Millennial scale cyclicity identified in the Holocene geomagnetic dipole field variation: implications for the geodynamo

Constraining past changes in the strength and configuration of the geomagnetic field provides a valuable perspective on the processes that govern the geodynamo. Here we present an updated dipole tilt reconstruction for the last 9000 years based on paleomagnetic records from five globally well-distributed sites. We identify a dominant 1350-year cyclicity in the dipole tilt variations and two preferred states of the dipole axis with north geomagnetic pole longitudes confined to either c. 120° West or c. 30° East. The dipole tilt reconstruction is compared to regional geomagnetic field intensity estimates based on archeomagnetic data from the northern hemisphere. The comparison suggests that the dipole tilt variation during the last 3000 years may have been closely related to changes in the magnetic field beneath North America and western Eurasia. We propose that the apparent preferred states of the dipole axis can be associated with different configurations of the four semi-stationary high latitude flux lobes observed at the core mantle boundary in long-term time averages of paleomagnetic field models. The tilt episodes towards 30° East in the northern hemisphere are perhaps related to the appearance of a high intensity flux lobe beneath western Eurasia. The dipole tilt reconstruction is also compared to millennial-scale variations in the length of day (LOD), reconstructed from ancient records of eclipses. The two records show a remarkable in-phase relationship that indicates a general decrease in the LOD during episodes of large dipole tilt. We find no straightforward way to link the changes in LOD and dipole tilt, yet the similar periodicity exhibited by both records points to the importance of core flow variations on these time scales.