Paleo Slide Set: Coral Paleoclimatology:Natural Record of Climate change for High School Student
Positive x-radiograph collage of Galapagos Pavona clavus coral.

Here is an X-ray of a coral sample. You can see lines called growth bands, very much like those in most trees. 
A light and a dark band together represent one year's growth. The amount of seasonal growth is determined 
by temperature, cloud coverage and the amount of nutrients present. As you can see, in some years the corals 
grew more than others, due to climatic events. The ring from winter growth is darker because growth is slowed 
and the coral skeleton becomes more compacted. The ring from spring and summer growth is lighter because growth 
speeds up and the skeleton formed is less dense. Paleoclimatologists count the bands to determine the age of 
the sample. They can also identify significant weather events by looking at the uneven bands. 

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