Coral Reefs as Climate Archives

Science Center Objects

USGS scientists use coral reefs as archives for reconstructing climate change during the Holocene (past 10,000 years). Coral reefs provide proxy information about rates of sea level change in the past, and individual coral colonies can be used to reconstruct the annual cycle of temperature and salinity variations for up to three centuries.

Coral reefs provide an important record of environmental conditions in the subtropical Atlantic Ocean and Caribbean Sea. Cores taken of the reef substrate or framework reveal information about the history of sea level change, as well as the history of reef growth or degradation over centuries and millennia. The chemical composition of individual coral skeletons reflects changes in environmental factors such as seawater temperature, salinity, and pH (acidity) over time scales of months to centuries. If scientists understand how each environmental factor influences the coral skeleton, they can reconstruct the environment in which the coral grew.

As corals grow, they deposit a hard skeleton made up of the mineral aragonite, a form of calcium carbonate (CaCO3), which supports the softer tissues that comprise the coral polyp. X-rays of coral skeleton slabs show alternating light and dark layers (bands) that are the result of seasonal changes in growth. A couplet of light and dark layers (bands) represents 1 year of growth and can be used to determine the age of the coral by counting back and down the coral from the known year it was sampled.

A SCUBA diver beside a Massive starlet coral on the sea floor at Dry Tortugas National Park

A USGS diver beside a Massive Starlet (Siderastrea siderea) coral colony in Dry Tortugas National Park.  Photo credit: Ilsa Kufner, USGS. (Public domain)

X-radiograph and coral image of an Orbicella faveolata

An x-radiograph (top) and photograph (bottom) of an Orbicella faveolata specimen collected from Dry Tortugas National Park in 2008. Note the annual banding couplets of high-and-low density bands. Figure modified from Flannery and Poore, 2013.

(Public domain.)