Page 191 - Earth's Climate Past and Future

CHAPTER 9 • Insolation Control of Ice Sheets 167

IN SUMMARY, evidence from δ O signals indicates
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that ice sheets have fluctuated at orbital cycles of
approximately 23,000, 41,000, and 100,000 years
during the long history of northern hemisphere
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glaciation. Unfortunately, these δ O signals also
contain a large temperature overprint that makes it
difficult to constrain the actual size of the ice sheets.
For this reason, independent confirmation is needed
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to confirm that δ O signals provide a reasonably
accurate history of ice volume.

9-7 Confirming Ice Volume Changes: Coral Reefs
and Sea Level

The oceans provide an independent measure of ice
volume—the fossil remains of coral reefs. Today most
coral reefs grow in warm tropical ocean water and prefer
clear water near small islands (Figure 9–15) rather than
water muddied by river runoff from large continents.
Coral reefs grow near sea level, and the species most
useful to climate scientists (such as Acropora palmata)
grow at or just below sea level. Reefs have strong struc-
tural frameworks that remain intact long after individual
coral organisms have died and that preserve records of
past sea level positions.
As sea level rises and falls, coral reefs migrate ups-
lope and downslope. In effect, ancient reefs function as
dipsticks that measure the past level of water in the
world ocean. Over orbital cycles of tens to hundreds
of thousands of years, fluctuations in sea level result FIGURE 9-15 Coral reefs Coral reefs form in clear, shallow
mainly from changes in the amount of water extracted waters in warm tropical seas. (Ian Cartwright/PhotoDisc.)
from the ocean and stored in ice sheets on land. As a
result, the sea level history recorded by the coral reef
dipsticks is a record of ice volume on land.
Old coral reefs can be dated by radiometric decay of ice on Earth was as small as it is now. In fact, the
methods. Their skeletons contain small amounts of amount was slightly smaller, because the “extra” 6 m of
234 U, which slowly decays to 230 Th (Chapter 2). This sea level requires that some of today’s ice on Greenland
dating technique is well suited for use during the last or Antarctica or both had melted at that time.
several hundred thousand years. The sea-level record Unfortunately, all other coral reefs that grew during
from dated coral reefs can be compared with changes in the last 150,000 years formed when sea level was below
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the δ O signal covering the last 150,000 years (see the modern position because of the greater amount of
Figure 9–14). seawater tied up in ice sheets. These other reefs must
Ocean islands in tectonically stable regions like today lie submerged deep on the underwater slopes of
Bermuda have a prominent fossil coral reef that dates to these islands and beyond easy reach. To circumvent
125,000 years ago and lies about 6 m above modern sea this problem, marine scientists have turned to ocean
level. Reefs of this age are unique during the last islands in a different tectonic setting: areas where tec-
150,000 years; they are the only indication of a sea level tonic uplift has raised older reefs that formed below
higher than today. This evidence agrees with the marine today’s sea level and has exposed them above sea level
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δ O record in Figure 9–14: the only δ O minimum (Figure 9–16).
comparable to the modern value within the last 150,000 The two most intensively studied of these islands
years dates to between 130,000 and 120,000 years ago. are Barbados in the eastern Caribbean Sea and New
Both types of evidence agree that this was the only Guinea in the western Pacific Ocean. Both islands are
interval in the last 150,000 years when the amount rimmed by prominent coral reef terraces that have been