Post- Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 27

Ken Verosub published an important review paper
on sedimentary paleomagnetism in 1977. In the
paper, Verosub made a huge conceptual leap about
the paleomagnetism of sedimentary rocks. If deep-sea
marine sediments carry an accurate record of the
geomagnetic fi eld, as exemplifi ed by the study of
Opdyke & Henry (1969), then they must have acquired
their remanence by post-depositional remanent
magnetization. This conclusion makes sense if you
assume all syn-depositional remanence is inac-
curate (as the very early re-deposition experiments
seemed to show), but doesn't follow from the later
very slow re-deposition experiments discussed in
Chapter 2 .
Verosub goes on to suggest that the mechanism for
a pDRM, for marine sediments at least, is bioturbation.
There was contradictory evidence in the literature
when Verosub proposed this pDRM mechanism. Some
workers (Keen 1963; Harrison 1966) stated that bio-
turbation would destroy a sediment's initial DRM.
Other workers investigated the nature and accuracy
of a pDRM caused by post-depositional disturbance
with different illustrative experiments. Kent (1973)
conducted a simple stirring of high-water-content
marine sediments in the laboratory to show that bio-
turbation could reset the magnetization of a sediment
and accurately record the direction of the geomagnetic
fi eld over a range of inclinations. Tucker (1980)
followed up with 'stirred remanent magnetization'
experiments and a more sophisticated laboratory
set-up than that used by Kent (1973), but came to
essentially the same conclusion: stirring, a model of
bioturbation, could reset the magnetization of a sedi-
ment with a magnetization proportional in strength to
the ambient magnetic fi eld. Based on these results, a
stirring remanence has been the starting point for
compaction/inclination shallowing laboratory experi-
ments (Deamer & Kodama 1990; Sun & Kodama
1992), assuming it is the correct model for an accurate
marine sedimentary paleomagnetism.
Steve Graham (1974) did one of the fi rst in situ
bioturbation/DRM experiments by measuring the
magnetization of tidal fl at sediments from a heavily
bioturbated region of San Francisco Bay. The sedi-
ment's magnetization accurately recorded the direc-
tion of the local geomagnetic fi eld. The chemistry of
the sediments, the evidence of heavy bioturbation and
subsequent laboratory re-deposition experiments with
the sediment convinced him that bioturbation caused
the sediments to acquire an accurate pDRM.

Brooks Ellwood (1984) reported another fi eld experi-
ment investigating bioturbation as a mechanism for
pDRM acquisition. He sprinkled a 1 mm thick layer of
magnetite in a trench dug in tidal fl at sediments on
Sapelo Island, Georgia in a zone of active bioturbation.
He then measured the sediment's magnetization peri-
odically. It took 50 days for the magnetization to stabi-
lize in the sediments, but the magnetization was 20°
shallower than the local magnetic fi eld. After 211 days
and intense bioturbation during the summer, the sedi-
ment's magnetization began to approach the magneti-
zation of the local geomagnetic fi eld. Ellwood concluded
that bioturbation does not reset a sediment's magneti-
zation unless it is very intense. He notes that the bio-
turbation in the tidal fl ats of Sapelo Island is probably
more intense than in deep-sea marine sediments.
In contrast to this is the laboratory experimental
work conducted by Katari et al . (2000) in which
natural marine sediments were allowed to be biotur-
bated by polycheate worms introduced to the sedi-
ments in the laboratory. Despite active and obvious
bioturbation for three weeks, the magnetization of the
sediments was not reset unless the sediment had been
resuspended in a fecal mound. From this, Katari et al .
concluded that bioturbation cannot easily reset a
sedimentary magnetization and is therefore not an
important cause of a pDRM. In fact, we see that in
subsequent investigations into pDRM Tauxe and her
colleagues (Katari et al . 2000) conclude that pDRM is
not important in causing the magnetization of natural
sediments.
The confl icting results from the different bioturba-
tion and stirring experiments cited here may be due to
the difference in time that bioturbation was allowed to
act on the sediments, i.e. 3 weeks versus 211 days or
potentially much longer (but undetermined) in Gra-
ham's (1974) experiment. However, Tauxe and her col-
leagues would suggest another explanation: many of
the pDRM experiments were fl awed because the natural
sedimentary fabric had been destroyed, i.e. by stirring
or experiment preparation (digging a trench) or drying
before measurement, and that an undisturbed fabric
comprising fl occulated organics and clay grains typi-
cally traps and immobilizes the magnetic mineral
grains directly at deposition. Payne & Verosub (1982)
had come to the same conclusion earlier based on
experiments in which samples, initially with high
water content, were turned in a magnetic fi eld at dif-
ferent points of drying. For samples with less than 60%
sand, they found that the samples did not acquire any