Post- Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 28

pDRM. Stober & Thompson ' s (1979) re - deposition
experiments with Finnish lake sediments also led them
to conclude that magnetic grains were immobilized
soon after deposition by the formation of organic gels.
Perhaps the simplest explanation is that the absence/
presence of a pDRM depends mostly on lithology. Pale-
omagnetists typically target the fi nest - grained sedi-
ments because that would ensure the quietest
depositional environment and the least disturbed sedi-
ment. The fi nest-grained sediments would have the
highest clay content and possibly a high organic frac-
tion, thus a strong sedimentary fabric immobilizing the
magnetic particles. As sediment grain size increases
and the fraction of quartz particles in a sediment
increases, the pore spaces are larger allowing post-
depositional magnetic particle movement and there is
less chance that organic or clay fl occules will trap the
magnetic grains.

Sasajima centrifuged sediment slurries for a range of
sediment densities. In these experiments, the ability to
acquire a post-depositional remanence decreases as
void ratio decreases or sediment density increases.
Hamano suggested that his results indicated that sedi-
ments acquired a pDRM at depths of 15 cm - 2.5 m in
the sediment column. Otofuji and Sasajima only
reported sediment densities for the acquisition of a
pDRM of 1.13 - 1.3 g/cc.
If this lock-in depth behavior was typical of natural
sediments, then geomagnetic fi eld behavior would
occur lower in the sediment column or 'earlier' than
the depositional age of the actual magnetic fi eld
changes. Furthermore, if the physical immobilization
or lock-in of the grains was a function of the magnetic
particle size and the size of the sediment's pore spaces,
then larger magnetic particles would be locked-in at
shallower depths in the sediment column than smaller
magnetic particles. For a grain size distribution of
magnetic particles, subpopulations of the magnetic
particles would record the geomagnetic fi eld at differ-
ent depths in the sediment column, thereby smearing
the record of geomagnetic fi eld variations.
Denham & Chave (1982) presented a theoretical
model of pDRM acquisition and lock-in depth in which
the sediment is considered to be a fl uid with viscosity
which increases with depth; the characteristic align-
ment time of the magnetic particles in the sediment
therefore increases exponentially with depth. This is
the approach used in the modeling discussed in obser-
vations of natural sediments discussed later in this
chapter. Katari et al . (2000) criticize this approach,
based on evidence that natural sediments do not have
a gradually increasing Newtonian viscosity with
depth. Instead, they favor the approach of Shcherba-
kov & Shcherbakova (1983) who considered that
sediments are better modeled with slow elastic strain
and plastic deformation. Their slow elastic strain
could allow post-depositional realignment, but no re-
alignment could occur due to plastic strain. This theo-
retical approach better fi ts the results of laboratory
experiments by Tucker (1980) and Katari et al . (2000) .
Recent re-deposition experiments yielded confl icting
views about lock-in depth and its thickness. Katari
et al . (2000) conducted experiments with natural,
undisturbed marine sediments and saw no evidence for
realignment with a post-depositional fi eld. Their work
with three weeks of bioturbation in the laboratory also
gave weak evidence for realignment of a sedimentary
magnetization after deposition. Based on these results

MECHANISM OF P DRM ACQUISITION

Although the dominance of a pDRM in sediments and
sedimentary rocks may be controversial, it is worth-
while discussing some of the concepts in the literature
about the acquisition of a pDRM and the observations
that these concepts are based on. If the simple model
of a pDRM is envisioned, that the magnetic particle
grains are mobile after deposition and can re-orient
parallel to the Earth's magnetic fi eld, then the porosity
of the sediment allows the mobility. As the sediment is
buried more deeply in the sediment column the poros-
ity decreases and, at some point in time after deposition
and at a certain depth in the sediment column, the
magnetic grains are immobilized and the pDRM is
physically ' locked in ' . The idea of a ' lock - in depth ' for
a pDRM originally came from an important laboratory
experiment conducted by Lovlie (1974) in which sedi-
ments were incrementally deposited in a column over
a period of time in the laboratory, and the declination
of the ambient magnetic fi eld was rotated by 180°
halfway through the experiment. Sediments deposited
10 cm below the point at which the fi eld was rotated
recorded the new fi eld direction.
Lovlie's experiments were followed by those of
Hamano (1980) and Otofuji & Sasajima (1981). Both
experiments showed basically the same behavior.
Hamano compacted both synthetic and natural sedi-
ments slowly in the laboratory and turned on a mag-
netic fi eld during a range of void ratios. Otofuji and