Page 206 - Earth's Climate Past and Future

182 PART III • Orbital-Scale Climate Change

~ Ice Deep ~ Ice Deep this cooling, the atmospheric CO concentration should
2
volume water volume water have fallen by ~20–30 ppm because of increased CO 2
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13
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13
δ O (‰) δ C (‰) δ O (‰) δ C (‰) solubility.
54 3 –1 0 1 5 4 3 –1 0 1 The altered salinity of the glacial ocean would also
0 1.0
have affected atmospheric CO but in the opposite
2
direction. CO dissolves more easily in seawater with a
2
lower salinity, but the average glacial ocean was saltier
than it is today because of the amount of freshwater
100,000 41,000 taken from the ocean and stored in ice sheets. Although
years years some high-latitude ocean surfaces (such as the North
Atlantic) became less salty during glaciations, the aver-
age salinity of the entire ocean increased by about
Myr ago 0.5 1.5 1.1‰, enough to cause an estimated glacial CO increase
2
of 11 ppm.
The 11-ppm CO increase caused by higher salinity
2
would have offset just under half of the 20–30 ppm
decrease caused by ocean cooling for a net CO drop of
2
~14 ppm. Most of the observed CO decrease of 90 ppm
2
must have occurred by means of other mechanisms.
10-7 Biological Transfer from Surface Waters
1.0 2.0
One way to move carbon from surface to deep waters is
FIGURE 10-9 Carbon transfer during glaciations A through biological activity. Photosynthesis occurs in
sediment core from the deep Pacific Ocean shows more sunlit surface waters where sufficient nutrients (phos-
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negative δ C values during glaciations (δ O maxima), phorus and nitrogen) are available to stimulate growth of
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mainly because C-enriched carbon is transferred from marine phytoplankton (Figure 10–10). CO is extracted
the land into the ocean. (Adapted from D. W. Oppo et al., 2
“A δ C Record of Upper North Atlantic Deep Water
13
During the Past 2.6 Million Years,” Paleoceanography 10
[1995]: 373–94.)

How Did the Carbon Get into the
Deep Ocean?

The answer to this question is complicated and not
yet resolved. Scientists are actively investigating several
Nutrient-depleted
factors that seem likely to have played a role in this
100 m mixing CO + H O CH O + O 2
transfer. Wind Photosynthesis
2
2
2
Nutrient
10-6 Increased CO Solubility in Seawater
2 Net export upwelling
Changes in the average temperature of the ocean dur- of carbon
ing glaciations would have altered the chemical solubil- and nutrients
ity of CO in seawater and thereby affected the amount
2
of CO left in the atmosphere. Because CO dissolves Oxidation of carbon
2 2 CH O + O CO + H O
more readily in colder seawater, atmospheric CO levels 2 2 2 2
2 and nutrients
will drop by ~10 ppm for each 1°C of ocean cooling.
As we will see in Chapter 12, surface-ocean tempera- FIGURE 10-10 Photosynthesis in the ocean Sunlight that
tures in the tropics cooled by an estimated average of penetrates the surface layers of the ocean causes
2°–4°C during the last glacial maximum 20,000 years photosynthesis in microscopic marine plankton. After the
ago and by larger amounts in many high-latitude plankton die, their organic tissue sinks to the seafloor and is
regions. The much larger volume of water in the deep oxidized to nutrient form. Upwelling returns the nutrients
ocean cooled by an average of 2°–3°C. As a result of (including organic carbon) to the ocean surface.