Paleo Slide Set: Coral Paleoclimatology
Positive x-radiograph collage of Galapagos Pavona clavus coral.

The polyp is seated in a pit in the coral skeleton composed of calcium carbonate (CaCO3) crystals 
secreted by the epidermis or skin of the lower half of the column. As long as the colony is alive, 
calcium carbonate is deposited beneath its living tissues. The colony lies entirely above the skeleton 
and, with its network of interconnected polyps, completely covers it. Many corals periodically lift 
their bases and produce a new floor to their cup, encapsulating a tiny portion of their skeleton 
and entirely sealing it off from any contact with sea water or living tissues. Over the course of 
many years, each polyp lifts itself hundreds of times, each time leaving even more skeleton behind. 
The density of the capsule left behind depends on the timing of its creation. Coral skeleton formed 
in winter has a different density than that formed in summer because of variations in growth rates 
related to temperature and cloud cover conditions. Thus corals exhibit seasonal growth bands very 
much like those in trees. Sometimes these bands are visible to the naked eye; usually, however, 
they are more visible in an x-ray like this. When paleoclimatologists drill a coral core, they can 
count the growth bands and date samples exactly. Long cores can cover several hundred years; 
this portion of a core from Urvina Bay in the Galapagos Islands covers the period from 1716 to 1735 
A. D. To best understand past climate, scientists need to be able to date their samples as accurately 
as possible. They need to know exactly when climatic changes occurred so that they can create realistic 
computer models of the global climate system.

Photo Credits:
Jerry Wellington
Department of Biology, University of Houston