Geology Reference
In-Depth Information
accurately measured with modern superconducting rock magnetometers,
making rock magnetic measurements very sensitive measures of paleoen-
vironmental conditions. In one of the case studies presented in Chapter 6,
the sensitivity of rock magnetics to paleoclimatic variations will be dem-
onstrated by a study of the Cretaceous Cupido Formation from Mexico in
which rock magnetics can detect astronomically forced cycles, even
though the repeating, shallowing-upward facies cannot.
Rock magnetic parameters have been successful measures of glacial-
interglacial cycles in loess, mainly in the Chinese Loess Plateau, but also in
Eastern Europe and Alaska (summarized in Evans & Heller 2003). Rock
magnetic measurements of European maar lake sediments have also
detected glacial-interglacial climate cycles.
Susceptibility
variations from
Lac Du Bouchet in France have been directly correlated to δ
18
O records
of glacial-interglacial cycles from the Pacific and Indian Oceans and
Greenland ice cores (Heller et al. 1998). Terrigenous input into the north-
western Indian Ocean can be tracked by magnetic susceptibility, and the
cyclic variations in susceptibility can be directly correlated to astronomical
calculations for northern hemisphere insolation (deMenocal & Bloemendal
1995). Susceptibility variations have also detected changes in paleoclimate
in Eocene marine sediments off Antarctica (Sagnotti et al. 1998). Various
studies of North Atlantic marine sediments have used rock magnetics to
study deglaciation (Stoner et al. 1995) and North Atlantic Deep Water
circulation (Kissel et al. 1999). These examples show that rock magnetic
parameters that measure a quantity as simple as the concentration of
magnetic minerals in sediment can easily detect changes as profound as
global paleoclimate.
Conducting a rock magnetic cyclostratigraphic study of a sedimentary
sequence is fairly straightforward. Most rock magnetic cyclostratigraphic
studies measure variations in the concentration of a depositional magnetic
mineral in a sequence of rocks. Magnetite (Fe
3
O
4
) is, in most cases, a
primary, depositional magnetic mineral. Therefore, erosional, transport,
and depositional processes as well as the depositional environment affect
its concentration, making magnetite the preferred target of cyclostrati-
graphic studies. Furthermore, magnetite has a relatively low magnetic
coercivity (for magnetic hardness, see Chapter 2) and its concentration is
easily measured by applying an
anhysteretic remanent magnetization
(ARM)
(for more details, see Chapter 2) to the cyclostratigraphy samples.
ARM, as will be shown in Chapter 2, also allows the researcher to target
the concentration variations of only one magnetic mineral (magnetite) in
the rock compared to the multiple mineral sources for magnetic suscep-
tibility, so the interpretation of any rock magnetic cycles recorded by an
ARM will be straightforward. However, as shown in the case studies
presented in Chapter 6, other rock parameters can be used with equal
success for identifying astronomically forced cycles in a sedimentary
sequence. The rock magnetic parameters used must be chosen on a case-
by-case basis.
