Orbital modulation of the Earth's magnetic field intensity

More than 20 years ago, on the basis of data from a Pacific sediment core, it was suggested that geomagnetic field intensity may vary with the Earth's orbital obliquity (centred on a period of ∼41 kyr) as a result of the effect of obliquity on precessional forces in the Earth's core1. It had also been proposed that precession plays an important role in the energy budget of the Earth's geodynamo2. But subsequent analyses indicated that the energy available from precession is at least an order of magnitude less than that required to drive the geodynamo3. Here, however, we report a spectral analysis of sedimentary records of relative geomagnetic palaeointensity from two North Atlantic sites which shows significant power both at orbital eccentricity (∼100 kyr) and obliquity (41 kyr). The eccentricity power is also present in bulk magnetic properties (such as susceptibility) and is therefore attributable to lithological variations controlled by eccentricity-driven climate change. The obliquity power, however, is not apparent in bulk magnetic properties, and seems to be a property of the geomagnetic field itself, thus providing evidence for the orbital forcing of geomagnetic field intensity.