Rock Magnetism - Rock Magnetic Cyclostratigraphy - page 27

a rock. It is preferable to be able to target a specific population of magnetic
grains for cyclostratigraphic analysis in order to better understand how
climate cycles were encoded. For this reason, measuring the concentration
variations of specific ferromagnetic minerals is preferred for cyclostratigra-
phy studies.
The rock magnetic parameter of choice for many of the earliest rock
magnetic cyclostratigraphic studies (Latta et al. 2006; Kodama et al. 2010) is
anhysteretic remanent magnetization (ARM). ARM is a laboratory-applied
remanence. A sample is placed in an alternating magnetic field that decays
to zero over approximately a minute. The initial peak, alternating field used
in most laboratories is approximately 100 millitesla (mT). The application of
an alternating magnetic field that decays to zero is the procedure used for
standard alternating field demagnetization, and modified alternating field
demagnetizers are usually used to apply ARMs. The modification involves
the ability to apply a small, constant, biasing magnetic field during the
ramp-down of the alternating field, so the sample is left with a remanence
that is subsequently measured on a rock magnetometer. The constant
biasing magnetic fields used for ARM application are usually about the same
strength as the Earth's magnetic field (~50 μT). Since the strength of ARM is
dependent on the strength of the biasing field, many workers normalize the
ARM by the biasing field's intensity. The field-normalized ARM is denoted
the ARM susceptibility, χ ARM =ARM/H b , where H b is the biasing field, and
allows comparison of the results from different laboratories. The power of
using ARM or χ ARM for cyclostratigraphic studies is that it measures the
concentration variations of only the ferromagnetic minerals in a sample.
Since most laboratories cannot apply ARMs in peak alternating fields higher
than about 100 mT, most ARM measurements are limited to lower coer-
civity ferromagnetic minerals like magnetite, titanomagnetite, or greigite.
Paramagnetic and diamagnetic minerals do not contribute to an ARM.
Similarly, if very high coercivity magnetic minerals are present in the rock,
e.g., hematite or goethite, they will typically not be activated by a 100 mT
alternating field and will not contribute significantly to an ARM. While
ARM or χ ARM are usually interpreted to measure concentration variations in
a sedimentary sequence, χ ARM is strongly dependent on magnetite grain size
(Peters & Dekkers 2003). This relationship is to a large extent the result of
SD magnetite grains smaller than about 0.1 µm acquiring χ ARM s approxi-
mately 10 times stronger than magnetite grains in the 0.1-10 µm size range,
where the ARM acquired is not a strong function of grain size. This strong
sensitivity of small SD magnetite grains to ARM application should be
considered when interpreting ARM rock magnetic cyclostratigraphies. For
small SD magnetite grains <0.1 µm in size, χ ARM will be strongly dependent
on grain size, but the flat response of χ ARM to grain sizes in the 0.1-10 µm
range indicates that for these larger grains χ ARM is more of a concentration-
dependent parameter. The magnetic grain size of the magnetic minerals
should always be checked with the magnetic parameter ratios discussed
below, to see if the ARM variations could be due to grain size variations or