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| Titel |
A New High-Resolution Record of the Blake Geomagnetic Excursion from ODP Site 1062 |
| VerfasserIn |
Mark Bourne, Conall Mac Niocaill, Gideon Henderson, Alex Thomas, Mads Knudsen |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036754
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| Zusammenfassung |
| We present a high resolution record of the Blake geomagnetic excursion from Ocean Drilling
Program (ODP) Site 1062 on the Blake-Bahama Outer Ridge. The excursion is recorded in
three separate cores, with the high average sedimentation rate (10 cm ka-1) at
this location allowing the determination of transitional field behaviour during the
excursion.
A complex geometry is observed for the excursional geomagnetic field. The directional
records show an initial deviation from the expected directions across an interval of 1 m that
achieves a completely reversed state, and then returns to normal polarity. A second, although
less well-defined, short-lived phase of anomalous directions is observed immediately
following the first event in two of the three cores. Measurements of the magnetic
susceptibility show little variation through the core indicating that the concentration and grain
size of the remanence carriers remains relatively constant throughout the studied interval.
Measurements of the S-Ratio and remanence coercivity also remain constant through
the core sections of interest, and indicate magnetite to be the primary remanence
carrier.
The relatively homogeneous sediment enables the determination of two relative
palaeointensity proxies by normalizing natural remanent magnetization measurements using
artificially induced magnetizations (anhysteretic remanence, ARM and isothermal
remanence, IRM). These records are consistent between all three cores. The relative
palaeointensity proxies suggest that the Earth’s magnetic field decreased substantially in
intensity up to 70 ka prior to the initial event, before reaching an intensity minimum
coinciding with the directional excursion maximum. A second palaeointensity minimum is
also observed after the excursional event with no associated directional change. These
features are consistent with global palaeointensity stacks.
A preliminary age model based on an oxygen isotope stratigraphy, and an average
sedimentation rate, implies an age of c. 122 ka for the Blake event, in broad agreement with
published ages of the excursion. The age model also gives a minimum duration for the
directional excursion of ~9 ka. This duration is similar to that proposed for some full polarity
reversals, and appears to suggest that there is no clear distinction between the duration of
reversals and excursions. Our age model, however, assumes a constant sedimentation rate
between two tie-points from the oxygen isotope stratigraphy. We are currently refining the
age model by use of measurements of 230Th concentrations in the sediments in order to
assess possible variations in the sedimentation rates through the core sections of
interest.
Unlike previous records of the Blake excursion that show American longitudinal
preference, the transitional virtual geomagnetic poles (VGPs) obtained from Site 1062 exhibit
Atlantic and African longitudinal preference. This would suggest that the dipolar component
accounts for a reduced proportion of the transitional geomagnetic field. The paths show a
high proportion of transitional VGPs in regions associated with low seismic velocities in the
lower mantle. Therefore, unlike recent records of the Iceland Basin Excursion,
the Site 1062 paths do not support the hypothesis of a link between lower mantle
heterogeneities and VGP distribution; whereby transitional states preferentially
coincide with ‘metastable’ points associated with regions of high seismic velocity. |
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