How reliable is the sharp increase in field strength at the start of the Permo-Carboniferous Reversed Superchron?
Abstract
The 50 million year transition from the peak in reversal frequency in the Middle Jurassic ( 170Ma), associated with a weak geomagnetic field, to the stable and apparently strong field during the Cretaceous Normal Superchron (CNS: 84-121Ma), represents a dramatic change in time-averaged geomagnetic field behaviour during the Mesozoic Era. Recent work from Siberia suggests that the geomagnetic field was also weak prior to the Permo-Carboniferous Reversed Superchron (PCRS: 265-310Ma); with V(A)DM values of 4-81 ZAm2 between 364-398Ma. To determine how long this weak field may have persisted, new sites from the UK, the Midland Valley ( 410Ma) and Kinghorn ( 335Ma) have been sampled. These sites have previously been used for determining the pole position of Baltica (Q values of 7 and 6) and carry their ChRM in (titano-)magnetite. The samples were measured using Thermal and Microwave Thellier-IZZI protocol with partial thermo-remanent magnetisation (pTRM) checks. Accepted Arai plots show positive pTRM checks, a clear relation between distinct primary directional and palaeointensity components and little to no zig-zagging. The results from these sites suggest that a relatively weak field may have persisted for 100Ma before the PCRS, with VDM values of 6-57 ZAm2 for the two localities. This would be consistent with recent work that suggests that the Mesozoic Dipole low may have persisted for more than 100Ma before the CNS, from the Jurassic to around the Permo-Triassic boundary ( 140-250Ma). Integrating these new results with updated PINT data from 200-500Ma suggests that after this persistently weak field, there was a sharp increase in field strength at the start of the PCRS. However, QPI analysis of the sites during this period shows that the high field values during the PCRS come from only a few studies with moderate-low demonstrated reliability and so further work may be needed to see if this transition is reliable.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMDI32A..04H
- Keywords:
-
- 1507 Core processes;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1510 Dynamo: theories and simulations;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1521 Paleointensity;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1522 Paleomagnetic secular variation;
- GEOMAGNETISM AND PALEOMAGNETISM