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Figure 10.6. Comparison of inverse solar input to 65 N ( Figure 9.11 ) with the slope of
SPECMAP ( Figure 5.4 ) over the past 800,000 years. Note that the solar curve is plotted
inversely so that higher solar intensities lie lower on the vertical scale (adapted fromRoe, 2006).
Imbrie ice accumulation model) to obtain a function proportional to ice sheet
volume that can be compared with ocean sediment data. Figure 10.5 shows the
result of such a comparison.
An alternative approach was suggested by Roe (2006). 1
In this approach one
differentiates benthic ocean sediment data to obtain dV
dt instead of integrating
insolation. The slope of ocean sediment data is interpreted as the rate of change of
the ice volume, and this is compared directly with the variability of insolation on
the same time scale. One of Roe's results is shown in Figure 10.6 . The agreement
between the slope of the SPECMAP curve and the solar curve is impressive.
Nevertheless, a few caveats are in order. First, the fact that SPECMAP was
tuned to the solar intensity curve forces the oscillations in dV
=
dt to match the
oscillations in the solar curve. Second, the SPECMAP curve is not very precise,
and assessing slopes on nearly vertical trends can be tricky. Some chartsmanship
is involved. Third, as we pointed out in Section 5.4, the time scale for SPECMAP
is believed to contain serious errors prior to 600 kybp . Fourth, as shown in
Section 5.1, Lisiecki et al. (2008) discussed the assumption that benthic d 18 O
represents a phase of changing ice volume despite the fact that benthic d 18 Ois
also affected by deepwater temperature change. However, they put forth a basis
for assuming that the benthic d 18 O can be interpreted as representing ice volume.
They also said: ''Generating a robust age model for benthic d 18 O or ice volume
=
1 Seemingly unknown to Roe, Nigel Calder came up with a similar idea back in 1974. See http://
calderup.wordpress.com/2010/07/10/milankovitch-back-to-1974/
 
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