Environments - Geochemistry: An Introduction - page 182

a sunny summer's day. Homonuclear diatomic molecules such as N 2 and O 2 do not absorb
infrared, visible, or ultraviolet solar radiation because they remain symmetrical with a zero
electric dipole, no matter how hard they are hit by photons. Such collisions are described as
elastic. Nitrogen and oxygen therefore do not block the sunlight, they are optically trans-
parent over most of the solar electromagnetic spectrum, and do not contribute to warming.
In contrast, the symmetry of H 2 O, CO 2 ,CH 4 , and O 3 vibrations changes when photons
collide with these molecules: new vibrational and rotational patterns reflect absorption of
electromagnetic radiation and the whole gas phase warms up, at least as long as atmo-
spheric pressure is strong enough for frequent molecular collisions. The reflected radiation
is therefore absorbed by H 2 O, CO 2 ,CH 4 , and ozone and warms up the atmosphere. This
is the greenhouse effect. The history of the Earth's climate is therefore for a large part that
of its atmospheric CO 2 .

9.1.1 Quaternary climates

This effect can be seen clearly in the composition of Quaternary sediments where the extent
of carbonate preservation varies with the depth of deposition in keeping with the pattern
of glaciations ( Fig. 9.1 ) that recur every 100 000 years. Carbonate preservation clearly

decreases and CCD rises during the high-CO 2 peaks of interglacial periods. This pattern is
similar to the pattern of fluctuations in marine oxygen isotopes as recorded by the

≈

18 Oof
the calcite of deep-water foraminifera. During glacial times, a greater quantity of ice with
very negative

δ

18 O values is stored in the polar ice caps and seawater oxygen is enriched
in 18 O ( Fig. 9.2 ) . Because the enrichment of 18 O in rain and snow is stronger during cold
periods, the glacial

δ

18 O maxima in seawater and the carbonates in isotopic equilibrium
with it (maximum ice volume) are the mirror image of

δ

18 O minima in ice cores.

δ

4.2

4.4

4.6 20 − 80%

4.8

0 − 20%

0 100 200 300 400 500 600 700

Age (ka)

Figure 9.1 Preservation of carbonates in Quaternary sediments at various depths. The different shades of

grey refer to the percentages of carbonate preservation shown. The zone between the two curves
mainly depicts the transition of the carbonate compensation depth (CCD). During glacial periods
(G), the CO 2 content of the atmosphere was lower, the ocean less acidic, the CCD sank, and the
calcareous fraction of sediments increased (after Farrell and Prell, 1989 ) .