Orbital Forcing
There are several separate components of orbital
variation. The present orbital behavior of the earth
includes the following cyclic changes (Imbrie 1985):
1. Variations in orbital eccentricity (the shape of the
earth's orbit around the sun); several "wobbles",
which have periods of 413, 100, and 54 ka.
2. Changes of up to 3" in the obliquity of the ecliptic,
with a period of 41 ka.
3. Precession of the equinoxes. The earth's orbit
rotates like a spinning top, with a period of 23 ka.
This affects the timing of the perihelion (the position of closest approach of the earth to the sun on
an elliptical orbit), which changes with a period of
19 ka.
Each of these components is capable of causing
significant climatic fluctuations, given an adequate
degree of global sensitivity to climate fo rcing. For
example, when obliquity is low (rotation axis nearly
normal to the ecliptic), more energy is delivered to
the equator and less to the poles, giving rise to a
steeper latitudinal temperature gradient and lower
seasonality. Variations in precession alter the structure of the seasonal cycle by moving the perihelion
point along the orbit. This changes the earth-sun
distance at every season, thus changing the intensity
of insolation at each season. "For a given latitude
and season typical departures from modern values
are on the order of ±5%" (Imbrie 1985). Because the
forcing effects have different periods they go in
and out of phase. One of the major contributions
of Milankovitch was to demonstrate these phase
relationships on the basis of laborious time-series
calculations. These can now, of course, be readily
carried out by computer. The success of modern
stratigraphic work has been to demonstrate the existence of curves of temperature change and other
variables in the Cenozoic record that can be correlated directly with the orbital variations. For this
purpose, sophisticated time-series spectral analysis
is performed on various measured parameters, such
as oxygen-isotope content or cycle thickness. This
approach has led to the development of a special type
of quantitative analysis termed cyclostratigraphy
(Fischer et al. 1990). Many examples of sedimentary
cycles generated by orbitally fo rced sea-level change
or climate change have now been published. The
record is particularly clear in successions controlled
by Cenozoic glacioeustasy and in certain shallowmarine platform carbonates and lacustrine rhythms.
In the case of nonmarine deposits, the effe cts of
orbital forcing are thought to be much more subtle
and difficult to prove. Forcing may be felt through
445
the climatic control of rainfall and evaporation,
upon which depends the rate of erosion, the rate of
production of detritus and its type, and the rate of
sediment transport. de Boer and Smith (1994b) suggested that orbital forcing in nonmarine settings
may be particularly important in the 30-40" latitude
belt, where the effects on precipitation and evaporation are thought to be the most pronounced.
However, the effe cts are difficult to demonstrate,
particularly where tectonism and sea-level change
may have introduced complications into the stratigraphic record.
H. Olsen (1990, 1994) described a Devonian fluvial section in East Greenland containing prominent
fining-upward cycles that show systematic variations in thickness. Variations in channel sand body
thickness are shown in Fig._ I2.4. The average thickness Of the fluvial fining-upward cycles is 20 m, with
a range of 13-30 m. Channel sandstones vary from 2
to 13 m in thickness. Bundles of 3-6 cycles can be
defined, as can thicker trends that trace out megacycles. Megacycles range from 79 to 135 m, with an
average of 101 min thickness. H. Olsen (1990, 1994)
calculated disharge values for the channels using a
suite of geomorphic equations based on channel
dimensions. The results are shown in Fig. 12.5 and
demonstrate the same cyclic variability. H. Olsen
( 1990, 1994) argued that the simplest explanation for
the rhythmic variations is changes in discharge
brought about by orbital forcing of climate changes,
and he attempted to fit the pattern of thickness and
discharge variations to patterns of orbital variation.
However, Algeo and Wilkinson (1988) offered a
warning:
"Despite an often-claimed correspondence betw"een cycle
and Milankovitch orbital periods, factors independent of
orbital modulation that affect cycle thickness and sedimentation rate may be responsible for such coincidence.
For example, nearly all common processes of sediment
transport and dispersal give rise to ordered depositional
lithofacies sequences that span a relatively narrow range of
thicknesses ... Further> long-term sediment accumulation
rates are generally limited by longHterm subsidence rates,
which converge to a narrow range of values for very differ�
ent sedimentary and tectonic environments (Sadler 1981).
In essence, the spectra of real-world cycle thicknesses
and subsidence rates are relatively limited, and this in
turn constrains the range of commonly determined cycle
periods. For many cyclic sequences, calculation of a
Milankovitch�range period may be a virtual certainty, regardless of the actual generic mechanism of cycle forma�
tion."
