Page 190 - Earth's Climate Past and Future

166 PART III • Orbital-Scale Climate Change

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This long δ O record shows two trends: (1) a grad- Glaciation δ 0 Interglaciation
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ual drift toward more positive values and (2) numerous 0
cyclic-looking oscillations between positive and nega-
tive values. Both features reflect some combination of
changes in temperature and fluctuations in ice volume
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(Appendix 1). Changes toward more positive δ O val-
ues indicate more ice on the land and/or a cooling of 41,000 years
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deep-ocean temperatures. More negative δ O values
indicate smaller ice sheets and/or warmer deep-ocean 50,000 100,000 years
temperatures. ~
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Before 2.75 Myr ago, the δ O values were relatively Years ago
negative (+3.5‰ or larger) and no ice-rafted debris was
present. During this interval, northern hemisphere ice
sheets either did not exist or never reached the size 23,000 years
needed to send large numbers of icebergs to the North 100,000
Atlantic south of Iceland. The smaller variations in
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δ O during this interval probably reflect temperature
changes in the deep waters.
Beginning 2.75 Myr ago, significant amounts of ice-
rafted debris appeared in the record, an indication that
ice sheets were now present at least sporadically. This
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debris accumulated during intervals of positive δ O 150,000
values, which occurred mainly at a regular cycle of 18
41,000 years (see Figure 9–13). This part of the record FIGURE 9-14 Ice sheet δ O changes over the last
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suggests that ice sheets were now forming during inter- 150,000 years A multicore combined δ O record covering
vals of low summer insolation but that all or most of the the last 150,000 years shows 23,000-year and 41,000-year
oscillations in addition to the larger oscillation near 100,000
ice probably disappeared during the subsequent sum- years. (Adapted from D. Martinson et al., “Age Dating and the
mer insolation maxima. Orbital Theory of the Ice Ages: Development of a High-Resolution
This regime of 41,000-year cycles persisted for the 0 to 300,000-Year Chronostratigraphy,” Quaternary Research 27
first two-thirds of the interval of northern hemisphere [1987]: 1–29.)
glaciation from 2.75 to 0.9 Myr ago. The forty or more
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δ O oscillations that can be detected during this inter-
val indicate at least forty episodes of glaciation. The
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slow background shift of the δ O signal toward more that begins during the major glaciation near 150,000
positive values during this interval also indicates a grad- years ago (Figure 9–14). Near 130,000 years ago, an
ual underlying drift into a colder world. abrupt shift occurred into an interglacial interval that
Beginning near 0.9 Myr ago and becoming more lasted until 120,000 years ago. Like the modern inter-
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obvious after 0.6 Myr ago, the character of the δ O glaciation, this interval had no ice-rafted debris in
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record changes (see Figure 9–13). Maximum δ O values North Atlantic sediments, because northern ice sheets
increase in amplitude but are spaced farther apart, indi- were not present except on Greenland.
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cating that ice sheets persisted for longer intervals of Between 125,000 and 80,000 years ago, the δ O
time and grew larger in a colder world. These glacial signal oscillated several times between values that indi-
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intervals come to an end during abrupt δ O decreases cate more or less ice and colder or warmer tempera-
that indicate rapid ice melting and ocean warming. Over tures. The spacing of these oscillations at approximately
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the last 0.6 Myr, there have been six of these large δ O 23,000 years confirms the presence of the orbital pre-
maxima, each followed by an abrupt deglaciation (called cession signal in this record. The two later glacial max-
a termination) at an average spacing near 100,000 years. ima near 63,000 and 21,000 years ago are separated by
Almost hidden in the highly compressed record shown about 42,000 years, an indication that the 41,000-year
in Figure 9–13 are smaller 41,000-year and 23,000-year orbital tilt signal is also present in this record.
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δ O oscillations that persist during the last 0.9 Myr as The rapid transition between 17,000 and 10,000
secondary cycles superimposed on the larger oscillations years ago marks a second abrupt deglaciation, the first
near 100,000 years. since 130,000 years ago. These terminations are the
To get a clearer sense of the character of these later most prominent marker of the longer-period oscilla-
cycles, we zoom in on the most recent part of a record tions near a period of 100,000 years.