Was the Earth's Magnetic Field Weak in the Late Devonian?
Friday, 19 December 2014
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.