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linearity of the DRM intensity response to fi eld strength
and the accuracy of the direction of the DRM. Their
theoretical modeling and experimental results would
suggest that freshwater sediments can be very accurate
recorders of geomagnetic fi eld directions but not of
relative paleointensities, while marine sediments will
do better as relative paleointensity recorders but may
suffer from inclination shallowing. The basic idea
behind this observation is that the orientation of small
fl occules at low salinities is controlled predominately
by the geomagnetic fi eld but the DRM response to the
fi eld strength is non-linear. However, the orientation of
large fl occules that result from higher salinities, i.e.
those in marine conditions, are controlled mainly by
hydrodynamic forces. The damping effect of the hydro-
dynamic forces makes the response of the DRM of large
fl oc-dominated sediments (marine sediments) more
linear to the fi eld strength, but can lead to directional
inaccuracy. Moreover, small fl uctuations in fl oc size,
particularly for the small fl ocs of freshwater sediments,
can have a large effect on DRM effi ciency (another
reason for inaccurate relative paleointensity variations
for freshwater sediments).
The proposal that fl occulation affects the DRM
acquisition mechanism has its roots in Shcherbakov &
Shcherbakova's (1983) theoretical treatment of mag-
netic and non-magnetic particles sticking together
(coagulating) either during diffusion or gravitational
settling. The only sticking mechanism mentioned in
Shcherbakov and Shcherbakova's paper is van der
Waals forces. Diffusion is predicted to be more impor-
tant for particles (non-magnetic and magnetic) less
than 1 μm in size, and kinematic processes are more
important for larger particles (> 1 μm) settling through
smaller particles. Anson & Kodama (1987) independ-
ently realized the importance of clay particles and
magnetite sticking to each other and proposed an elec-
trostatic sticking mechanism. They used it to explain
inclination shallowing observed in their laboratory
compaction experiments. Sun & Kodama (1992) pro-
vided a more evolved model for inclination shallowing
than that of Anson & Kodama, based on more sophis-
ticated experimental evidence in which magnetite
particles become attached to clay particles by electro-
static and van der Waals forces when sediment is
initially deposited. As the clay fabric in a sedimentary
rock develops soon after deposition, clay domains are
formed and the magnetite particles become embedded
in the clay domains. Clay domains have been observed
in natural marine clay-rich sediments (e.g. Bennett
et al
. 1981) and are essentially agglomerations or fl oc-
cules of clay particles sticking together by electrostatic
forces. For clay-rich marine sedimentary rocks, the
fl occules of Tauxe
et al
. (2006) and Mitra & Tauxe
(2009) may well be the clay domains of the Sun &
Kodama (1992) model. The only difference between
the models is when the magnetite grains become
attached to the clay grains and are incorporated into
the clay domains. The Tauxe fl occulation model sug-
gests that this happens before deposition during
settling. This scenario is proposed initially in Shcherba-
kov & Shcherbakova ' s (1983) theoretical paper. Sun &
Kodama (1992) did not specify the timing but implied
that attachment happened at or soon after deposition;
this was probably because they visualized the sedimen-
tary particles being so widely dispersed in the water
column that they were essentially non-interacting
until touchdown.
The syn- or post-depositional attachment of mag-
netite particles to clay grains would undoubtedly
inhibit post - depositional realignment. The observation
that the magnetization does not decrease during criti-
cal point drying of a recently deposited high-water-
content sediment (Sun & Kodama 1992) would suggest
that the magnetic grains are fi rmly fi xed to the clay
particles or embedded in clay domains, even in a very-
high-porosity sediment soon after deposition. The lack
of post-depositional realignment of magnetic particles
was also an important implication of the model of
Tauxe
et al
. (2006) . The early laboratory re - deposition
experiment by Johnson
et al
. (1948) that showed a
linear dependence between fi eld and DRM intensity
was conducted with glacial clays and fresh water, so it
is unlikely that fl occulation was important. Electro-
static or van der Waals attachment of magnetite par-
ticles to unfl occulated clay particles, as envisioned by
Sun & Kodama (1992), was the probable misalignment
mechanism.
DRM IN HEMATITE-BEARING ROCKS
The magnetization mechanism of hematite-bearing
sedimentary rocks, in particular red beds, has long
been controversial (see discussion of the red bed con-
troversy in Butler 1992). Both a DRM and a chemical
remanence (CRM) have been proposed for the paleo-
magnetism of red beds with a variety of evidence sup-
porting each idea. Some workers have shown different
inclinations in the topset or bottomset beds when
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