Page 189 - Earth's Climate Past and Future

CHAPTER 9 • Insolation Control of Ice Sheets 165

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ocean) and one horizontal dimension (latitude); changes linked changes in δ O to changes in orbital insolation.
in the other horizontal dimension (longitude) are omit- They found that orbital periods were clearly present in
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ted. The goal of these coupled models is to simulate the δ O changes over the last 300,000 years and that the
linked changes in ice sheets and the atmosphere-ocean δ O changes lagged behind changes in summer insola-
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system. tion forcing by several thousand years, equivalent to the
lag of ice volume behind summer insolation predicted
Northern Hemisphere Ice Sheet History by Milankovitch.
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The first continuous and detailed δ O record of the
The history of glaciation in the northern hemisphere entire 2.75 Myr of northern hemisphere glacial history
has been reconstructed during the last four decades. was compiled in the late 1980s by isotopic analysis of
The two most definitive kinds of evidence of this his- benthic foraminifera from the North Atlantic Ocean
tory have come from the ocean. 18
(Figure 9–13). The core from which this δ O record
was taken also contains ice-rafted debris from ice sheets
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9-6 Ice Sheet History: δ O Evidence
on the adjacent continents.
At first thought, it might seem that the best records of
past glaciations would be found on continents where the
ice sheets actually existed. Ice erodes underlying sedi-
ments and bedrock and deposits long moraine ridges More ice Less ice
Colder
Warmer
containing unsorted sediment called till (Chapter 2). temperature temperature
Unfortunately, these deposits are of little use in recon- δ 0 (‰)
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structing long-term glacial history because each succes- 5 4 3
sive glaciation erodes and destroys most of the sediment 0
left by the previous ones. The few undisturbed deposits 100,000
that remain are isolated fragments beyond the reach of years 100,000-year
radiocarbon dating. cycles dominant
Continuous records of glacial history come from 0.5
ocean basins where sediment deposition is uninterrupted.
Ocean sediments contain two key indicators of past Transition
interval
glaciations: (1) ice-rafted debris, a mixture of coarse and
fine sediments delivered to the ocean by melting icebergs 1.0
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that calve from ice sheet margins; and (2) δ O records
from the shells of foraminifera, which provide a quantita- 41,000
tive measure of the combined effects of changes in ice vol- Myr ago years
ume and in the temperature of ocean water (Appendix 1). 1.5
These signals accumulate layer by layer in sediments on
the ocean floor. 41,000-year
Decades ago, the marine scientists Cesare Emiliani cycles dominant
and Nick Shackleton pioneered the use of oxygen-isotope 2.0
ratios recorded in the shells of marine foraminifera to
study past climates. In the 1950s and 1960s Emiliani ana- 41,000
lyzed δ O records extending back a few hundred thou- years
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sand years and interpreted the δ O variations primarily as 2.5
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a record of past temperature changes. In the late 1960s
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Shackleton proposed instead that the δ O signals were Slow drift in trend First ice rafting
2.75 Myr ago
mostly a record of changing global ice volume, with only
a small overprint from temperature changes. (Current FIGURE 9-13 Evidence of ice sheet evolution: δ O A
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thinking is that the effect of ice volume on δ O signals sediment core from the North Atlantic Ocean reveals a long
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lies somewhere between these two views and is slightly δ O record of ice volume and deep-water temperature
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larger than the temperature effect in most regions). Using change. No major ice sheets existed before 2.75 Myr ago, after
new mass spectrometers capable of analyzing very small which small ice sheets grew and melted mainly at a cycle of
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samples, Shackleton published detailed δ O signals based 41,000 years until 0.9 Myr ago. Since that time, large ice sheets
on bottom-dwelling (benthic) foraminifera and extended grew and melted at intervals near 100,000 years. The diagonal
our knowledge of glacial history much further into the white line shows a gradual long-term δ O trend toward more
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past. ice and colder temperature. (Adapted from M. E. Raymo, “The
In 1976 James Hays and John Imbrie joined with Initiation of Northern Hemisphere Glaciation,” Annual Reviews of
Shackleton to write a landmark paper that conclusively Earth and Planetary Sciences 22 [1994]: 353–83.)