Geography Reference
In-Depth Information
isotope ratio of shells of microscopic plankton (which
refl ects the volume of water in the oceans and ice sheets) was
compared with the predictions of the theory using statistical
techniques, and a close match was found. Since then, further
successful tests have been made on, for example, coral
reef sequences from Barbados (which refl ect the sea-level
variations that accompanied the growth and shrinkage of
the ice sheets) and ice cores from Antarctica and Greenland
(refl ecting changes in properties of the atmosphere).
The second advance was the reconstruction from marine
sediments of continuous records of the actual climatic changes
that affected the Earth over this same time interval. Information
about environmental change is contained in the remains of
microscopic organisms that have steadily accumulated, often
without major breaks and mostly unaffected by subsequent
erosional events, on the ocean fl oor. This source of information
could not be tapped, however, before the technology had been
invented to retrieve successfully sediment cores from the deep
oceans. For most of the fi rst half of the 20th century, the strong
belief, based on terrestrial evidence from Europe and elsewhere,
was that there had been only four glaciations during the
Quaternary. It is now known that there were more than 10 times
this number of glacial episodes within the current Ice Age. The
marine evidence, which allowed Milankovitch's theory to be tested
defi nitively, today provides a more-or-less complete temporal
framework for the shorter, often discontinuous records that are
available from the terrestrial Earth surface.
The effect of these advances was to draw physical geographers,
along with many other natural environmental scientists, into
reconstructing the pattern, timing, and effects of these major
climatic events. In the terrestrial context, there was fi rst an
emphasis on reconstructing vegetation change from mires based
