Geoscience Reference
In-Depth Information
Figure 10.27. Frequency spectra of EPICA Dome C Antarctic ice core data (Jouzel et al.,
2003).
at Vostok, the spectrum for the most recent 400,000 years has a major peak
corresponding to eccentricity and a secondary peak corresponding to obliquity,
but there does not appear to be a peak corresponding to precession. Prior to
400 kybp , there is a peak that could be associated with precession. As in the case
of Vostok, there are some tantalizing correspondences with solar variability, but
there remains enough disagreement that the matter is hardly settled.
One possible explanation for the lack of a precession peak in the frequency
spectrum of ice core data relates to the discussion given in Section 10.2.1, in
which it was suggested that ice sheet buildup reacts to the 22,000-year precession
oscillations of solar intensity by growing when the amplitude of oscillations is
small and diminishing when the amplitude of oscillations is large. Thus, it may
well be that precession oscillations act as a sort of carrier signal as in AM radio,
and this carrier signal is amplitude-modulated by the variability of eccentricity and
obliquity. The 22,000-year period has no consequence for climate change except to
carry the signal for the 41,000 and 100,000-year variability of obliquity and eccen-
tricity. Hence, from this point of view the 22,000-year period due to precession is
not important when examining the frequency spectrum of ice core data; what is
important is the amplitude of precession oscillations, and these are dictated by
variations in obliquity and eccentricity.
The SPECMAP representation of ocean sediment data was given in Figure
5.4 . Comparison of solar input to high latitudes with ocean sediment data was
illustrated by M&M for the SPECMAP. The spectrum for the SPECMAP is
shown in the left half of Figure 10.28 . As can be seen from this figure, all three
solar frequencies are represented. The SPECMAP chronology was based on tuning
to solar variations as expressed in the Imbrie ice model (see Figure 9.14 ) , so it is
not surprising that solar frequencies appear. The spectrum corresponding to the
Imbrie model is shown in the right half of Figure 10.28 . In the Imbrie model, the
lowest frequency corresponds to a period of about 400,000 years, and the peak
 
Search WWH ::




Custom Search