652
Bleil and von Dobeneck
ing techniques often fail, prospects should be rather
encouraging. On the other hand, for mixed lithologies and thus large areas of the world oceans there
is at present little reason to be optimistic.
Conclusions
For the first time a succession of at least 5 or 6
different polarity events within the younger half of
the normal polarity Brunhes Chron have been documented at equatorial latitudes. Paleomagnetic
analyses of sediment sequences from the Ceara
Rise revealed repeated complete 180 0 shifts of
ChRM declinations from northern to southern
directions indicating full reversals of the Earth's
magnetic field and implying an unusually high variability of the geodynamo system during the late
Quaternary. Fluctuations in relative paleointensities
are directly associated with polarity transitions.
Minima typically coincide either with singular short
reversal episodes amid opposite polarities or with
reversal boundaries.
Converted into time series on basis of a common high-resolution chronostratigraphy, the
paleomagnetic data sets ofthe different sites show
similar general patterns in polarity and relative
paleointensity, but also remarkable inconsistencies.
Polarity events occur both during glacial and interglacial periods. There is no conclusive evidence
for a systematic correspondence of relative paleointensities to orbital frequencies in the precession or
obliquity band and hence a link to climate changes.
The coherence of a regional polarity stack for
the Ceara Rise with published results, mostly from
high latitudes, appears moderate at best. While an
about equal number of events is observed for the
last 380 kyr ofthe Brunhes Chron, the discrepancies in age assignments are too large and especially too variable to be reasonably explained by
an inaccurate dating. Similarly, the accumulated
Ceara Rise relative paleointensity record shows
only marginal agreements with a recent global scale
synthesis of pelagic sediment data from mid latitudes.
Model calculations based on the idea of a
delayed remanence acquisition qualitatively
account for all these findings. They demonstrate
that Earth's magnetic field characteristics are
recorded in the remanent magnetization of sediments by gradual processes critically depending
on sediment composition. Each lithology and its
variability through time acts as an individual filter
controlling the depth position, where in the sediment column polarity transitions are established in
the ChRM relative to the paleosurface of their
actual occurrence. As a result, intermittent complete by-passing and/or interference phenomena
will potentially produce ghost images of polarity
events in ChRM directional data. Uniform high
sedimentation rate terrigenous deposits should be
almost perfectly suited to register details of the
Earth's magnetic field history, whereas low to
medium sedimentation rate carbonates and variable sediment compositions may be extremely difficult to decipher appropriately. The model suggests
that Brunhes Chron and possibly also older relative paleointensity profiles are essentially dominated
by field reversal effects masking changes in Earth's
magnetic field strength. In summary, perspectives
that polarity event and/or relative paleointensity
records can be used as a standard high-resolution
chronostratigraphic method for mid and low latitude pelagic deposits are not very promising.
Acknowledgments
We thank captain and crew ofRIV METEOR for
their very efficient support during cruise M 16/2.
Norbert R. Nowaczyk co-ordinated most of the
laboratory work. Various discussions with Karl
Fabian and comments by John Tarduno and an
anonymous reviewer substantially improved the
manuscript. This study was funded by the Deutsche
Forschungsgemeinschaft (Sonderforschungsbereich 261, contribution no. 188).
Data are available under www.pangaea.de/
Projects/SFB261.
References
Bleil U (1987) Quatemary high latitude magnetostratigraphy. Polar Res 5:325-327
Bleil U, Gard G (1989) Chronology and correlation of
Quaternary magnetostratigraphy and nannofossil
biostratigraphy in Norwegian-Greenland Sea sediments. GeolRundschau 67:1173-1187
Bleil and von Dobeneck
ing techniques often fail, prospects should be rather
encouraging. On the other hand, for mixed lithologies and thus large areas of the world oceans there
is at present little reason to be optimistic.
Conclusions
For the first time a succession of at least 5 or 6
different polarity events within the younger half of
the normal polarity Brunhes Chron have been documented at equatorial latitudes. Paleomagnetic
analyses of sediment sequences from the Ceara
Rise revealed repeated complete 180 0 shifts of
ChRM declinations from northern to southern
directions indicating full reversals of the Earth's
magnetic field and implying an unusually high variability of the geodynamo system during the late
Quaternary. Fluctuations in relative paleointensities
are directly associated with polarity transitions.
Minima typically coincide either with singular short
reversal episodes amid opposite polarities or with
reversal boundaries.
Converted into time series on basis of a common high-resolution chronostratigraphy, the
paleomagnetic data sets ofthe different sites show
similar general patterns in polarity and relative
paleointensity, but also remarkable inconsistencies.
Polarity events occur both during glacial and interglacial periods. There is no conclusive evidence
for a systematic correspondence of relative paleointensities to orbital frequencies in the precession or
obliquity band and hence a link to climate changes.
The coherence of a regional polarity stack for
the Ceara Rise with published results, mostly from
high latitudes, appears moderate at best. While an
about equal number of events is observed for the
last 380 kyr ofthe Brunhes Chron, the discrepancies in age assignments are too large and especially too variable to be reasonably explained by
an inaccurate dating. Similarly, the accumulated
Ceara Rise relative paleointensity record shows
only marginal agreements with a recent global scale
synthesis of pelagic sediment data from mid latitudes.
Model calculations based on the idea of a
delayed remanence acquisition qualitatively
account for all these findings. They demonstrate
that Earth's magnetic field characteristics are
recorded in the remanent magnetization of sediments by gradual processes critically depending
on sediment composition. Each lithology and its
variability through time acts as an individual filter
controlling the depth position, where in the sediment column polarity transitions are established in
the ChRM relative to the paleosurface of their
actual occurrence. As a result, intermittent complete by-passing and/or interference phenomena
will potentially produce ghost images of polarity
events in ChRM directional data. Uniform high
sedimentation rate terrigenous deposits should be
almost perfectly suited to register details of the
Earth's magnetic field history, whereas low to
medium sedimentation rate carbonates and variable sediment compositions may be extremely difficult to decipher appropriately. The model suggests
that Brunhes Chron and possibly also older relative paleointensity profiles are essentially dominated
by field reversal effects masking changes in Earth's
magnetic field strength. In summary, perspectives
that polarity event and/or relative paleointensity
records can be used as a standard high-resolution
chronostratigraphic method for mid and low latitude pelagic deposits are not very promising.
Acknowledgments
We thank captain and crew ofRIV METEOR for
their very efficient support during cruise M 16/2.
Norbert R. Nowaczyk co-ordinated most of the
laboratory work. Various discussions with Karl
Fabian and comments by John Tarduno and an
anonymous reviewer substantially improved the
manuscript. This study was funded by the Deutsche
Forschungsgemeinschaft (Sonderforschungsbereich 261, contribution no. 188).
Data are available under www.pangaea.de/
Projects/SFB261.
References
Bleil U (1987) Quatemary high latitude magnetostratigraphy. Polar Res 5:325-327
Bleil U, Gard G (1989) Chronology and correlation of
Quaternary magnetostratigraphy and nannofossil
biostratigraphy in Norwegian-Greenland Sea sediments. GeolRundschau 67:1173-1187