Paleo Slide Set: Polar Ice Cores
The ratio between the heavier H218O and lighter H216O water molecules in the ice

The ratio between the heavier H218O and lighter H216O water molecules in the ice is expressed as 
the departure from a standard --d 18O=1000x(18O/16Oice -18O/16O std). Snow falling on the Greenland 
Ice Sheet under colder temperatures is more negative, that is it contains more of the light H216Omolecules. 
Thus, a plot of the d18O of snow versus temperature shows an excellent correlation. Since d18O is primarily 
temperature-dependent, it serves as a paleothermometer providing important information about past climate. 

Photo Credits:
Graphic produced by Thomas Andrews using data from Johnsen et al. (1989) 
NOAA Paleoclimatology Program