Long-Term Changes in the Geomagnetic Field

Monika Korte [monika@gfz-potsdam.de], GeoForschungsZentrum Potsdam, Germany.

    Records of cosmogenic isotopes like 14C and10Beare widely used as proxies to study past insolation variability and climate change. The isotope production rates, however, are also influenced by the strength of the geomagnetic field and its fluctuations, which has to be taken into account. Here I present the results of recent studies of the global geomagnetic field evolution of the past 7000 years and discuss implications for cosmogenic isotope studies.

Continuous global geomagnetic field models have been developed recently based on archeomagnetic data and lake sediment records. While data distribution and quality still limit their temporal and spatial resolution, they provide robust estimates of the large scale field evolution. They suggest that previous virtual axial dipole moment (VADM) studies overestimated the actual dipole moment by about 19%. Moreover, VADM results had to be averaged over at least a few hundred year intervals to minimize the influence of the non-dipole field contributions. The global models offer a higher temporal resolution and all results suggest that significant variations have occurred on shorter timescales. For cosmogenic isotope studies this might imply that a larger part of centennial scale variations than previously thought could be due to changes in the geomagnetic field rather than solar activity.