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CHAPTER 10 • Orbital-Scale Changes in Carbon Dioxide and Methane 185
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pumping and glacial productivity to alter atmospheric At one extreme are the relatively positive δ C values
CO concentrations remains unclear. Some scientists (> 0.8‰) in the North Atlantic Ocean near 2000–4000 m
2
have suggested that the dust from the land may deliver depth. These values are positive because North Atlantic
other key elements that stimulate ocean productivity. Deep Water is formed from surface waters that have
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been greatly enriched in C by photosynthesis (see
10-8 Changes in Deep-Water Circulation Box 10–1). A plume of positive δ C values at depths of
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2000–4000 m defines the core of this southward flow
Another mechanism for transferring more carbon to the
deep ocean is to change the pattern of deep-ocean circu- (Figure 10–14A).
lation. Today, most of the water that fills the deeper In contrast, the waters that form in the Antarctic
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oceans forms in the subpolar North Atlantic Ocean or in region and flow to a wide range of depths have δ C val-
the Southern Ocean (companion Web site, pp. 24–25). ues lower than 0.5‰. Because photosynthetic fraction-
North Atlantic Deep Water flows southward through the ation of carbon isotopes in the Southern Ocean is
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Atlantic basin, while colder and denser Antarctic Bottom incomplete, extremely positive δ C values do not develop
Water fills the much larger volume in the Pacific and in the surface waters that feed the deeper flow. The
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Indian Oceans. contrast between the low-δ C water from the Antarctic
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Evidence from δ C measurements in bottom- and the high-δ C water from the North Atlantic makes
dwelling foraminifera indicates that the circulation pat- them relatively easy to trace.
tern during glacial times was different. We saw earlier that
the average δ C composition of the entire ocean became
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more negative during glacial times, but in this case, our 0 Atlantic Ocean
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focus is on regional δ C variations in the ocean. Varying Modern δ C values
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degrees of photosynthesis (see Box 10–1) in different 1
polar areas of the ocean give inorganic carbon distinc- +0.5 +0.7
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tively different δ C values in north polar versus south +0.9
polar areas. As a result, the deep waters that form from Water depth (km) 2
surface waters in these regions begin their downward trip +1.1
with distinctively different δ C values (Figure 10–13). 3
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4
Atlantic
Ocean
A 40°S 20° 0° 20° 40°N
Pacific Antarctica 0
Ocean 60°N Last glacial maximum
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δ C values
1 +0.9
Water depth (km) +0.2 +0.6
60°S 2 –0.2
60°N 60°S Indian 3
60°S Ocean
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δ C of –0.4
ocean water 4
> 0.8 ‰
0–0.8 ‰ 40°S 20° 0° 20° 40°N
< 0 ‰ B Latitude
20°N
FIGURE 10-14 Change in deep Atlantic circulation during
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FIGURE 10-13 Modern deep-ocean δ C patterns In glaciation In contrast to (A) the modern distribution of δ C
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today’s ocean, photosynthesis and carbon isotope in the Atlantic Ocean, (B) the axis of high-δ C water formed
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fractionation drive δ C values higher in surface waters in the north flowed south at shallower levels during the last
compared to deep waters. (Adapted from C. D. Charles and R. glacial maximum. (Modified from J.-C. Duplessy and E. Maier-
G. Fairbanks, “Evidence from Southern Ocean Sediments for the Reimer, “Global Ocean Circulation Changes,” in Global Changes in
Effect of North Atlantic Deepwater Flux on Climate,” Nature 355 the Perspective of the Past, ed. J. A. Eddy and H. Oeschger [New York:
[1992]: 416–19.) Wiley, 1993].)