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CHAPTER 17 • Climatic Changes Since the 1800s  311


                                                            away from the coast of Hudson Bay (Figure 17–4B). The
                                                                                           14
                                                            highest-elevation ridge dates to 7000  C years ago, just
                                                            after the last ice melted from the area and ocean water
                                                            flooded back into the depression in Hudson Bay caused
                                                            by the ice. The stair-step series of beach ridges shows that
                                                            the land beneath the former ice sheet has been rising for
                                                            thousands of years, with the rates gradually slowing as the
                                                            bedrock memory of the ice sheet load has weakened.
          Bedrock rising                                       The second major group of tide gauge responses
          Bedrock sinking                                   shows a relatively fast rate of sea level rise. This group is
          Ocean basins                                      clustered in a halo pattern surrounding the former ice
               sinking
                                                            sheets but extending well beyond the ice margins (see
        FIGURE 17-2 Patterns of sea level change Relative sea level  Figure 17–2). In North America, these gauges are found
        today is changing in several ways regionally because of bedrock  along the east coast from southern New England south
        movement. Bedrock is rapidly rising in areas formerly covered  to Florida, and in Europe they are found in a narrower
        by thick ice and sinking in regions surrounding the former ice  band across England, France, and northern Germany.
        sheets. Farther from the ice sheets, ocean basins are sinking  Today’s rapid rise of sea level in these regions is also
        under the added weight of meltwater. (Adapted from A. M.  caused by a memory of the glacial maximum ice sheets,
        Tushingham and W. R. Peltier, “Ice–3G: A New Global Model of  even though these areas were not located directly
        Late Pleistocene Deglaciation Based upon Geophysical Predictions  beneath the ice loads. During glacial times, the deep
        of Post-Glacial Relative Sea level Changes,” Journal of Geophysical  rock displaced from beneath the center of the ice sheet
        Research 96 [1991]: 4497–4523.)                     load had to go somewhere. Flowing outward beyond
                                                            the margins of the ice sheets, it caused an increase in the
        flowed back into this region, causing the depressed  elevation of the land, called a  peripheral forebulge
        bedrock to rise gradually toward its former elevation.  (Figure 17–5). As a rough analogy, the weight of a
        Even now, thousands of years after ice melting, rates  person sitting on a partly inflated air mattress in water
        of relative sea level fall caused by ongoing bedrock  will depress the center of the air mattress into the water,
        rebound are as high as 10 mm per year.              but the excess air pushed to the edges of the mattress
           As a result of this slow bedrock rebound, ancient  will cause the edges to bulge up out of the water.
        beach ridges surround the lower-lying parts of Hudson  After the ice sheets melted, the rock displaced
        Bay (Figure 17–4A). The modern beach is at sea level, and  beyond the ice margins gradually flowed back into the
        older beach ridges occur at successively higher elevations  region where the ice sheet had been. This return flow








                          Ice
                                               Higher  True global
                             Sinks                     sea level rise
                   Bedrock
                                                 0
                    Bedrock flows out
         A  Last glaciation (21,000 years ago)
                                    Global       Sea
                                   sea level    level
                        Bedrock                change
                      rebounding    rising            Relative
                                                      sea level           FIGURE 17-3 Bedrock rebound and sea
                                                      drop caused         level fall In the regions where ice sheets
                                                      by land rising      once were present, relative sea level is
                                                                          rapidly falling today. Bedrock in these
                    Bedrock flowing back               Observed local     areas is still rebounding in response to the
                                                       sea level drop     earlier melting of ice, and the rebound of
                                                Lower
                                                    1900            2000  the land overwhelms the true global rise of
        B  Today                                    C       Year          sea level.
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