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CHAPTER 10 • Orbital-Scale Changes in Carbon Dioxide and Methane  187


           On a global average basis, this mechanism acts slowly  where decaying plant matter uses up the available oxy-
        (over thousands of years) because of the time required to  gen and creates the necessary reducing conditions.
        dissolve deep-sea carbonates and because of the slow  Most natural wetlands are located in the tropics and in
        overturn of the deep ocean. Deep-ocean regions experi-  boreal (circum-Arctic) regions.
        enced different and often opposing changes in CaCO     Methane concentrations in ice cores show a series
                                                      3
        dissolution during glaciations (increases in most of the  of cyclic variations between maxima of 650–700 parts
        Atlantic, decreases in most of the Pacific). Because these  per billion (ppb) and minima of 350–450 ppb (Figure
        changes more or less canceled each other out at a global  10–16). Ice flow models show that the peaks in methane
        scale, little if any change in the average CO  –2  content  concentration fall very close to times of maxima in
                                              3
        of deep or surface waters occurred.                 northern hemisphere summer insolation at the 23,000-
           Nevertheless, regional changes in CO  –2  could also  year cycle of orbital precession. Small adjustments in
                                            3
        have altered atmospheric CO concentrations. Today,  estimated ages within the ice cores bring the CH
                                  2                                                                       4
        deep water from North Atlantic sources comes to the  peaks into full alignment with July insolation maxima.
        surface in the Antarctic region (see Figure 10–14A).  This midsummer timing for methane peaks is sup-
        Because today’s North Atlantic deep water is a relatively  ported by the age of the most recent CH maximum in
                                                                                               4
        non-corrosive water mass, it dissolves relatively little
        CaCO on the seafloor and delivers relatively small
              3
        concentrations of CO  –2  to the surface waters of the
                            3
        Southern Ocean. As a result, dissolved CO concentra-                  July insolation (30°N)
                                             2                                          2
        tions can remain at relatively high levels in the South-                    W/m
        ern Ocean and the overlying atmosphere.                         0  440      480       520
           During glaciations, however, the water that formed
        in the North Atlantic sank to much shallower depths
        and did not intersect as much of the seafloor (see Figure                              Insol
        10–14B). Instead, an expanded area was bathed by                                       CH 4
        southern-source water that was more corrosive, dis-
        solved more CaCO , and eventually returned more
                          3
        CO  –2  to Antarctic surface waters. The geochemists
            3                                                     100,000
        Wally Broecker and Tsung-Hung Peng proposed that
        this change in circulation would have reduced the con-
        centration of dissolved CO in the Southern Ocean
                                 2
        along with the amount of CO in the overlying atmos-        Years ago
                                  2
        phere. They estimated that this mechanism, called the
        polar alkalinity hypothesis, might explain as much as
        40 ppm of the observed 90-ppm decrease in atmos-
        pheric CO during glacial times. Because the deep flow     200,000
                 2
        in the Atlantic Ocean is relatively rapid, this mechanism
        can influence atmospheric CO concentrations within a
                                  2
        few hundred years.
          IN SUMMARY, several factors probably contributed to
          the reduction in atmospheric CO concentrations
                                      2
          during glacial intervals: reduced CO solubility in      300,000
                                        2
          colder waters, greater biological pumping of carbon
          from surface to deep waters, and changes in
          deep-ocean circulation. The size of the latter two
          contributions is highly uncertain, and this problem            300   400    500   600   700
          is currently an area of intense investigation.                      Vostok methane (ppb)
                                                            FIGURE 10-16 Methane and monsoons The methane
        Orbital-Scale Changes in CH                         record from Vostok ice in Antarctica shows regular cycles at
                                        4
                                                            intervals of 23,000 years and closely resembles the monsoon
        In contrast to the oxidized carbon common in most of  response to low-latitude insolation forcing. (Left: Adapted
        Earth’s environments, the carbon in methane (CH ) is
                                                    4       from W. F. Ruddiman and M. E. Raymo, “A Methane-Based Time
        in a reduced form that is produced when oxygen is   Scale for Vostok Ice: Climatic Implications,” Quaternary Science
        absent. Most natural methane originates in wetlands,  Reviews 21 [2003]: 141–55.)
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