Page 192 - Earth's Climate Past and Future

168 PART III • Orbital-Scale Climate Change

global sea level reached in the last 125,000 years was
somewhere between –110 m and –125 m below the
Present-day sea level modern level near 20,000 years ago. The timing of this
sea level minimum correlates with the largest δ O max-
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Sea level during Live coral reef imum in Figure 9–14, again confirming that δ O is a
reef deposition good index of ice volume.
(lower glacial level) The sea-level minimum from 20,000 years ago and
Initial deposition the +6-m maximum level from 125,000 years ago can
serve as anchor points to remove the effect of uplift on
A Deposition of coral reef
these tectonically active islands and to calculate changes
in sea level during the intervening interval (Box 9–2).
This method reveals that the reefs at 82,000 and 104,000
years ago were formed when sea level was lower than
today by an estimated 17 m. This estimate falls about
High interglacial
sea levels Sea level 15% of the way from full interglacial to full glacial sea
fluctuations Fossil reef levels, about the same as the relative (proportional)
change in δ O between the minimum value 125,000
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years ago and the maximum value 20,000 years ago
Low glacial sea levels
(see Figure 9–14). This agreement provides even more
Initial uplift
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confirmation that δ O is a good index of ice volume,
B Subsequent changes
despite the temperature overprint known to be present.
Each 10-m change in global sea level results in an
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isotopic (δ O) change of 0.8–1.1‰.
Exposed IN SUMMARY, coral reefs that formed during the last
fossil reef 150,000 years confirm that the δ O signal is a
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reasonable proxy for ice sheet size. The ages of the
High sea level most prominent δ O minima correspond to the
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(interglacial) ages of coral reefs formed during high stands of sea
Uplift continues level caused by reduced ice volume, and the
amplitudes of the sea level changes estimated from
C Present
the reefs correspond to the relative changes in ice
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FIGURE 9-16 Gradual uplift of coral reefs (A) Coral reefs volume inferred from the δ O signal.
may initially form on the edge of an uplifting island at times
when sea level lies below its modern position. (B) As time Is Milankovitch’s Theory the Full Answer?
passes, uplift steadily raises the island and the fossil reef toward
higher elevations, while sea level moves up and down against the As noted earlier, the 2.75-Myr history of northern hemi-
island in response to changes in ice volume. (C) Today old fossil sphere glaciation shown in Figure 9–13 has two defining
reefs can lie well above sea level as a result of uplift. characteristics. One feature is the long sequence of δ O
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oscillations, each of which lasted for tens of thousands
of years. As noted previously, this aspect of the signal
is linked to orbital forcing of glacial cycles. Milutin
lifted out of the ocean since they formed (Figure 9–17). Milankovitch, who died in 1958, did not live to see these
One of the major reefs exposed on both Barbados and remarkable records of glacial history. During his life-
New Guinea dates from 125,000 years ago, the same time, the only records available were those from glacial
age as the last interglacial reefs on tectonically stable moraines on land, and most of those were deposited dur-
islands. Two other reefs date from about 104,000 and ing or after the most recent glaciation 20,000 years ago.
82,000 years ago. The ages of the two younger reefs on Until well into the 1960s, most glacial geologists thought
Barbados and New Guinea match the ages of two that only four or five glaciations had occurred, but the
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prominent minima in the δ O signal in Figure 9–14. marine δ O record in Figure 9–13 revealed some fifty
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These reefs record oscillations toward higher sea levels glacial maxima. The other obvious feature in this δ O
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because of ice melting, but the melting was not as com- record is the slow, persistent increase in δ O values. This
plete as today and sea levels were not as high. trend is part of the gradual cooling that has been under-
Recent technological advances that permit drilling way for 55 Myr and that led to northern hemisphere
of deeply submerged reefs have shown that the lowest glaciation (Chapter 6).