QUESTION:
How can scientists use measurements of different oxygen isotopes from ice
cores to determine temperatures at the time when the ice was frozen?

        Sincerely,
        Ken D., Freeport, Maine 

ANSWER from Scott Borg, Program Director                                 
            Office of Polar Programs -- Antarctic Geology & Geophysics
Program        
            National Science Foundation
            Mon Mar 31 15:41:32 1997


Dear Ken,

Naturally occurring oxygen has three isotopes with atomic masses of 16, 17,
and 18.  Most oxygen is O-16: O-17 is real rare and O-18 only accounts for
about 0.2% of the oxygen on earth.  Because oxygen is an essential element
of water, all three of the isotopes are found in all types or phases of
water (liquid, vapor, and ice).  Oxygen is also present as free O-2
molecules in the atmosphere.  It turns out that the amount of O-18 relative
to O-16 in water vapor in the atmosphere (amongst other things, the
atmosphere is a mixture of water vapor and gaseous oxygen) is dependent on
temperature.  This is called a temperature-dependent mass fractionation.
So, by measuring the ratio of O-18 to O-16 in the ice, scientists can
estimate the temperature of formation of the ice.  This technique assumes
that the ice equilibrated with the atmosphere and that the isotopic
composition has not been altered after formation.  The temperature
dependence of oxygen isotopic composition of water (or ice) in the
atmosphere has been experimentally determined.

Scott Borg