Post- Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 29

they present a model of deep-sea DRM acquisition in
which a layer of mixing - a homogeneous layer of
thickness L at the sediment-water interface - is where
detrital magnetic particles are incorporated into the
sediment column.
Directly below this is a thin layer dL ; this is vanish-
ingly thin for Katari et al . (2000), where pDRM realign-
ment can occur. Below this are the historical layers
that provide a stable record of geomagnetic fi eld vari-
ations. In their modeling, Katari et al . assume a homo-
geneous layer about 2 cm thick. Carter-Stiglitz et al .
(2006) have conducted laboratory re-deposition exper-
iments that provide a similar, but different, picture of
deep-sea DRM acquisition (Fig. 3.1). In their experi-
ments, Carter - Stiglitz et al . ' froze ' the magnetic parti-
cles in their re-deposited sediments in place by adding
a gelatin to the sediment that solidifi ed below 20°C.
They could examine the degree of magnetization at
different concentrations (or water contents) of sedi-

ment. Based on their work, they saw pDRM acquisition
at concentrations m s /( m s + M water ) between 44 and 50%
and very weak DRM effi ciency at higher concentra-
tions. They revisit Katari et al .'s model and indicate that
the layer of pDRM acquisition dL could have a fi nite
thickness. In their model, the lock-in depth would be
dependent on the rate at which the sediment column
is dewatered by burial compaction.

EVIDENCE OF P DRM IN NATURAL
SEDIMENTS

Although laboratory re-deposition experiments can be
very helpful, particularly because different conditions
can be isolated and controlled, the best evidence for or
against a signifi cant post - depositional lock - in of sedi-
mentary magnetization must come from observations
of real sediments. The evidence of post-depositional
lock-in can be divided into two general categories.
First, there are observations of discrete offsets between
the expected depth of some distinctive geomagnetic
event and the observed depth of the paleomagnetic
recording of the event. Almost all of the studies in the
literature have used the observation of the Matuyama-
Brunhes polarity transition boundary (MBB) in marine
sediments, the most recent reversal of the geomagnetic
fi eld at 780 ka. The second type of evidence used to
support a pDRM lock-in depth comes from modeling of
the assumed smoothing of geomagnetic fi eld varia-
tions by the lock-in of magnetization over a given
depth (and magnetic grain size) range. We will take a
quick look at the studies supporting (or not supporting)
a lock-in depth in each category, and then criticisms of
those studies.
de Menocal et al . (1990) is the fi rst study to observe
an offset in the MBB compared to its expected position
with respect to a well-dated tie point in the oxygen
isotope paleoclimate record in deep-sea marine sedi-
ment cores. The basic approach used was to compare
the depth of the MBB in eight cores with the depth of
the oxygen isotope interglacial stage 19.1 (Fig. 3.2).
The robustness of the approach depends on the 8 cores
having different sediment accumulation rates (which
they do, varying over 1-8 cm/kyr). Essentially, the
depth of the MBB with respect to oxygen isotope stage
19.1 is plotted as a function of sediment accumulation
rate. The data fall on a straight line, but have a y -
intercept from which the lock-in depth is determined.
The slope of the line is the age difference between stage

Fig. 3.1 Mixed layer L envisioned as due to bioturbation
and zone of pDRM lock-in dL . The magnetization of the
sediment is locked in at a depth L + dL below the top of the
sediment column and becomes part of the historical or
archived record of the geomagnetic fi eld. Figure is from
Carter - Stiglitz et al . (2006) and represents the model of
magnetization of Katari et al . (2000) for marine sediments.
For Katari et al ., the lock-in layer dL is vanishingly small. For
Carter - Stiglizt et al . and others the lock-in depth dL has a
fi nite thickness. Some observations of natural sediments
suggest it is 15 cm thick. Reprinted from Earth & Planetary
Science Letters , 245, B Carter-Stiglitz et al ., Constraints on
the acquisition of remanent magnetization in fi ne - grained
sediments imposed by re-deposition experiments, 427-437,
copyright 2006, with permission from Elsevier.