Geology Reference
In-Depth Information
Box 2.1 Isotopic changes in the ocean due to glaciation.
The history of climatic change in Quaternary times is primarily a function of the growth and
decay of ice sheets. The key record of changing volumes of glaciers through time comes from
the isotopic record of the oceans. During glaciations, water evaporated from the ocean is stored
in ice sheets (see figure). The ratio of two common isotopes of oxygen, 16 O and 18 O, is different
in ice sheets than in the ocean as the result of a fractionation process. When water evaporates
(primarily near the equator), the lighter isotope ( 16 O) is preferentially evaporated, causing the
ocean to become enriched in 18 O. During condensation and precipitation, 18 O is preferentially
removed from the water vapor, so that during its poleward transport, the remaining vapor
becomes increasingly enriched in 16 O. Precipitation at high latitudes is therefore strongly
depleted in 18 O compared to “standard mean ocean water”. (SMOW). This causes the ice sheets
to be isotopically “light” compared to SMOW (lower 18 O/ 16 O ratio). Conversely, the oceans
become isotopically heavier (higher 18 O/ 16 O ratio) due to the storage of “extra” 16 O in the ice
sheets. The more ice stored on land, the isotopically heavier the ocean becomes. Foraminifera
that grow in equilibrium with the sea water record its isotopic composition. Consequently, the
stratigraphic record of isotopic changes displayed by foraminifera during the Quaternary can
be interpreted as a record of changing ice volumes.
enrichment in 16 O
poleward vapor transport
snow
18 O /16 O precipitation > 18 O /16 O evaporation
18 O /16 O << SMOW
18 O /16 O evaporation < SMOW
high latitude ice sheet
18 O /16 O >> SMOW
enrichment in 18 O
equatorial ocean
Isotopic changes due to sequestration of water in ice sheets.
Fortunately, within the past few decades, the
history of Quaternary sea level has been recon-
structed with increasing accuracy (Bloom et al. ,
1974; Chappell, 1974; Chappell et al. , 1996;
Lambeck and Chappell, 2001). During maximum
glacial conditions, the volume of water stored in
ice sheets on land caused average sea level to be
lowered by more than 100 m. On the other hand,
during the peak of the previous interglaciation
at about 125 ka, there was apparently less ice on
Earth than there is today, and mean sea level
was approximately 6 m higher than it is today.
The most detailed reconstructions of the growth
and decay of ice sheets have been derived from
variations in the oxygen isotopic composition of
seawater (Box 2.1). The removal of water from
the ocean via evaporation and the sequestering
of this isotopically lighter (lower 18 O/ 16 O ratio)
water in ice sheets caused the oceans to become
isotopically heavier (higher 18 O/ 16 O ratio) during
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