Paleo Slide Set: Coral Paleoclimatology:Natural Record of Climate change for High School Student
Coral reefs only grow under certain conditions.

Coral reefs exist in the warm tropics at latitudes between 23.5N and 23.5S. Sometimes coral will grow 
above or below these latitudes if there are warm currents such as the Gulf Stream that pull warm water 
north or south from the tropics. At the equator, the sun's rays are most direct and shine for a longer 
period of the year than at other latitudes. The tropics, between the Tropic of Cancer in the south and 
the Tropic of Capricorn in the north, are a range in which the sun is directly overhead during a large 
part of the year keeping the waters at a constant, warm temperature.

The closer to the surface of the water, the more sunlight the corals receive, which is essential for 
photosynthesizing algae. Yet the coral cannot live above water for long. The depth at which coral can 
exist also depends on the cloudiness of the water. The waters in the Caribbean tend to be clear because 
there are not a lot of dissolved nutrients floating in the currents promoting plankton growth. In other
parts of the world the water is not as clear due to high plankton activity, therefore the sun's rays do 
not penetrate as deeply.

Large amounts of nutrients such as nitrogen and phosphorous, brought in by the ocean waves are essential 
for the growth of coral. However, even though there are a lot of nutrients flowing out of the mouths of rivers, 
corals do not grow in these areas because the water is clouded with mud or silt. Corals also cannot survive
near or in freshwater sources, such as a river or a lake. 

Photo Credits:
Sarah H. Dawson 
NOAA Paleoclimatology Program