Page 205 - Earth's Climate Past and Future

CHAPTER 10 • Orbital-Scale Changes in Carbon Dioxide and Methane 181

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terrestrial vegetation is tagged with a negative δ C We can use a mass balance calculation to estimate
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value averaging –25‰, whereas the large amount of the effect of adding very negative ( C-enriched) carbon
inorganic carbon in the ocean has an average value to the deep sea during maximum glaciations:
near 0‰. Although a small fraction of the terres-
trial organic carbon delivered to the ocean remains as (38,000) (0‰) + (530) (–25‰) = (38,530) (x)
Inorganic Mean C added Mean Glacial inorganic Mean
organic matter, most of it is converted to inorganic C in ocean δ C from land δ C carbon total δ C
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carbon that retains the very negative –25‰ δ C
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composition. where x is the δ C value of glacial inorganic carbon in
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During glacial times (Figure 10–8A), C-rich organic the ocean and the sizes of the carbon reservoirs shown
carbon transferred from the land to the deep ocean and are in billions of tons of carbon. Solving for x, we find
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converted to inorganic form should have made the δ C that the mean δ C value of inorganic carbon in the
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value of the inorganic carbon in the ocean slightly more glacial ocean should have shifted from ~0‰ to –0.34‰
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negative. These lower δ C values should correlate with because of the addition of C-rich carbon transferred
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the more positive δ O values resulting from more from the land.
glacial ice and colder deep-ocean temperatures. The This estimated –0.34‰ shift has been tested by
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opposite pattern should have occurred during inter- comparing it to δ C values in the CaCO shells of
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glacial times (Figure 10–8B), with more positive δ C glacial-age benthic foraminifera in cores distributed
values because of the return of the negative carbon to across the world ocean over a range of water depths.
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the land and more negative δ O values because of the The measurements show the δ C composition of the
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melting of O-rich ice. deep waters region by region. With all these analyses
combined into a global average value, the estimated
δ C change for the entire ocean is –0.35‰ to –0.4‰.
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In view of the uncertainties involved, the good agree-
ment between the two methods indicates that carbon
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isotopes are useful tracers of past carbon shifts among
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This evidence suggests that we should also be able
to trace carbon transfers during earlier glacial cycles
using δ C changes measured in the shells of benthic
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Deep ocean: foraminifera. Because the Pacific contains by far the
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high δ O largest volume of water of Earth’s oceans, scientists
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low δ C
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have used changes in its deep-water δ C values through
time as the best single “quick” index of average δ C
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changes in the global ocean. The longer-term δ C
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A Glacial climate record in Figure 10–9 shows the basic relationship
expected from the transfers shown in Figure 10–8: large
amounts of terrestrial carbon with negative (–25‰)
16 O 13
12 C δ C values were transferred into the ocean every time
ice sheets grew, and then were returned to the land
when the ice melted.
The most negative δ C values are spaced at inter-
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vals close to 100,000 years during the last 0.9 Myr and
Deep ocean: at cycles of 41,000 years prior to 0.9 Myr ago. Although
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low δ O the correlation between the two signals is far from per-
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high δ C fect, time series analysis confirms that the δ C (carbon
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transfer) and δ O (ice volume) signals have varied at
the same periods and with the same approximate timing
over millions of years.
B Interglacial climate
On closer inspection, many of the δ C variations
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FIGURE 10-8 Glacial transfers of C and O (A) During shown in Figure 10–9 exceed the –0.35 to –0.4‰ shift
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glaciations, C-enriched organic matter is transferred from the that can easily be attributed to transfers of carbon from
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land to the ocean at the same time that O-enriched water land to sea. This unexpectedly large δ C response tells
vapor is extracted from the ocean and stored in ice sheets. us that additional factors must have been affecting
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(B) During interglaciations, C-rich carbon returns to the land oceanic δ C values through time. The source of these
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as O-rich water flows back into the ocean. variations is not known.