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170 PART III • Orbital-Scale Climate Change

BOX 9-2 LOOKING DEEPER INTO CLIMATE SCIENCE
Sea Level on Uplifting Islands

oral reefs on islands that are slowly being lifted out of This calculation removes the 6-m difference in sea
Cthe ocean by tectonic processes can be used as sea level between today and 125,000 years ago to isolate
level dipsticks if the overprint caused by uplift can be the average effect of the gradual uplift between the
removed. Ancient coral reefs now sit on the emerged two times. With the mean uplift rate determined (and
flanks of islands at elevations defined relative to modern assumed to have remained constant over the last 125,000
sea level. The present elevation is the result of two factors: years), the amount of uplift that has occurred since a reef
(1) the position of sea level at the time the reef formed of any intermediate age formed can be corrected in order
(the difference from the modern position), and (2) the to derive an estimate of sea level at the time that it
amount of uplift of the island since the reef formed. formed:
The amount of subsequent uplift of each reef depends
S = h – (U)(t)
on its age (older reefs have undergone greater uplift) and
on the rate of uplift of the island (reefs of a given age have
where S is the relative sea level at the time the older
undergone more uplift on fast-rising islands than on slow-
reef formed (in meters), h is the present elevation of the
rising ones). Although the present elevations of these
older reef (in meters), U is the mean uplift rate in meters
older reefs are measured from present sea level, modern
per 1000 years, and t is the time elapsed since the reef
sea level is just one position in a pattern of continual rises
formed (in thousands of years).
and falls through time.
With these equations, the differing effects of uplift
First the average rate of uplift (U) is calculated for the
that occurred on any island can be removed. For example,
island under study by using the last interglacial reef on the
New Guinea is being uplifted at a rate close to 2 m per
island as a reference point. Both the age of this reef
1000 years, while Barbados is rising at a rate of 0.3 m per
(125,000 years) and the sea level at the time it formed (6 m
1000 years. A coral reef dated to 82,000 years ago will
higher than today) are known. The calculation is
have been uplifted by 150 m on New Guinea since it
formed, but only by 25 m on Barbados. With the correc-
(Ht – 6)
U = —————— tion for local uplift rates, the 82,000-year reef formed on
125
Barbados must have formed when global sea level was
where U is the mean uplift rate in meters per 1000 about 17 m lower than it is today. The same kind of calcu-
years and Ht is the present elevation (in meters) of the lation shows that the 104,000-year reef formed at roughly
125,000-year reef on the island under study. the same relative sea level on New Guinea.

point remained over the Arctic Ocean at all times (Figure To some degree, this small-glaciation interval agrees
9–19B). North America and Eurasia remained too warm with the Milankovitch theory, which predicted discrete
for ice sheets to form. intervals of glaciation, each lagging just behind individ-
Small Glaciation Phase (2.75–0.9 Myr ago) By ual summer insolation minima and ending during subse-
2.75 Myr ago, global cooling had altered the position quent summer insolation maxima. On closer inspection,
of the equilibrium-line threshold which now began to however, the δ O oscillations during this period are not
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interact with the summer insolation curve. At intervals fully consistent with his theory. Insolation changes at
of 41,000 or 23,000 years, summer insolation minima high northern latitudes show much larger variations at
crossed the equilibrium line threshold (Figure 9–19C), the 23,000-year precession cycle than at the 41,000-year
the climate point moved southward over the continents tilt cycle, both in monthly changes and in the caloric
(Figure 9–19D), and ice sheets began to grow. But summer index used by Milankovitch (Chapter 7). Yet the
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before these ice sheets could grow very large, summer δ O signal during the small glaciation phase is domi-
insolation had already begun to increase toward the nated by very strong 41,000-year variations, with much
next maximum, and the equilibrium line had moved weaker changes at 23,000 years (see Figure 9–13). A
northward off the continents. As a result, the ice sheets plot of the relative amplitude of these orbital cycle varia-
melted away. tions using the power spectrum method (see Chapter 7)