The Geomagnetic Field during Palaeozoic Time

Abstract

The Palaeozoic reversal sequence is represented by complex rhythms with characteristic times of (5–7) × 107, (1–4) × 107and (1–2) × 106y. The first order units recognized are Arginian (R) 590–535 m.y., Siberian (Rn) 535–485 m.y., Baikalian (Nr) 485–430 m.y., Sayanian (Rn) 430–360 m.y., Donetsk (Rn) 360–300 m.y., Kiaman (R) 300–240 m.y., and Illawarra (NR) 240–215 m.y. Three types of Palaeozoic field transitions have been recognized: 1. “sudden” reversals (single or repeated), 2. reversals with a considerable dispersion of palaeomagnetic directions, 3. reversals with loop-wise rotations of the field. The latter may result from the concurrence of axial and equatorial dipoles. Reversals usually occur with a 4 to 10 times lower field intensity. The time span for the complete transition ranges from 8 × 103to 3 × 105y. Peculiarities of the reversals have been traced over strike distances of 1600 km. Angular constituents of paleosecular variation have components with characteristic times of (4–8) × 103y. and 3 × 104y. The latter is characterized by standard deviations of 6 to 10 degrees in stable field epochs and up to 20 degrees in intervals of frequent reversals. The field of the geocentric axial dipole is a good first approximation of time-averaged field. However, the occasional presence of an irreversible equatorial dipole, controls the shape of geomagnetic reversals of the 3rd type. © 1980, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.