Geology Reference
In-Depth Information
Figure 7.6
KLY-3s
Kappabridge susceptibility
meter that can be used to
measure bulk magnetic
susceptibility of
cyclostratigraphy samples.
Lehigh University
paleomagnetism laboratory.
paleomagnetism laboratory, so that any ARM applied after a high-field IRM
application will be difficult to measure since the background will be much
stronger than the ARM.
The following sequence of measurements is just a suggestion, but it has
proven useful in the rock magnetic cyclostratigraphy studies done in the
Lehigh University paleomagnetism laboratory:
•
Natural remanent magnetization (NRM): NRM measurements won't
usually be able to detect climate cycles but they may be able to detect
heterogeneously distributed magnetic overprinting either by viscous
or chemical effects. Secondary chemical overprints (growth of
secondary magnetic minerals) would affect the quality and veracity
of the cyclostratigraphic record.
•
Magnetic susceptibility (χ): Magnetic susceptibility measurements are
typically made on a susceptibility meter, not a rock magnetometer
(Figure 7.6). It measures the magnetization induced in a rock sample
during the application of small magnetic field, similar in magnitude
to the geomagnetic field. A susceptibility measurement is a fast
measurement that gives composite concentration variations for all the
magnetic minerals: ferromagnetic, paramagnetic, and diamagnetic, in
the rock sample. In the Stirone River section study (Gunderson et al.
2012), susceptibility measurements gave the best cyclostratigraphic
signal, better than ARM, so susceptibility measurements should
always be made in a cyclostratigraphy study. Susceptibility may also be
combined with ARM measurements in some studies to monitor
magnetic mineral particle size.
•
IRM acquisition experiments: IRM acquisition experiments, in which
a sample is exposed to higher and higher DC magnetic fields in an
impulse magnetizer and its remanence is measured at each field step,
can be an important way to identify the ferromagnetic mineralogy in
