Milankovitch Cycles
251
their maxima. Such supply of dust by wind has implications for deep-sea sedimentation - perhaps (as dust brings phosphate and iron) even for productivity in the ocean
(and hence for atmospheric C02; see Sect. 9.4.2).
9.3.5 Milankovitch Cycles and Dating. As pointed out by Emiliani, the isotopic
variations clearly indicate some sort of regular cycling such as could be produced by
the Milankovitch mechanism, which invokes regular variations in the Earth's orbital
parameters as a cause for the succession of ice ages separated by warm periods (Fig.
9.6). The hypothesis of Milutin Milankovitch (1879-1958) states that long-term fluctuations in the radiation received from the Sun, during summer seasons, in the high
latitudes of the northern hemisphere, have controlled the occurrence of ice ages over
the last 600 000 years.
For Emiliani's suggestion to be tested, a time scale for the isotopic variations was
needed. Three dating methods are available: (1) C-14 dating of the uppermost portion
of the record, and extrapolation downward. This method is not very reliable. (2)
Uranium dating of corals which grew during the last high stand of sea level, a datum
that can be correlated with the warm peak in the Isotope Stage 5 (Fig. 9.4 c). The best
estimates for this age are near 124000 years. From this we obtain an average sedimentation rate, which is more reliable than the one based on radiocarbon. (3)
Magnetic reversals - the same that are recorded in the cooling basalt of the spreading
sea floor (Sect. 1.8) - are recorded also in deep-sea sediments. The last major boundary between magnetic epochs (the Brunhes and the Matuyama) is dated at
790000 years ago. (An earlier date of 730 000 is now abandoned). This date can be
recovered in very long cores only. It allows interpolation of the ages for the isotope
variations.
The time scale derived from methods (2) and (3) is the one used in the comparison
between isotope record and Milankovitch curve in Fig. 9.6. A certain similarity of the
curves is obvious. Spectral analysis - that is, a search for the frequencies contained in
the isotopic record - shows that the following periods are strongly represented: near
20000 years, near 40000 years, and near 100000 years (see next Section). These are
the main periods contained in the Milankovitch irradiation curve. Thus, the evidence
is strong that irradiation of the northern hemisphere is the dominating factor in
controlling the frequencies of the Pleistocene climatic fluctuations.
What are these frequencies? They describe the motions of the Earth's rotational
axis and the change in shape of its path around the sun. The axis is not stationary in
space, and does not always point to the North Star as in the present. Instead, it
describes a circle, of which the North Star is one point. The circle is completed once
in about 21 000 years. This is the precession (Fig. 9.6). Also, the inclination of the
Earth's axis to the plane of its orbit changes through time. It is 66 1/2 0 at present, but
varies between about 65° and 68° once in 41 000 years. This is the obliquity variation. Obliquity is very important, because high obliquity obviously means warm
summers and cold winters, and vice versa. Finally, the Earth's orbit about the sun is
not a circle but an ellipse (as Johannes Kepler, 1571-1630, showed). The ratio between the long and the short axis varies through time. This is the eccentricity variation. One cycle takes about 100000 years. These, then, are the elements governing
251
their maxima. Such supply of dust by wind has implications for deep-sea sedimentation - perhaps (as dust brings phosphate and iron) even for productivity in the ocean
(and hence for atmospheric C02; see Sect. 9.4.2).
9.3.5 Milankovitch Cycles and Dating. As pointed out by Emiliani, the isotopic
variations clearly indicate some sort of regular cycling such as could be produced by
the Milankovitch mechanism, which invokes regular variations in the Earth's orbital
parameters as a cause for the succession of ice ages separated by warm periods (Fig.
9.6). The hypothesis of Milutin Milankovitch (1879-1958) states that long-term fluctuations in the radiation received from the Sun, during summer seasons, in the high
latitudes of the northern hemisphere, have controlled the occurrence of ice ages over
the last 600 000 years.
For Emiliani's suggestion to be tested, a time scale for the isotopic variations was
needed. Three dating methods are available: (1) C-14 dating of the uppermost portion
of the record, and extrapolation downward. This method is not very reliable. (2)
Uranium dating of corals which grew during the last high stand of sea level, a datum
that can be correlated with the warm peak in the Isotope Stage 5 (Fig. 9.4 c). The best
estimates for this age are near 124000 years. From this we obtain an average sedimentation rate, which is more reliable than the one based on radiocarbon. (3)
Magnetic reversals - the same that are recorded in the cooling basalt of the spreading
sea floor (Sect. 1.8) - are recorded also in deep-sea sediments. The last major boundary between magnetic epochs (the Brunhes and the Matuyama) is dated at
790000 years ago. (An earlier date of 730 000 is now abandoned). This date can be
recovered in very long cores only. It allows interpolation of the ages for the isotope
variations.
The time scale derived from methods (2) and (3) is the one used in the comparison
between isotope record and Milankovitch curve in Fig. 9.6. A certain similarity of the
curves is obvious. Spectral analysis - that is, a search for the frequencies contained in
the isotopic record - shows that the following periods are strongly represented: near
20000 years, near 40000 years, and near 100000 years (see next Section). These are
the main periods contained in the Milankovitch irradiation curve. Thus, the evidence
is strong that irradiation of the northern hemisphere is the dominating factor in
controlling the frequencies of the Pleistocene climatic fluctuations.
What are these frequencies? They describe the motions of the Earth's rotational
axis and the change in shape of its path around the sun. The axis is not stationary in
space, and does not always point to the North Star as in the present. Instead, it
describes a circle, of which the North Star is one point. The circle is completed once
in about 21 000 years. This is the precession (Fig. 9.6). Also, the inclination of the
Earth's axis to the plane of its orbit changes through time. It is 66 1/2 0 at present, but
varies between about 65° and 68° once in 41 000 years. This is the obliquity variation. Obliquity is very important, because high obliquity obviously means warm
summers and cold winters, and vice versa. Finally, the Earth's orbit about the sun is
not a circle but an ellipse (as Johannes Kepler, 1571-1630, showed). The ratio between the long and the short axis varies through time. This is the eccentricity variation. One cycle takes about 100000 years. These, then, are the elements governing