High-resolution record of the Laschamp geomagnetic excursion at the Blake-Bahama Outer Ridge

Abstract

SUMMARY Geomagnetic excursions are brief deviations of the geomagnetic field from behaviour expected during ‘normal secular’ variation. The Laschamp excursion at ∼41ka was one such deviation. Previously published records suggest rapid changes in field direction and a concurrent substantial decrease in field intensity associated with this excursion. Accurate dating of excursions, and determination of their durations from multiple locations, is vital to our understanding of global field behaviour during these deviations. We present here high-resolution palaeomagnetic records of the Laschamp excursion obtained from two Ocean Drilling Program (ODP) Sites, 1061 and 1062 on the Blake-Bahama Outer Ridge (ODP Leg 172). High sedimentation rates (∼30‐40cmkyr −1 ) at these locations allow determination of transitional field behaviour during the excursion. Palaeomagnetic measurements of discrete samples from four cores reveal a single excursional feature, across an interval of 30cm, associated with a broader palaeointensity low. We determine the age and duration of the Laschamp excursion using a stratigraphy linked to the δ 18 O record from the Greenland ice cores. This chronology dates the Laschamp excursion at the Blake Ridge to 41.3ka. The excursion is characterized by rapid transitions (less than 200yr) between stable normal polarity and a partially reversed polarity state. The palaeointensity record is in good agreement between the two sites, revealing two prominent minima. The first minimum is associated with the Laschamp excursion at 41ka and the second corresponds to the Mono Lake excursion at ∼35.5ka. We determine that the directional excursion during the Laschamp at this location was no longer than ∼400yr, occurring within a palaeointensity minimum that lasted 2000yr. The Laschamp excursion at this location is much shorter in duration than the Blake and Iceland Basin excursions.