Abstract

Geomagnetic excursions have been reported for the past 25 years in both sedimentary and igneous rocks of Brunhes age and from widespread geographic localities. They comprise sequences of paleo-magnetic directions that are anomalous in that they depart widely from the range of geomagnetic north directions recorded through historic time; they have sometimes been interpreted as records of aborted reversals of polarity of the main geomagnetic dipole. The search for "excursions" sought to provide a set of stratigraphic markers. The case of the Laschamp "excursion," described in lava flows from the Chaîne des Puys (Massif Central, France), is analyzed here through a new sequential record of paleosecular variation recovered from sedimentary cores collected in Lac du Bouchet, a maar lake about 100 km from the Laschamp site. The absence of anomalous directions indicates that this excursion lasted for only a few centuries. This constitutes a warning to stratigraphers who attempt to use excursions as marker events, and it gives an insight on the behavior of Earth's geodynamo on the scale of 102 to 103 yr.

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