Geology Reference
In-Depth Information
After the samples are brought to the laboratory, they must be trimmed to
fit into standard paleomagnetic/rock magnetic plastic sample boxes (nom-
inally 2 cm × 2 cm × 2 cm in size). The samples should be weighed because
the magnetic measurements of intensity must be normalized by sample
mass for an accurate cyclostratigraphy. Alternatively, small diameter
(~11 mm) 15 mm long cores can be drilled from the samples. After the
samples are trimmed or cored, placed in sample boxes and weighed, they
are ready for rock magnetic analysis.
If ARM is the rock magnetic parameter selected for the cyclostratigraphy,
it is applied to each sample using a specialized piece of equipment, an
alternating field demagnetizer modified so that a small, constant, biasing
magnetic field can be applied during demagnetization . After application of
the ARM, the sample's magnetic intensity is measured in a rock magnetom-
eter. Usually a superconducting rock magnetometer is used for speed of
measurement and high accuracy. The measurement of each sample takes on
the order of about 5-7 minutes for both the ARM application and the
magnetic intensity measurement. Five hundred samples can be processed in
about 50-60 hours, once they have been trimmed and weighed.
Time series analysis (Chapter  4) is used to deconstruct the ARM data
series into its constituent frequencies. Determining whether any cycles
observed are Milankovitch (astronomically forced) is the most difficult
part of the study. Ideally, some independent control on time is available to
unequivocally identify astronomically forced cycles. Biostratigraphy or
magnetostratigraphy (Chapter  3) are two important ways of doing this.
The time assignment can be at a fairly coarse scale, just high enough
resolution to allow the identification of the longest (~100 or 405 kyr eccen-
tricity) astronomically forced cycles targeted for detection in the sampling.
Once these cycles are identified, the series can be tuned either to a theoret-
ical insolation series (e.g., Laskar et al. 2004) or to a simple sinusoid at that
frequency to remove the effects of varying sedimentation rates in the
record. Time series analysis of the tuned series can determine whether
the  shorter astronomically forced cycles (precession, obliquity ) are more
pronounced in the power spectrum, thus providing further evidence that
astronomically forced cycles have been identified. The identification of
astronomical cycles has now translated the coarse resolution biostratig-
raphy or magnetostratigraphy to a high-resolution chronostratigraphy for
the sedimentary sequence.
1.3 The Significance of Rock Magnetic
Cyclostratigraphy
The realization of a high-resolution chronostratigraphy using rock
magnetic cyclostratigraphy has the potential to be a transformative chro-
nostratigraphic technique for the Earth sciences. High-resolution time
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