Constraining global, long-term variations of the geomagnetic field by multi-proxy data

The geomagnetic field varies on a range of spatial and temporal scales. Global, spatial variations can be conveniently studied using empirical spherical harmonic models. One characteristic of the long-term variations is geomagnetic excursions - events associated with strong directional deviations and low field intensities. Recent modelling efforts provided global insights into the field variations over the past 100 ka and, in higher resolution, over the past 70 ka including the Norwegian-Greenland Sea, Laschamps, and Mono Lake excursions. However, the models are limited by the spatial/temporal distribution and uncertainties of paleomagnetic sediment and volcanic data. Because the cosmogenic radionuclide production rates depend on the geomagnetic field, these records provide independent constraints on long-term geomagnetic field variations. In this work, we explore the possibility to complement the paleomagnetic data with cosmogenic radionuclide production rate records and build the first multi-proxy model of the geomagnetic field. A global compilation of 10Be records from sediments and ice cores is used to analyze regional/global variations in comparison with paleomagnetic data, then converted to geomagnetic field intensity to serve as input data to the global model. The geomagnetic field predictions from two models, one based on paleomagnetic data only and the other additionally including 10Be records, are compared to investigate the influence of the cosmogenic isotopes on the modelling results. The Laschamps excursion is the most prominent feature in the past 100 ka, recorded globally in the paleomagnetic records, and represented as an approx. doubling in the 10Be production rate records. We discuss the paleosecular variation, dipole moment, and field morphology, as well as the potential of the presently available compilation of cosmogenic isotope records to improve global geomagnetic field reconstructions.