The past millionyears 67Deep cores have been drilled out of the ice at several locations in both
Greenland and Antarctica. At Russia’s Vostok station in east Antarctica,
for instance, drilling has been carried out for over twenty years. The
longest and most recent core reached a depth of over 3.5 km; the ice
at the bottom of the hole fell as snow on the surface of the Antarctic
continent over 400 000 years ago.
Small bubbles of air are trapped within the ice. Analysis of the com-
position of that air shows what was present in the atmosphere for the
Paleoclimate reconstruction from isotope data
The isotope^18 O is present in natural oxygen at a concentration of about 1
part in 500 compared with the more abundant isotope^16 O. When water
evaporates, water containing the lighter isotope is more easily vapor-
ised, so that water vapour in the atmosphere contains less^18 O compared
with sea water. Similar separation occurs in the process of condensation
when ice crystals form in clouds. The amount of separation between the
two oxygen isotopes in these processes depends on the temperatures at
which evaporation and condensation occur. Measurements on snowfall
in different places can be used to calibrate the method; it is found that the
concentration of^18 O varies by about 0.7 ofa part per thousand for each
degree of change in average temperature at the surface. Information is
therefore available in the ice cores taken from polar ice caps concern-
ing the variation in atmospheric temperature in polar regions during the
whole period when the ice core was laid down.
Since the ice caps are formed from accumulated snowfall which
contains less^18 O compared with sea water, the concentration of^18 Oin
water from the oceans provides a measure of the total volume of the
ice in the ice caps; it changes by about one part in 1000 between the
maximum ice extent of the ice ages and the warm periods in between.
Information about the^18 O content of ocean water at different times is
locked up in corals and in cores of sediment taken from the ocean bot-
tom, which contain carbonates from fossils of plankton and small sea
creatures from past centuries and millennia. Measurements of radioac-
tive isotopes, such as the carbon isotope^14 C, and correlations with other
significant past events enable the corals and sediment cores to be dated.
Since the separation between the oxygen isotopes which occurs as these
creatures are formed also depends on the temperature of the sea water
(although the dependence is weaker than the other dependencies consid-
ered above) information is also available about the distribution of ocean
surface temperature at different times in the past.^6