Was the Earth's Magnetic Field Weak in the Late Devonian?

Abstract:

Very few data exist to describe geomagnetic field behaviour in the Late Devonian (LD). Samples, which have recently been Ar-Ar dated to 364-377 Myr ago, of LD-aged volcanics and instrusives from the Viluy large igneous province in Siberia are investigated. These units have already demonstrated reliable, palaeomagnetic directions consistent with the retention of a primary remanence. Microwave Thellier-type palaeointensity experiments (mostly IZZI protocol with partial thermoremanent magnetization checks) were performed on 55 samples from 16 sites, of which, 12 samples from 4 sites provide satisfactory paleointensity data. Arai plots are strongly concave-up in shape but multiple lines of evidence support that this is caused by a strong component of magnetisation overprinting a weak primary magnetisation rather than by lab-induced alteration or multidomain behaviour. The samples display corresponding distinct directional components, positive pTRM checks and little or no zig-zagging of the Arai plot. Furthermore, the results of non-heating pseudo-Thellier experiments support the existence of a strong component overprinting a much weaker one. The site-mean paleointensities, ranging from 5.3-11.1 µT and which correspond to a virtual axial dipole moments (VADMs) of (1.0-2.1) ×10²² Am², indicate that the LD was a time of extremely weak magnetic field intensity. It provides the evidence that the superchron state, between ~310 and 265 Myr ago, is preceded by very weak field in the LD (~60 Myr before the superchron). If low dipole moment can be considered an indicator of high reversal frequency (as appears to be the case in the mid-Jurassic) then our results support that rapid transitions between reversal hyperactivity and superchron states are a recurring feature in the palaeomagnetic record, potentially linked to simultaneous episodes of true polar wander.