Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 114

Lake Baikal, ice cores from the polar regions and
mid-latitude continental glaciers and lake water depth
records from the Triassic of the Newark Basin in
eastern North America (Kukla et al. 1990 ; Hooghiem-
stra et al. 1993 ; Mommersteeg et al. 1995 ; Olsen 1997 ;
Thompson et al. 1997 ; Williams et al. 1997 ). Hinnov
(2000) indicates nearly continuous records of orbitally
forced stratigraphy back to the Oligocene.
The astronomically forced Milankovitch cycles
which we attempt to detect magnetically in the rock
have periods of 20 kyr, 40 kyr, 100 kyr and 400 kyr.
These variations are due to changes in the direction of
the Earth's spin axis in space (precession), the degree
of tilt of the Earth's spin axis with respect to its orbital
plane around the Sun (obliquity) and the ellipticity of
its orbit around the Sun (eccentricity). Over the past
million years, precession has had an average period of
21 kyr but is actually made up of three major periods
(19 kyr, 22 kyr and 24 kyr) and is due to the gravita-
tional pull on the Earth's tidal bulge that causes its spin
axis to precess. Precession simply changes where the
seasons fall in Earth's orbit around the Sun and acting
alone could not, if the Earth's orbit were perfectly cir-
cular, change insolation. However, when it is coupled
with ellipticity changes in the Earth's orbit, it has a
marked effect on insolation. If for example northern
hemisphere winter occurs at aphelion, when the Earth
is furthest from the Sun in an elliptical orbit, a very cold
winter results. On the other hand, if northern hemi-
sphere winter occurs at perihelion, Earth's closest
approach to the Sun, northern hemisphere winter is
moderated. Therefore, precession only effects insola-
tion by being modulated by eccentricity.
Eccentricity, the ellipticity of the Earth's orbit, has
varied with a nominal 100 kyr period (short eccentric-
ity) and a 405 kyr period (long eccentricity) over the
past million years. Short eccentricity is really made up
of two periods close to 125 and 95 kyr. By itself, eccen-
tricity only causes very small (0.1%) variations in inso-
lation; when coupled with precession the effects are
much bigger (several percent).
Obliquity is the third signifi cant orbital variation in
insolation. It has had a period of 41 kyr over the past
million years and is due to small variations in the tilt
of the Earth's spin axis with respect to the plane of the
ecliptic, the plane defi ned by the Earth's orbit around
the Sun. The variation is small (from 21.5 to 24.5°)
and the Earth's tilt is currently about 23.5°. The tilt of
the Earth's spin axis is what causes the seasons.

The periods of these variations change back through
geologic time. Eccentricity (particularly long eccentric-
ity, 405 kyr) stays fairly constant but the periods of
precession and obliquity become much shorter, mainly
because the Earth's rotation rate was faster in the past
and the Moon was closer to the Earth. As the Earth's
rotation has slowed down and the Moon has moved
further away, precession and obliquity periods have
increased.
An astronomically forced interpretation of rhythmic
lithologic variations and rock magnetic variations is
not always straightforward and may not always be
warranted. One of the greatest stumbling blocks to an
orbitally forced interpretation of cyclostratigraphy is
understanding or accepting how minute variations in
insolation at the top of the atmosphere can cause
essentially linear changes in global climate, given the
complexity and non-linearity of Earth's climate system
(Rial et al. 2004 ).
There are some that argue, quite strenuously, that
orbitally forced stratigraphy does not exist and any
patterns observed are due to circular reasoning or
unwarranted assumptions (Bailey 2009). Bailey
(2009) takes issue with the tuning of cyclostrati-
graphic records to a particular Milankovitch frequency,
a standard cyclostratigraphic analytical technique (see
for example Preto et al. 2001) as a blatant example of
circular reasoning. We will demonstrate tuning and
how it can be used to identify Milankovitch forcing (the
Cupido and Arguis formations). The resulting evidence
of orbitally forced stratigraphy can be convincingly
strong, but there are also examples where tuning
has somehow produced the appearance of Milanko-
vitch cycles despite being in confl ict with more compel-
ling evidence that they do not exist (the Latemar
controversy).
Another pitfall to the assumption of orbitally forced
cyclostratigraphy is the potential presence of autocy-
cles in a given depositional environment. Autocycles in
a platform carbonate environment will be considered
in our coverage of the rock magnetic cyclostratigraphy
of the Cupido Formation (see section on Cupido Forma-
tion below). Finally, the complexities of sediment trans-
port to the depositional basin may work to 'shred' any
climate or environmental signal so that energy injected
into a system at one frequency can be smeared across
a range of frequencies. High frequencies are much
more prone to being affected (Jerolmack & Paola
2010 ).