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particularly if the magnetostratigraphy can be easily correlated to the
Geomagnetic polarity time scale
(Gradstein et al. 2004; Gradstein et al.
2012). Since the measurement of a magnetostratigraphy is an important
way of assigning time to a stratigraphic section so that
Milankovitch
cycles
are more easily identified, in many rock magnetic cyclostrati-
graphic studies, a magnetostratigraphy has been determined as a part of
the study, and in doing so has demonstrated that the magnetic minerals
are likely to be primary, depositional minerals. In some cases, a previous
paleomagnetic or magnetostratigraphic study has shown that the paleo-
magnetism is depositional in age and that the rocks are perfect for a rock
magnetic cyclostratigraphic study. In some cases, poor paleomagnetic
results do not necessarily rule out depositional magnetic minerals. For
instance, Precambrian and Paleozoic rocks of the southern California-
western Nevada region are dominated by secondary viscous magnetiza-
tions (Gillett & Van Alstine 1982) that are carried by depositional
minerals. The rocks have a poor magnetostratigraphy but have yielded
a reasonable rock magnetic cyclostratigraphy (see Chapter 6; Minguez
et al. 2014).
In the rock magnetic cyclostratigraphies described in this topic, the
emphasis is on the permanent or remanent magnetizations of the rocks.
Successful cyclostratigraphies have resulted from the measurement of
magnetic susceptibility variations throughout a sedimentary sequence (see,
for example, Mayer & Appel 1999; Ellwood et al. 2011; Gunderson et al.
2012); however, as will be shown in Section 2.4, the remanent magnetization
measurements typically used for rock magnetic cyclostratigraphy can be
targeted for specific magnetic minerals or subpopulations of a magnetic
mineral. The induced magnetization that is measured in a susceptibility
measurement is the composite of the induced magnetizations for the differ-
ent magnetic minerals in a sample: iron-rich silicates or clays, calcite or
quartz grains, and the ferromagnetic mineral grains of either very small or
large grain size. The susceptibility signal cannot be easily deconstructed, so
the source of the cyclostratigraphic signal is usually poorly known and the
mechanism by which the environmental or climate process was encoded
remains somewhat of a mystery.
2.2
Types of Magnetism
Understanding the concept of a magnetic dipole moment is critical to under-
standing magnetization measurements for rock magnetic cyclostratigraphy.
The magnetization or magnetism of any material can be envisioned as a dipole
moment. A dipole moment has two magnetic poles, a north (+) pole and a
south (-) pole that are always paired. The magnetic moment vector points
from the south pole toward the north pole (Figure 2.1). Unlike an electric
dipole moment, in which positive and negative electric charges become paired,