Algeo and Wilkinson (1988) studied cyclicity in
more than 200 stratigraphic units, determining
periodicities from cycle thickness, sedimentation
There are several separate components of orbital
variation. The present orbital behavior of the earth
includes the following cyclic changes (Imbrie 1985):
1. Variations in orbital eccentricity (the shape of the
earth's orbit around the sun); several "wobbles",
which have periods of 413, 100, and 54 ka.
2. Changes of up to 3" in the obliquity of the ecliptic,
with a period of 41 ka.
3. Precession of the equinoxes. The earth's orbit
rotates like a spinning top, with a period of 23 ka.
This affects the timing of the perihelion (the position of closest approach of the earth to the sun on
an elliptical orbit), which changes with a period of
19 ka.
Each of these components is capable of causing
significant climatic fluctuations, given an adequate
degree of global sensitivity to climate fo rcing. For
example, when obliquity is low (rotation axis nearly
normal to the ecliptic), more energy is delivered to
the equator and less to the poles, giving rise to a
steeper latitudinal temperature gradient and lower
seasonality. Variations in precession alter the structure of the seasonal cycle by moving the perihelion
point along the orbit. This changes the earth-sun
distance at every season, thus changing the intensity
of insolation at each season. "For a given latitude
and season typical departures from modern values
are on the order of ±5%" (Imbrie 1985). Because the
forcing effects have different periods they go in
and out of phase. One of the major contributions
of Milankovitch was to demonstrate these phase
relationships on the basis of laborious time-series
calculations. These can now, of course, be readily
carried out by computer. The success of modern
stratigraphic work has been to demonstrate the existence of curves of temperature change and other
variables in the Cenozoic record that can be correlated directly with the orbital variations. For this
purpose, sophisticated time-series spectral analysis
is performed on various measured parameters, such
as oxygen-isotope content or cycle thickness. This
approach has led to the development of a special type
of quantitative analysis termed cyclostratigraphy
(Fischer et al. 1990). Many examples of sedimentary
cycles generated by orbitally fo rced sea-level change
or climate change have now been published. The
record is particularly clear in successions controlled
by Cenozoic glacioeustasy and in certain shallowmarine platform carbonates and lacustrine rhythms.
In the case of nonmarine deposits, the effe cts of
orbital forcing are thought to be much more subtle
and difficult to prove. Forcing may be felt through
445
the climatic control of rainfall and evaporation,
upon which depends the rate of erosion, the rate of
production of detritus and its type, and the rate of
sediment transport. de Boer and Smith (1994b) suggested that orbital forcing in nonmarine settings
may be particularly important in the 30-40" latitude
belt, where the effects on precipitation and evaporation are thought to be the most pronounced.
However, the effe cts are difficult to demonstrate,
particularly where tectonism and sea-level change
may have introduced complications into the stratigraphic record.
H. Olsen (1990, 1994) described a Devonian fluvial section in East Greenland containing prominent
fining-upward cycles that show systematic variations in thickness. Variations in channel sand body
thickness are shown in Fig._ I2.4. The average thickness Of the fluvial fining-upward cycles is 20 m, with
a range of 13-30 m. Channel sandstones vary from 2
to 13 m in thickness. Bundles of 3-6 cycles can be
defined, as can thicker trends that trace out megacycles. Megacycles range from 79 to 135 m, with an
average of 101 min thickness. H. Olsen (1990, 1994)
calculated disharge values for the channels using a
suite of geomorphic equations based on channel
dimensions. The results are shown in Fig. 12.5 and
demonstrate the same cyclic variability. H. Olsen
( 1990, 1994) argued that the simplest explanation for
the rhythmic variations is changes in discharge
brought about by orbital forcing of climate changes,
and he attempted to fit the pattern of thickness and
discharge variations to patterns of orbital variation.
However, Algeo and Wilkinson (1988) offered a
warning:
"Despite an often-claimed correspondence betw"een cycle
and Milankovitch orbital periods, factors independent of
orbital modulation that affect cycle thickness and sedimentation rate may be responsible for such coincidence.
For example, nearly all common processes of sediment
transport and dispersal give rise to ordered depositional
lithofacies sequences that span a relatively narrow range of
thicknesses ... Further> long-term sediment accumulation
rates are generally limited by longHterm subsidence rates,
which converge to a narrow range of values for very differ�
ent sedimentary and tectonic environments (Sadler 1981).
In essence, the spectra of real-world cycle thicknesses
and subsidence rates are relatively limited, and this in
turn constrains the range of commonly determined cycle
periods. For many cyclic sequences, calculation of a
Milankovitch�range period may be a virtual certainty, regardless of the actual generic mechanism of cycle forma�
tion."
Algeo and Wilkinson (1988) studied cyclicity in
more than 200 stratigraphic units, determining
periodicities from cycle thickness, sedimentation