Global dynamics of the geomagnetic field in the 15-70 kyr period: New results from selected paleomagnetic sediment records

Abstract:

Reconstructions of the geomagnetic field on long timescales are important to understand the geodynamo processes in the Earth's core. The geomagnetic field exhibits a range of variations on these timescales that vary from normal, dipole-dominated secular variations to geomagnetic excursions and reversals. These transitional events are associated with very low intensities and significant directional deviations, and polarity changes in case of reversals. Recent progress in modeling the long-term geomagnetic field provided models covering the past 100 kyr and a shorter period (20 kyr) containing the Laschamp (41 ka) and Mono Lake excursions. A new, global, geomagnetic field model spanning the past 70 kyr (named GGFSS70) is built from selected sediment records from globally distributed sites. The dataset contains only high-quality records with good age controls. The model exhibits a better temporal resolution, and provides more consistent global variations and an enhanced view of the processes on these timescales. Using the GGFSS70 model we studied the dipole moment variations over the past 70 ka, the paleosecular variation (PSV) index, dipole vs. non-dipole variations and the core-mantle boundary field morphologies over the geomagnetic excursions. GGFSS70 indicates that the axial-dipole component changed sign for about 500 years in the middle of the Laschamp excursion. This reversing increases the intensity at the Earth's surface and creates a double intensity low over the Laschamp excursion, which is observed in high-resolution intensity records. The Mono Lake (34.5 ka) and Norwegian-Greenland Sea (65 ka) excursions both show excursional values of the PSV index (which quantifies both the intensity and field directions) and dipole moment lows, though not as pronounced as for the Laschamp excursion.