The Laschamp geomagnetic ‘event’

Abstract

SHORT lived geomagnetic ‘events’ provide useful stratigraphic markers; they may enhance climatic and evolutionary changes^1,2 and they provide stringent parameters for geomagnetic models. During such events the virtual geomagnetic pole undergoes excursions outside the usual range of secular variation but they are usually brief, generally lasting about 10⁵ yr, so that their magnetic record in deep sea sediments is often lost because of post-depositional remagnetisation³ and because of biological activity in the upper few decimetres of the sediments⁴. In lake sediments, where sedimentation is more rapid, the magnetic record is better preserved. Dating is, however, difficult because secular variations in atmospheric carbon isotopes and irregular distributions in the environment are known to produce errors. Furthermore, geomagnetic events, the levels of which may have been displaced by bioturbation⁴, are usually dated by assuming uniform sedimentation between ¹⁴C dated horizons. The Laschamp event, in particular, seems to have been detected at 12 locations although the reported age (Table 1) varies between 7,000 and 17,000 yr BP, a range outside the quoted error for individual determinations, but within the realistic error in dating.