Page 209 - Earth's Climate Past and Future
P. 209

CHAPTER 10 • Orbital-Scale Changes in Carbon Dioxide and Methane  185


                                                                                                  13
        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
                                                                                   13
                                                            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
                                                                                        13
                                                            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
                                                                                                    13
        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
                                                                                      13
        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
                                                                                   13
        Indian Oceans.                                      contrast between the low-δ C water from the Antarctic
                                                                        13
                           13
           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
                   13
        more negative during glacial times, but in this case, our  0           Atlantic Ocean
                          13
        focus is on  regional  δ C variations in the ocean. Varying          Modern δ C values
                                                                                    13
        degrees of photosynthesis (see Box 10–1) in different  1
        polar areas of the ocean give inorganic carbon distinc-     +0.5 +0.7
                      13
        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
                               13
                                                               4
                                        Atlantic
                                        Ocean
                                                            A         40°S     20°    0°     20°    40°N

                Pacific    Antarctica                          0
                Ocean                              60°N           Last glacial maximum
                                                                       13
                                                                      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
          13
         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
                                                                                                       13
                                       13
        FIGURE 10-13 Modern deep-ocean δ C patterns In      glaciation In contrast to (A) the modern distribution of δ C
                                                                                            13
        today’s ocean, photosynthesis and carbon isotope    in the Atlantic Ocean, (B) the axis of high-δ C water formed
                       13
        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].)
   204   205   206   207   208   209   210   211   212   213   214