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Rapid directional changes associated with a 6.5 kyr-long Blake geomagnetic excursion at the Blake-Bahama Outer Ridge

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  • Mark Bourne, University of Oxford, Storbritannien
  • Conall McNiocaill, University of Oxford, Storbritannien
  • Alexander Thomas, University of Oxford, Storbritannien
  • Mads Faurschou Knudsen
  • Gideon Henderson, University of Oxford, Storbritannien
Geomagnetic excursions are recognized as intrinsic features of the Earth's magnetic field. High-resolution records of field behaviour, captured in marine sedimentary cores, present an opportunity to determine the temporal and geometric character of the field during geomagnetic excursions and provide constraints on the mechanisms producing field variability. We present here the highest resolution record yet published of the Blake geomagnetic excursion (similar to 125 ka) measured in three cores from Ocean Drilling Program (ODP) Site 1062 on the Blake-Bahama Outer Ridge. The Blake excursion has a controversial structure and timing but these cores have a sufficiently high sedimentation rate (similar to 10 cm ka(-1)) to allow detailed reconstruction of the field behaviour at this site during the excursion. Palaeomagnetic measurements of the cores reveal rapid transitions (<500 yr) between the contemporary stable normal polarity and a completely reversed state of long duration which spans a stratigraphic interval of 0.7 m. We determine the duration of the reversed state during the Blake excursion using oxygen isotope stratigraphy, combined with Th-230 excess measurements to assess variations in the sedimentation rates through the sections of interest. This provides an age and duration for the Blake excursion with greater accuracy and with constrained uncertainty. We date the directional excursion as falling between 129 and 122 ka with a duration for the deviation of 6.5 +/- 1.3 kyr. The long duration of this interval and the fully reversed field suggest the existence of a pseudo-stable, reversed dipole field component during the excursion and challenge the idea that excursions are always of short duration.
TidsskriftEarth and Planetary Science Letters
Sider (fra-til)21-34
Antal sider14
StatusUdgivet - 1 jun. 2012

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