Geology Reference
In-Depth Information
Grains that fall into the central region of the Day plot are called PSD
grains. Much natural magnetic grain behavior falls into this category. These
are small MD grains that behave as SD grains. Various explanations have
been offered to account for this behavior. These explanations include (1)
that the magnetization of the domain wall separating the domains of a two
domain grain gives the grain a net magnetization or (2) that the domain
walls are restricted by vacancies in the crystal lattice from positioning
themselves to exactly cancel out the grain's overall moment or (3) that the
spin moments in adjacent Fe atoms aren't exactly parallel but take on com-
plicated “vortex” or “flower” configurations.
2.5
Environmental Magnetic Parameters
2.5.1
Individual Environmental Magnetic Parameters
In rock magnetic cyclostratigraphy, the main goal is to measure a rock
magnetic parameter, throughout a sedimentary sequence, that will have
encoded a climate-driven process. Environmental magnetic studies provide
many examples of rock magnetic parameters being used as climate proxies,
but the best rock magnetic parameters for cyclostratigraphic studies should
be fairly easy to measure, because typically 1000s of samples need to be
processed, and fairly easy to interpret.
Magnetic susceptibility has been used in many environmental magnetic
studies to detect climate variations. It is a straightforward measure of the
concentration of magnetic minerals in a sample and is a quick and easy
measurement. It has been successful in delineating the loess-paleosol
sequences in the Chinese Loess Plateau, which can be directly correlated to
the δ
18
O climate signal of glacial-interglacial cycles in marine sediments
(Liu et al. 2012). A recent rock magnetic cyclostratigraphic study of Plio-
Pleistocene marls from the Stirone River section in Italy (Gunderson et al.
2012) shows that susceptibility can be an excellent recorder of Milankovitch
cycles in marine sediments. Data compiled by Peters and Dekkers (2003)
shows that magnetic susceptibility is not strongly dependent on magnetic
particle size for the important natural magnetic minerals. Susceptibility
measurements are typically made with a susceptibility meter, using an
inductance bridge circuit, that measures the magnetization induced in a
sample while an alternating magnetic field with strength similar to the
Earth's (50-100 μT) is applied. An alternating field is used to avoid giving
the sample a viscous magnetization.
Despite its success in cyclostratigraphic studies, the interpretation of a
magnetic susceptibility cyclostratigraphic signal is not straightforward.
Magnetic susceptibility measures the concentration variations of not only
ferromagnetic minerals, i.e., magnetite, hematite, and Fe sulfides, but also
the paramagnetic Fe-rich silicates and diamagnetic quartz and carbonate in
