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186     PART III • Orbital-Scale Climate Change


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           A second process that affects the regional δ C pat-  northern hemisphere ice sheets and the pattern of deep-
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        tern is called  δ C aging. As deep waters flow from  water flow in the North Atlantic Ocean.
        their source regions to other parts of the world ocean,  These changes in the pattern of deep-water circula-
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        their δ C values gradually become more negative. The  tion could have affected CO concentrations in the
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        gradual shift results from the continual downward rain  atmosphere in several ways. One possibility is that
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        of  C-rich carbon (δ C = –22‰) from surface waters  stronger overturn of Antarctic-source waters at inter-
        along the path of deep-water flow (see Box 10–1).   mediate depths could have delivered more nutrients to
           This aging effect is evident mainly in the Pacific  surface waters and stimulated more biological produc-
        Ocean, where the flow of deep water is slow enough to  tivity and downward pumping of carbon. The carbon
        give the water time to age significantly (see Figure 10–13).  sent to the deep ocean could then reside there “hidden”
        In contrast, deep water circulates so quickly through  from the atmosphere.
        much of the Atlantic Ocean that it has little time to age.  A second potential link to atmospheric CO is
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        The variations in δ C values within the deep Atlantic  through the carbonate chemistry of the ocean. Concen-
        are determined mainly by physical mixing of water from  trations of dissolved CO in the surface waters of the
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        the North Atlantic and Antarctic sources.           world ocean are linked to the amount of the carbonate
           During the maximum glaciation 20,000 years ago,  ion CO 3 –2  present. These ions are produced when corro-
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        the  δ C pattern in the deep Atlantic Ocean changed  sive deep water dissolves CaCO on the seafloor. When
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        considerably (Figure 10–14B). The tongue of high-δ C  the CO 3 –2  is returned to surface waters by ocean circula-
        water that today lies at 2000–4000 m shifted upward to a  tion, it combines chemically with dissolved CO in the
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                                                                                                         –1
        depth of 1500 m or less and was replaced by low-δ C  surface waters to produce the bicarbonate ion HCO .
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        waters from the south. This pattern suggests that water  This process removes CO from surface waters. As a
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        chilled in northern latitudes during glacial times did not  result, if a larger amount of CaCO is dissolved on the
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        become dense enough to sink as deep as it does today  seafloor, the CO concentrations will be reduced in
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        but reached only intermediate depths. The lower δ C  surface waters and in the overlying atmosphere.
        values below 2000 m indicate the presence of a water
        mass that originated near Antarctica and flowed north.
           These changes in deep circulation patterns can be             Ice volume
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        traced through time by examining  δ C records from              max       min  North Atlantic
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        benthic foraminifera in tropical Atlantic sediment               δ O (‰)    deep water %
        cores. The equatorial Atlantic seafloor at 3000–4000 m        0  5   4   3  0   50  100
        is a good place for this kind of study because it lies in
        the region and at the depths that were strongly affected
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        by  δ C changes between the glacial and interglacial        0.2
        patterns (see Figure 10–14).
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           Measurements of δ C values from this region can be                                 100,000
                                                                                                years
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        used to calculate the relative contributions of high-δ C    0.4
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        water formed in the North Atlantic and low-δ C water
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        formed in the Southern Ocean. Changes in δ C values        Myr ago  0.6
        in these source areas through time can be determined by
        analyzing benthic foraminifera in cores from those
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        regions. The  δ C values measured in the tropical           0.8
        foraminifera can then be expressed as percentage contri-
        butions from these two sources. This analysis removes
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        the whole-ocean δ C change caused by carbon transfers       1.0
        from land to sea and isolates the relative contributions
        from the northern and southern sources through time.
           A long record from the tropical Atlantic shows           1.2
        oscillations in the percentage of northern-source and
        southern-source water (Figure 10–15). Northern-source  FIGURE 10-15 Changing sources of Atlantic deep water
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        waters generally dominated during δ O minima (inter-  The percentage of deep water originating in the North Atlantic
        glaciations), while Antarctic-source waters dominated  and flowing to the equator during the last 1.25 Myr has
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        during δ O maxima (glaciations). The measured fluc-  been consistently lower during glaciations than during
        tuations in deep-water sources occurred mainly at a  interglaciations. (Adapted from M. E. Raymo et al., “The Mid-
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        period near 100,000 years after 0.9 Myr ago. This δ C  Pleistocene Climate Transition: A Deep-Sea Carbon Isotopic
        trend indicates that a link exists between the size of  Perspective,” Paleoceanography 12 [1997]: 546–59.)
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