Post- Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 31

ing of a 190 ka geomagnetic excursion in Iceland
Basin cores. They compare the depth of the directional
changes of the excursion with the 10 Be record in
the core. 10 Be production in the upper atmosphere
is inversely proportional to the strength of the geo-
magnetic fi eld, so the low geomagnetic intensities
expected during a geomagnetic excursion should be
matched with a peak in 10 Be production that is then
deposited and recorded in the ocean's sediment.
Knudsen et al . see no offset, suggesting no detectable
lock - in depth.
Liu et al . (2008) essentially repeated the approach of
de Menocal et al . (1990) and Tauxe et al . (1996) , but
limited their core-to-core comparisons to pairs of cores
from the same oceanic basins. They considered water
masses to differ from oceanic basin to basin, thus
affecting the isotopic composition of the water and
hence the isotopic composition of the shells made from
the water. Since the depth of the fl uctuations in the
isotopic composition of the shells is ultimately what
the depth of the MBB was being compared to, they felt
that restricting the comparisons to a given basin would
give more accurate results. When they used this
approach they fi nd an offset of the MBB by < 20 cm
from its true position with respect to the oxygen iso-
topic record; for Liu et al ., this offset however includes
both the bioturbated mixing layer and the pDRM lock-
in depth. They argue that most of the offset is due to
mixing by bioturbation and not true lock-in depth,
which they suggest may only be 1 cm or so thick.
Finally, Suganuma et al . ' s (2010) study of the posi-
tion of the MBB directional and paleointensity changes
in marine sediment cores in the western Pacifi c com-
pared to the 10 Be record of paleointensity variations
seems to be clear-cut evidence of a 15 cm lock-in depth
for the pDRM recorded by these deep-sea sediments.
However, the match of paleomagnetic paleointensity
variations and 10 Be variations is not straightforward
and could affect the accuracy of the 15 cm offset deter-
mination. Furthermore, in Knudsen et al . ' s (2008)
study of the Iceland Basin, the sediment accumulation
rate was nearly an order of magnitude higher (11-
24 cm/kyr) than the western Pacifi c Ocean sediments
studied by Suganuma et al . (2010) ( < 1 cm/kyr). These
differences are one possible cause of the different
results of these two similar studies. Suganuma et al .
(2011) continued their pDRM lock-in studies with
modeling of lock-in depth for both low-sediment-
accumulation-rate cores from the Pacifi c (∼ 1 cm/kyr)
and high sedimentation rates from the North Atlantic

(∼10 cm/kyr), and found evidence for a lock-in depth
of 17 cm.
Lund & Keigwin (1994) is the fi rst study to provide
evidence of a pDRM lock-in depth by the 'smoothing
of the paleomagnetic record' approach. The main evi-
dence in support of a 10 - 20 cm lock - in depth comes
from assuming that the marine sedimentary record of
paleosecular variation of the geomagnetic fi eld over
the past 10,000 years has been smoothed with respect
to lake sediment data recording paleosecular variation
over the same period. The lake records used by Lund
and Keigwin are from Minnesota and Great Britain.
Their records are mathematically smoothed assuming
10 and 20 cm thick smoothing (lock-in) zones and
then compared to the marine sediment paleomagnetic
records from the Bermuda Rise measured by Lund and
Keigwin. The comparisons are made for both inclina-
tion and declination.
Kent & Schneider (1995) compare three cores with
records of the MBB and different sediment accumula-
tion rates. The high - sediment - accumulation - rate cores
show a double dip in paleointensity before and during
the MBB, while the slow - sediment - accumulation - rate
core shows only one dip. They show that they can
reproduce the slow - sediment - accumulation - rate core
dip record by sampling the record of the fast-
accumulation-rate core at a slower rate, but also by
assuming a pDRM lock-in depth of 16 cm.
Hartl & Tauxe (1996) obtain a different result with
the same approach as Kent and Schneider, by adding
fi ve new studies to Kent and Schneider's study.
Hartl and Tauxe show that detailed alternating fi eld
demagnetization and a more sophisticated determina-
tion of paleointensity in their records helps to resolve
two dips in paleointensity, even in the low-sediment-
accumulation-rate cores. They therefore argue that the
need to assume a lock-in depth to explain only one dip
in paleointensity in the slow - sediment - accumulation -
rate cores is no longer needed.
These studies are presented in such detail in order to
give a better sense of the quality and character of the
evidence supporting, or not supporting, a fi nite lock
in-depth; the major point is that all of the studies make
assumptions and somewhat complicated analyses to
observe lock-in depth. Tauxe et al . (2006) make some
good criticisms of some of these studies, essentially
repeating the arguments of Tauxe et al . (1996) and
Hartl & Tauxe (1996), but we will examine their criti-
cism of Lund and Keigwin in a little more detail simply
because this study is the fi rst and one of the best pieces