41 kyr obliquity cycle. An interval of transition followed from 1.1 to 0.676 Mya; then
came a series of six major glaciations with overall periodicity at (quite roughly)
100,000 years with continuing oscillations of ice volume (Fig. 16.11) corresponding
to the shorter 41 and 23 kyr oscillations. It is thought, although uncertainty remains,
that the longer cycling became possible as a result of persistence of extensive parts of
the northern ice sheets through the warm phases of the shorter cycles, accelerating
glacier recovery with a return of cooler summers via the feedbacks mentioned above.
Only when the eccentricity maximum put the summer solstice (and perhaps also the
tilt) at the perihelion was summer warming sufficient to ablate the ice sheets almost
completely over a period of a few thousand years. Moreover, almost as soon as this
combination of orbital and rotational conditions passed, ice sheets re-advanced in
somewhat intermittent fashion for nearly the full 100 kyr cycle period. Thus, large and
usually growing northern ice sheets have been present through much of the past 675
kyr. Re-advance during the 100 kyr cycles was not slower than the retreats, but
advances were more intermittent than the major ablations, taking longer overall.
Fig. 16.11 A consensus “stack” of ^18 O content estimates from carbonate in marine
sediment cores. The data, stratigraphically aligned and averaged, are proportional
estimates as δ^18 O (‰), plotted on an inverted scale to show intervals of low terrestrial
ice volume and low ^18 O content as peaks. Age estimates are from various sources,
including ^14 C in carbonate, magnetic stratigraphy shown as black and white bars
(Brunhes, etc.). Primary responses to Earth’s orbital variations in different eras are
indicated next to duration arrows. The small numerals are designations for “marine
isotope stages” (MIS), as for example, MIS-5 is the switch at the end of the
interglacial. The “stack”, termed LR04 for Lorraine Lisiecki and Maureen Raymo
who combined the data in 2004, is often used for comparison with variables in glacial
ice cores and new sediment profiles.
(After Lisiecki & Raymo 2005.)