Geoscience Reference
In-Depth Information
Figure 3.7. The last glacial-interglacial cycle (schematic). (A) is the last interglacial,
(B) the Last Glacial Maximum and (C) the present interglacial. The sea level fluctu-
ations associated with the expansion and contraction of the ice caps are simplified.
Note the slow, saw-toothed build-up to full glacial conditions and the short duration
of the last interglacial.
The inclination of the earth's axis (at present 23
°
27
ʹ
) also varies over time, from a
maximum tilt of 24
. When the tilt is at a maximum, the
summers in the higher latitudes tend to be hotter and the winters tend to be colder.
During times of minimum tilt, summers are less hot andwinters aremilder (
Figure 3.7
).
Mild cloudy summers offer a better chance for snow to persist in high northern latitudes
than do hot dry summers. This obliquity cycle of 41,000 years is therefore the main
control over seasonality (Williams et al.,
1998
).
The third cycle (often referred to as the precession of the equinoxes or, more
simply, the precessional cycle) reflects the changing season of the year when the
earth is nearest to the sun and is governed by the direction in which the spin axis of
the earth points in space. The precessional cycle is quite variable in duration, with a
mean period calculated by Milutin Milankovitch (
1941
) for the last 1 million years
of 21,000 years, although he noted that it had ranged from 16.3 ka to 25.8 ka in that
time. James Croll (
1875
) was aware of the role of the precessional cycle in altering
the seasonal distribution of heat received at the earth's surface, but his ice age model,
although prescient, failed to explain synchronous glacial and interglacial cycles in
both hemispheres, and it postulated too short a duration for each cycle.
The dominant cycles evident in late Pliocene deep-sea cores up until 2.6 Ma ago
were the 23 ka and 19 ka precessional cycles, after which the 41 ka obliquity cycles
dominated until about 0.7 Ma ago, when the 100 ka orbital eccentricity cycles became
dominant (Williams et al.,
1998
; Lisiecki and Raymo,
2005
; Clark et al.,
2006
; Lisiecki
and Raymo,
2007
). The interval known as the 'Middle Pleistocene Transition' began
about 1,250 ka ago and had ended by 700 ka ago, when the 100 ka cycles had become
dominant (Clark et al.,
2006
;Luthi et al.,
2008
).
°
36
ʹ
to a minimum of 21
°
59
ʹ
3.4.2 Long Pleistocene records of climatic change
Very few terrestrial records span the entire duration of the Pleistocene. The long pollen
records from the Bolivian Altiplano are one such archive. Another outstanding archive
