Biological Oceanography

(^) The orbital control of global ice volume (y) is generally demonstrated by the fit of a
model based on orbital variations, developed by Imbrie and Imbrie (1980), to data like
the δ^18 O stack. The model is:
(^) where x is the orbital forcing value (e.g. from Berger 1977) as the net sum at time t of
obliquity, precession, and eccentricity effects on insolation at 65°N, Tm is the mean
time lag of the ice response and b is a non-linearity parameter subtracted when ice is
growing (slowing growth) and added when it is not (speeding ablation). The fit with
Tm = 15 kyr and b = 0.6 (Fig. 16.12, Lourens et al. 2010) is mostly very good. The
orbital effect pulse at ∼150 kyr BP is also not present in ice core δD or δ^18 O data; the
theory is excellent but does not explain everything. There are also shorter-term,
smaller-scale variations, the Dansgaard–Oeschger variations seen in Greenland ice
cores, and the Antarctic isotopic maxima that appear to be coincident or somewhat
offset (Wolff et al. 2009; Fig. 16.13). They may be driven by variations in the oceanic
deep-ventilation considered next.
Fig. 16.12 Comparison of an Imbrie and Imbrie model (Tm = 15 kyr, b = 0.6; see text)
of global ice volume response to orbital variations (shaded time-series) with the LR04