Geology Reference
In-Depth Information
effects. Each process will be covered in a separate
chapter in the topic, along with an estimate of the
magnitude of its effect on the paleomagnetism of the
rock. For now, we'll give a brief overview of each.
If pDRM is important in a sedimentary rock, the
rock's magnetization is therefore more likely to be an
accurate record of the geomagnetic fi eld. It will
however appear to occur earlier in the sediment
column than its actual depositional age, so its timing
will be inaccurate. It is also highly likely that the paleo-
magnetic record of the fi eld will be smoothed, and the
record of high-frequency changes in the geomagnetic
fi eld will be lost. The degree of smoothing will depend
on the sediment accumulation rate; fast rates will mini-
mize the effect of smoothing.
Post-depositional remanence
An underlying assumption of post-depositional rema-
nence, based on laboratory experimental work and
fi eld observations, is that when sediments are fi rst
deposited the pore spaces between the non-magnetic
grains in the rock are larger than the submicron-
micron-sized magnetic mineral particles that give
the rock its magnetization. The magnetic grains
can therefore remain mobile and realign their mag-
netic moments with Earth's magnetic fi eld, thus
becoming more accurate in their recording of the fi eld
if they became disoriented during touchdown on the
sediment-water interface. As the sediment is buried
and becomes dewatered by compaction the pore spaces
decrease in size, trapping and immobilizing the mag-
netic mineral particles. Any sediment will have a grain
size distribution of magnetic particles, and the largest
magnetic particles in the sediment will become trapped
at shallower depths below the sediment-water inter-
face than smaller magnetic particles.
There are several results of this vision of post-
depositional remanence. First, the acquisition of the
sediment's magnetization, its post-depositional rema-
nent magnetization (pDRM), will occur lower in the
sediment column than where it was deposited. So even
though the pDRM was acquired after the sediment was
deposited, by locking in at a fi nite depth (typically
several centimeters to 20 cm according to some work;
deMenocal
et al
. 1990; Tauxe
et al
. 1996 ) the magneti-
zation acquisition time will appear to precede the depo-
sitional age of the sediment. Second, it has been argued
by some (see review by Verosub 1977) that pDRM is
more accurate than a syn-depositional DRM because
the grains have been able to reorient after touchdown
to be parallel to the ambient magnetic fi eld, and lessen
the disturbance that occurs at touchdown. Finally, in
this model of post-depositional remanence, different-
sized magnetic mineral grains are immobilized or
locked in at different depths. The result is that the pale-
omagnetic recording of the fi eld is smoothed over the
depth range in which all the magnetic grains are locked
into place by decreasing pore size (see summary in
Tauxe
et al
. 2006 ).
Burial compaction
At greater depths in the sediment column (from tens to
hundreds of meters) than depths which would cause
the lock-in of a pDRM, the dewatering due to burial
compaction can cause a bias in the magnetization
direction of sediments and sedimentary rocks. Experi-
mental data from our laboratory shows that burial
compaction can cause a shallowing of paleomagnetic
inclination of the order 10-20°, particularly for clay-
rich sediments. This effect was not fully appreciated in
the early days of paleomagnetism and even during its
highly productive middle age, because observations of
the DRM and pDRM of recent marine and lake sedi-
ments did not typically demonstrate any bias in the
recording of the geomagnetic fi eld inclination. One of
the classic experiments that informed paleomagnetists
was the observation by Opdyke & Henry (1969) that
cores collected at different latitudes from the world
ocean recorded inclinations entirely consistent with
their latitudes, following the dipole equation for the
GAD:
tan (
paleomagnetic inclination
)
=×
2
tan (
latitude
).
The cores sampled for these measurements were
only about 1-2 m in length however, so the effects of
burial compaction were not evident or important for
these recently deposited marine sediments.
Another reason that inclination shallowing caused
by compaction was not considered important by many
paleomagnetists is that it doesn't become obvious
unless the sedimentary rocks were deposited at inter-
mediate latitudes. Inclination shallowing follows a
tangent - tangent relationship (King 1955 ):
tan (
observed inclination
inclination at deposition
)
=
f
×
tan (
)
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