Page 226 - Earth's Climate Past and Future

202 PART III • Orbital-Scale Climate Change

proposed a specific mechanism by which such slow ice sheets. If these ice sheets were similar in extent
changes in bedrock configuration would have elicited a but smaller in volume, they must have been thinner
totally new resonant climatic response within the last than the ones that varied at or near the 100,000-year
million years. cycle.
A second, more credible explanation of the The glacial geologist Peter Clark proposed that the
~100,000-year oscillations focuses on delayed bedrock earlier ice sheets accumulated on top of soils that had
rebound during abrupt deglacial terminations caused by been developing for many millions of years before
the weight of large ice sheets. When the south-central northern hemisphere glaciation began. The weight of
portions of large ice sheets began to melt under rising the overlying ice melted the bottom layers of the ice
levels of summer insolation, the ice sheets would have sheets, and the meltwater trickled down into the soils.
retreated into the deep bedrock holes they had created Soils that are saturated with water are more easily
(see Chapter 9). The delay in bedrock rebound would deformed by the overlying ice and can cause the ice to
have kept the remaining ice in a warm environment and slip. Slipping would have moved large amounts of ice
accelerated the rates of melting. toward the ice sheet margins and southward into
This explanation requires that larger ice sheets warmer latitudes, where ice ablation rates were higher.
produce a slower rebound of the underlying bed- Between 2.75 and 0.9 Myr ago, frequent sliding may
rock than the smaller ice sheets that existed prior to have kept the ice sheet on North America low and thin,
0.9 Myr ago. The rate of bedrock rebound depends on subject to high ablation, and consequently small in
the viscosity (resistance to flow; see Chapter 4) of the volume (Figure 11–16A). Thin ice sheets would also be
material deep in the Earth that is “squeezed” outward easier to melt during even relatively weak insolation
from underneath the burden of the overlying ice maxima (as in the “small glaciation” phase of Chapter 9).
sheets. Higher-viscosity rock would return slowly, None of the original soil cover is now left across cen-
while lower-viscosity material would flow back more tral Canada because erosion by ice sheets has removed it.
rapidly. At this point, different models of the viscosity The surface is mostly bare bedrock, with scattered areas
of Earth’s mantle exist. Whether or not the larger and of coarse ice-eroded debris. With so little soft sediment,
thicker ice sheets of the 100,000-year world would have more recent ice sheets could not easily slide, and the
tapped more of the high-viscosity (slow-flow) response absence of sliding could have allowed them to grow
is unclear. much thicker (Figure 11–16B). Reconstructions of ice
Another explanation related to the underlying sheet thickness at the last glacial maximum 20,000 years
bedrock focuses on the character of the materials over ago indicate a broad interior region where the ice was
which the ice sheets moved and the way their move- thick and frozen to its base so that sliding was unlikely.
ment affected their thickness. Glacial geologists have Thicker ice sheets also stand a better chance of surviving
found several ice-deposited moraines in Iowa and through relatively weak insolation maxima and growing
Nebraska that lie beyond the geographic limits of the to larger size (the “large glaciation” phase of Chapter 9).
large ice sheet that existed at the most recent glacial Eroded material preserved both in old moraines and
maximum (20,000 years ago). Layers of volcanic ash in sediments pushed into the ocean show that ancient
date these older moraines to about 2 Myr ago, early in soils were the main type of debris eroded prior to the
the interval of the 41,000-year ice sheet cycles. These last 1.5 to 1 Myr, while freshly pulverized debris has
deposits prove that at least some of the smaller-volume been predominant since that time. This evidence
41,000-year ice sheets were already reaching maximum supports Clark’s hypothesis that the ancient soils
extents comparable to those of the later larger-volume were gradually eroded by the early ice sheets. This

Equilibrium line Accumulation Equilibrium line
N S N S
Accumulation Melting Melting
Sliding No sliding Sliding
Arctic Arctic
Ocean Ocean
A North American ice 2.75–0.9 Myr B North American ice 0.9–0 Myr

FIGURE 11-16 Ice slipping may control ice sheet volume (A) During earlier glaciations of
North America, ice sheets may have been thin because they slid on water-saturated soils toward
lower elevations and warmer temperatures. (B) Later, after ice sheets stripped off most of the
underlying soil, their central regions could grow higher because they no longer slid.