly visible in corals, there is a secondary banding structure that can sometimes be spotted that shows the monthly variation in the tides. The frequency of tides at a given spot on Earth depends both on the orbital period of our Moon and on the rotational rate of Earth. Ancient corals from 440 million years ago indicate that there were more tidal cycles per year, supporting the view of astronomers that Earth spun more quickly on its axis (and thus had shorter days) in the distant past than it does today. Thus coral data shows that Earth's rotation has slowed over the course of 440 million years from a rate 11% faster than today to the current rate that gives us our 24-hour days!

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Last modified: 9 April 2014
Created: 13 October 2010