Geoscience Reference
In-Depth Information
Figure 3.6. Orbital fluctuations.
ages are: 2.60 Ma to 1.80 Ma, 1.80 Ma to 780 ka, 780 ka to 125 ka and 125 ka to
11.7 ka (Gibbard et al.,
2010
).
3.4.1 Orbital periodicities during the Pleistocene
The history of the Pleistocene is one of alternating expansion and contraction of ice
caps and of associated expansion and contraction of deserts, with concomitant changes
in the rivers, lakes, glaciers and dunes in and around the deserts. The first two-thirds
of the Pleistocene was a time of low-amplitude, high-frequency climatic fluctuations,
with each glacial-interglacial cycle lasting about 40,000 years. The last third of the
Pleistocene, starting about 0.7Ma ago, was a period of high-amplitude, low-frequency
climatic changes, with each glacial-interglacial cycle lasting about 100,000 years.
The amount of solar radiation received from the sun in any given latitude depends on
three astronomically controlled variables (Milankovitch,
1920
; Milankovitch,
1930
;
Milankovitch,
1941
;Berger,
1981
). One is the distance of the earth from the sun. The
earth follows an elliptical path around the sun each year, with the sun not quite at
the centre of the ellipse (
Figure 3.6
). At present, when the earth is closest to the sun
(termed the perihelion), the distance is 147.1 million km. When furthest from the sun
(termed the aphelion), the corresponding distance is 152.1 million km. The shape of
the ellipse varies over time, being sometimes more elliptical and sometimes less so.
This cyclical change in what is termed the orbital eccentricity has a duration of 96,600
years and is responsible for a 3.5 per cent variation in solar radiation received in the
outer atmosphere.
