Hawkins, and three co-authors reported in mid-2021 that the magnetic field of our planet was relatively weak (less than half the strength of the long-term average field) for tens of millions of years between 332 and 416 million years ago; the similarities between this and a later period of low field strength provide further evidence for a 200- million-year-long cycle linked to deep Earth processes.
Variations in past geomagnetic field strength are important indicators of variation in deep Earth processes over hundreds of millions of years. Most other geophysical methods only provide a snapshot of the Earth’s recent interior, and deep Earth materials are poorly represented in the geological record. Recent measurements from Scotland (Northern United Kingdom), in addition to the existing datasets, show the field was relatively weak (less than half the strength of the long-term average field) for tens of millions of years between 332 and 416 Ma. The similarities between this and a later period of low field strength provide further evidence for a ∼200-My cycle linked to deep Earth processes.
The Mesozoic Dipole Low (MDL) is a period, covering at least ∼80 My, of low dipole moment that ended at the start of the Cretaceous Normal Superchron. Recent studies of Devonian age Siberian localities identified similarly low field values a few tens of million years prior to the Permo-Carboniferous Reverse Superchron (PCRS). To constrain the length and timing of this potential dipole low, the authors presented paleointensity estimates from Strathmore (∼411 to 416 Ma) and Kinghorn (∼332 Ma) lava flows, United Kingdom. Both localities have been studied for paleomagnetic poles (Q values of 6 to 7), and the sites were assessed for their suitability for paleointensity from paleodirections, rock magnetic analysis, and microscopy.
Thermal and microwave experiments were used to determine site mean paleointensity estimates of ∼3 to 51 μT (6 to 98 ZAm2) and 4 to 11 μT (9 to 27 ZAm2) from the Strathmore and Kinghorn localities, respectively. These, and all the sites from 200 to 500 Ma from the (updated) Paleointensity database (PINT15), were assessed using the Qualitative Paleointensity criteria (QPI). The procurement of reliable (QPI ≥ 5) weak paleointensity estimates from this and other studies indicates a period of low dipole moment (median field strength of 17 ZAm2) from 332 to 416 Ma. This “Mid-Paleozoic Dipole Low (MPDL)” bears a number of similarities to the MDL, including the substantial increase in field strength near the onset of the PCRS. The MPDL also adds support to the inverse relationship between reversal frequency and field strength and a possible ∼200-My cycle in paleomagnetic behavior relating to mantle convection.
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