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Fig. 3.3 Comparison of the Bermuda Rise (BR, Lund & Keigwin 1994) and Lake St Croix (LSC, Lund & Banerjee 1985)
paleosecular variation records as envisioned by Tauxe et al . (2006). The inclination records (on the left) are correlated
according to each core's age model as presented by Tauxe et al . (2006). The offset in absolute inclination is more than likely
due to the difference in latitudes between the sites. As Tauxe et al . point out, the major features in the paleosecular variation
show no offsets, arguing against a lock-in depth for the Bermuda Rise sediments. However, when declinations are compared
for the two records (on the right) using exactly the same correlation, the declination swing to the east in the Bermuda Rise
record is decreased in amplitude and increased in wavelength (smoothed) and offset down in the sediment column. This
behavior is usually observed for a pDRM lock-in depth, in this case about 8 cm in the Bermuda Rise record. (See Colour
Plate 2)
of evidence for smoothing of the paleomagnetic record
of geomagnetic fi eld variations by lock-in depth, thus
having a serious effect on paleomagnetic accuracy (the
theme of this topic).
Tauxe et al . (2006) make the very good case that, in
their comparison between the Bermuda Rise paleose-
cular variation record and the Lake St Croix PSV
record, that Keigwin and Lund did not use the correct
ages for the Bermuda Rise record. When the inclina-
tion records are compared using the correct ages there
is no evidence of smoothing or any offset, thus arguing
against any appreciable lock-in depth effects. A re-plot
of their fi gure 9 supports the comparison made by
Tauxe et al . (2006) of inclination records, but shows
an absolute shift between the inclination records of
about 5-8° with the Lake St Croix record being steeper.
This is most likely due to the 11.3° higher latitude
of Lake St Croix compared to the Bermuda Rise site,
thus inclination shallowing for the marine sediments
does not need to be invoked to explain the difference.
However, for a complete comparison, the declination
records should also be examined (Fig. 3.3). Both of the
records show a shift of declination towards the east.
The magnitude of the shift is much greater for Lake St
Croix (70°) than for the Bermuda Rise (10°), but the
shift occurs about 8 cm deeper in the marine record
than it does for Lake St Croix. The wavelength of the
eastern declination swing is broader and the amplitude
attenuated in the marine record. The complete com-
parison of PSV records at the two sites therefore sug-
gests no lock-in depth effects for inclination, but an
8 cm lock-in depth for declination with associated
smoothing. It could be argued that the offset in declina-
tion is simply due to the westward drift of non-dipole
magnetic fi eld features (a well-known feature of the
geomagnetic fi eld's secular variation) and the Bermuda
Rise's more easterly location with respect to Lake St
Croix. The inclination records show no evidence of
westward drift, however, so this explanation is probably
not correct.
After all the pDRM studies have been considered, we
have good evidence, experimental and observational,
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