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232 PART IV • Deglacial Climate Changes

Depth below modern sea level (m) FIGURE 13-3 Deglacial rise in sea level
120 110 100 90 80 70 60 50 40 30 20 10 0 Submerged corals off Barbados in the Caribbean
0 show the deglacial history of the rise in sea level
caused by the return of meltwater from the ice
sheets to the ocean. (Adapted from R. G.
Fairbanks, “A 17,000-Year Glacio-eustatic Sea Level
5,000 Record: Influence of Glacial Meltwater on the
Younger Dryas Event and Deep-Ocean Circulation,”
Nature 349 [1989]: 637–42, and from E. Bard et al.,
Years ago 10,000 14 C ages 30,000 Years Using Mass-Spectrometric U-Th Ages
“Calibration of the C Time Scale over the Past
14
from Barbados Corals,” Nature 345 [1990]: 405–10.)

15,000 Th/U ages

20,000

10,000 years ago, as expected. But the story is not that produce fastest rates of ice melting before the summer
14
simple because the C dates on the corals do not repre- insolation maximum.
sent their true ages.
The evidence reviewed in Box 13–1 indicates that 13-3 Glitches in the Deglaciation:
the dates of the rise in sea level from the thorium/ura- Deglacial Two-Step
nium method are more accurate than those from the
14 C method. The Th/U chronology shifts the timing of Another unexpected feature of the coral reef record is
the middle part of the deglaciation back in time by the observation that sea level did not rise smoothly
about 2000 years and the earlier parts of the deglacia- through the deglaciation (see Figure 13–3). It rose
tion by as much as 3500 years (see Figure 13–3). In con- quickly before 14,000 (Th/U or calendar) years ago,
trast, the timing of the summer insolation signal more slowly between 14,000 and 12,000 years ago, and
remains fixed by the independent (and highly accurate) then quickly again after 12,000 years ago. This pause in
astronomical time scale. As a result, the time of the melting rates gives the deglaciation pattern a distinctive
major rise in sea level shifts back in time compared to form sometimes called the deglacial two-step with a
the insolation curve. tempo of fast-slow-fast.
Does this earlier timing for the deglaciation invali- The pause in the rate of ice melting may have been
date the Milankovitch theory? In a larger sense, it does larger than it seems from the sea level curve in Figure
not. Milankovitch chose summer as the critical season 13–3. Measured sea level values along this curve can be
of insolation control of ice sheets, and the last deglacia- used to calculate the differences in sea level between
tion still occurred during a time when summer insola- successive intervals of time, and these differences
tion was higher than it is now, although somewhat provide a measure of the net rate of the flow of meltwa-
earlier in that interval than the Milankovitch theory ter from the ice sheets back to the ocean during
predicts. deglaciation. The meltwater influx signal calculated in
One factor that may contribute to this early this way (Figure 13–4) shows a slowing of melting rates
response is simply the large amount of ice that was during the middle of the deglaciation. Rates of ice melt-
available to melt. When summer insolation rose to val- ing were at least four to five times faster during the ear-
ues high enough to melt ice prior to the insolation max- lier and later intervals than during the pause in the
imum 10,000 years ago, the ice sheets were still very middle.
large. In contrast, when declining summer insolation The two-step deglaciation pattern tells us that the
fell back to the same values just after the maximum, glacial ice sheets were not just giant ice cubes steadily
much less ice was left to melt, no matter how warm the melting in warmer air masses under a strengthening
climate had become. This bias would naturally tend to summer sun. Instead, the ice sheets exhibited more