Page 285 - Earth's Climate Past and Future

CHAPTER 14 • Millennial Oscillations of Climate 261

14 C 14 C
years years
ago ago
0 0
1400 1400
10
10 2900 20 2800
Depth in core (cm) 30 4100 Depth in core (cm) 30 4300
20
5900
40
40
50
50
60 5800 60 8100
70
9500
70 80
7900 20 10 0 20 10 0
80 % grains Ice-rafted
30 20 10 300 200 100 0 B stained red grains/g sediment
% grains Volcanic grains/g
A stained red sediment

FIGURE 14-10 ∼1500-year cycle in North Atlantic ice rafting? Detailed analysis of the last
10,000 years from two widely separated sediment cores shows small maxima in ice-rafted debris
at intervals near 1500 years. Each ice-rafting peak contains volcanic glass fragments from
Iceland, as well as iron-stained quartz and other grains from regions farther north. (Adapted
from G. Bond et al., “A Pervasive Millennial-Scale Cycle in North Atlantic Holocene and Glacial
Climates,” Science 278 [1997]: 1257–66.)

may not even be the same phenomenon as the large Some scientists claim to have found a ~1500-year
fluctuations that occurred during times when ice sheets cycle in both the ice sheet and ocean records, but
were present. this evidence remains questionable. The intervals sepa-
rating major δ O oscillations in the Greenland ice
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Causes of Millennial Oscillations core are irregular, ranging from approximately 1000
years in length to 9000 years or more. Time-series
One important consideration is assessing the origin of analysis of the Greenland ice core δ O record indicates
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millennial oscillations is whether or not they occurred that only a very small fraction of the observed varia-
in cycles. If the oscillations were cyclic, they should be tions falls within a band centered near 1500 years. Fil-
useful for predicting the natural course of climate tering of the δ O signal at this period reproduces a
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change in future centuries, and at first glance, many few of the shorter-term δ O variations, especially
of the oscillations do look cyclic. The Dansgaard- those during the interval near 35,000 years ago, but
Oeschger oscillations shown in Figure 14–2 have been the cycle is not obvious in most of the rest of the
referred to as Dansgaard-Oeschger cycles, and the longer- record, where longer-duration oscillations dominate
term variations in polar foraminifera and ice-rafted (Figure 14–12).
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debris (which include the Heinrich events) are some- The C-dated evidence for a 1500-year cycle in ice
times called Bond cycles after the geologist Gerard Bond rafting in the North Atlantic Ocean during the last
who first detected them. In fact, however, the case for 10,000 years looks more convincing (see Figure 14–10),
strong cyclic behavior is not convincing. but the measured duration between ice-rafting maxima