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CHAPTER 12 • Last Glacial Maximum 213

12-2 How Large Were the Ice Sheets? sheets. Height and thickness are usually related in a
simple way: as an ice sheet grows, in time it weighs
The glacial geologists George Denton and Mikhail down the underlying bedrock by an amount equal to
Grosswald and the glaciologist Terry Hughes headed about 30% of its thickness (Chapter 9). As a result, the
the CLIMAP ice sheet reconstructions. Of the two height of the ice above the surrounding landscape
alternative reconstructions published, one broke with usually represents about 70% of its total thickness.
conventional thinking by portraying the ice sheets at Unfortunately, no simple method exists to measure or
their maximum plausible size (the “maximum recon- reconstruct changes in either the thickness or height of
struction”). Three aspects of this reconstruction proved ice sheets through time.
highly controversial. The CLIMAP maximum reconstruction showed
One debate arose over the lateral extent of the ice relatively high (thick) ice sheets, especially over North
sheets, which in most places were shown extending to America. This reconstruction was based on the assump-
the maximum limits that ice had reached at any time in tion that the ice sheets had existed at or near their max-
the history of northern hemisphere glaciation (see imum extent long enough to build up slowly to full
Figure 12–2). The southern limits of most continental equilibrium thickness and that they consisted of stiff ice
ice sheets at the last glacial maximum had not been in frozen to the underlying bedrock. These two assump-
much doubt for many years prior to the CLIMAP tions produced ice sheets with profiles that rose
reconstruction, but larger uncertainties remained about relatively steeply from their outer margins to high
whether or not higher-latitude ice margins actually elevations and great thicknesses in their central regions
reached the ocean. Some glacial geologists argued that (Figure 12–3A, B).
the ice sheets in the north were less extensive than they This aspect of the CLIMAP reconstruction is still
had been during earlier glaciations because they were controversial. One line of evidence has come from mea-
starved for moisture. The main reason for this uncer- surements of the size of the fall in global sea level
tainty was that the scarcity of organic carbon in the caused by water stored in the ice sheets. Coral reefs can
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cold, dry Arctic made it difficult to find C with which be used as dipsticks to measure past changes in sea level
to date the deposits. (Chapter 9), and reefs in different areas indicate a sea
Estimates of the northern ice margins generally dis- level drop of somewhere between 110 and 125 m during
agreed by a few hundred kilometers, amounts that were the last glacial maximum. The higher estimate of sea
important to arguments about the physical state of the level fall agrees with the large-volume ice sheets in
ice sheets in specific regions but actually rather small in the CLIMAP reconstruction, but the lower estimate
comparison with the full lateral dimensions of ice sheets falls close to the smaller estimate of ice volume (see
(thousands of kilometers) and with the size of grid boxes Table 12–1).
used in climate modeling (hundreds of kilometers on a If the ice sheets were as extensive as those CLIMAP
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side). As it turned out, careful C dating along the proposed but held a smaller volume of water taken from
northern margins of the ice sheets has confirmed that the ocean, one or more of the ice sheets must have been
most of them were indeed at or near their maximum thinner than the CLIMAP reconstruction indicated.
limits 21,000 years ago. Scientists naturally focused on the North American ice
A second major disagreement about the extent of sheet because of its huge size: if this ice sheet were thin-
glacial maximum ice sheets centered on marine ice ner by 30%, it would match the lower estimate of sea
sheets, the ice sheets that formed on shallow continental level change.
shelves with their bases lying below sea level (compan- Independent evidence supports the possibility of
ion Web site, p. 29). The CLIMAP reconstruction thinner North American ice. Ice sheets slip and slide
placed two marine ice sheets along the northern margin across soft water-laden sediments and unconsolidated
of Eurasia, a large one over the Barents Sea directly sedimentary rock (Chapters 9 and 11). Only the central
north of Scandinavia and a smaller one over the Kara part of the North American ice sheet rested on hard
Sea north of western Russia. This aspect of the recon- bedrock, while the southern margins lay on easily
struction caused years of controversy. Subsequent sur- deformed sediments at lower, warmer elevations. As a
veys of these shallow seas collected evidence (glacial result, the southern margins of the ice sheet were prob-
debris in sediment cores and depth soundings showing ably thinned by frequent sliding (Figure 12–3C). This
the extent of submerged moraine ridges) proving that at sliding could also have thinned the inner portion of the
the glacial maximum, a large marine ice sheet did exist ice sheet by drawing ice out toward the margins.
in the Barents Sea and perhaps in the westernmost Kara A second line of evidence comes from the amount of
Sea but not in the eastern Kara Sea or east of that region. bedrock rebound that has occurred since the ice sheets
Another controversial aspect of the CLIMAP melted. Bedrock weighed down by ice sheets has a slow
reconstruction was the thickness (and height) of the ice viscous response, and some of its rebound after the ice