Passage 18
Milankovitch proposed in the early twentieth century
that the ice ages were caused by variations in the Earth’s
orbit around the Sun. For sometime this theory was
considered untestable, largely because there was no suffi-
(5) ciently precise chronology of the ice ages with which
the orbital variations could be matched.
To establish such a chronology it is necessary to
determine the relative amounts of land ice that existed
at various times in the Earth’s past. A recent discovery
(10) makes such a determination possible: relative land-ice
volume for a given period can be deduced from the ratio
of two oxygen isotopes, 16 and 18, found in ocean sedi-
ments. Almost all the oxygen in water is oxygen 16, but
a few molecules out of every thousand incorporate the
(15) heavier isotope 18. When an ice age begins, the conti-
nental ice sheets grow, steadily reducing the amount of
water evaporated from the ocean that will eventually
return to it. Because heavier isotopes tend to be left
behid when water evaporates from the ocean surfaces,
(20) the remaining ocean water becomes progressively
enriched in oxygen 18. The degree of enrichment can
be determined by analyzing ocean sediments of the
period, because these sediments are composed of calcium
carbonate shells of marine organisms, shells that were
(25) constructed with oxygen atoms drawn from the sur-
rounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice
there was when the sediment was laid down.
As an indicator of shifts in the Earth’s climate, the