Post- Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 30

Fig. 3.2 Diagram of how de Menocal et al . (1990) and other workers determine the lock-in depth of a paleomagnetic marker
such as the Brunhes-Matuayama polarity transition (BMM) or a geomagnetic excursion. Essentially, the depth difference
between the paleomagnetic marker and an earlier time marker that doesn't have a lag in the sediment column (a foram layer,
a 10 Be-dated layer or a tektite layer) is plotted versus the sediment accumulation rate. Each data point comes from a different
core, sampling sediments with different accumulation rates. The y -intercept of the red line is a measure of the lock-in depth of
the paleomagnetic marker. Figure redrafted from Earth & Planetary Science Letters , 99, PB de Menocal et al ., Depth of
post-depositional remanence acquisition in deep-sea sediments: a case study of the Brunhes-Matuyama reversal and oxygen
isotope Stage 19.1, 1-13, copyright 1990, with permission from Elsevier. (See Colour Plate 1)

19.1 and the MBB. There is another assumption
thrown in, that bioturbation creates a mixing zone that
affects the isotope record as well as the magnetic
record, so the pDRM offset is in addition to the assumed
8 cm thick mixing zone caused by bioturbation. In the
end, de Menocal et al . see a lock-in depth of 16 cm
below the bioturbated mixing zone using this approach.
Tauxe et al . (1996) re-performed the de Menocal et
al . study, but added 11 more cores for a total of 19
cores. Using their expanded dataset and without
assuming a mixing model for bioturbation, Tauxe et al .
saw that pDRM offset was 1 cm or less. The different
results for the two studies could be partially due to the
MBB being moved upcore in the Tauxe et al . study as
the result of more rigorous alternating fi eld demagneti-
zation of the paleomagnetism in the sediments they
added to de Menocal's dataset.
Channell et al . (2004) saw a shift of the position of
the MBB to younger ages (by 4-5 kyr) in North Atlantic
high - sediment - accumulation - rate cores with respect

to the calibrated age for the MBB. Channell et al . argue
that the calibrated age of the MBB is based on its posi-
tion in slow - sediment - accumulation - rate cores, and
those cores are observed to have deeper lock-in depths
than fast-accumulation-rate cores. This is an observa-
tion made in de Menocal et al . ' s study, and would
suggest that lock-in depth has affected the calibration
of the MBB in slow - sediment - accumulation - rates cores
and to a lesser extent in Channell et al . ' s fast - sediment -
accumulation - rate cores. Tauxe et al . (2006) criticize
this interpretation, suggesting that the North Atlantic
cores studied by Channell et al . are of drift deposits and
their sediment has stayed in suspension in bottom cur-
rents for about 2 kyr before being deposited (not the
4 - 5 kyr offset Channell et al . observe), thus giving
the too-young age for the sediments (since the age of
the MBB in the core is based on the oxygen isotopic age
of the sediment).
Knudsen et al . (2008) don't look at the position of
the MBB, but at the depth of the paleomagnetic record-