ORBITAL MODULATION OF THE EARTHS MAGNETIC-FIELD INTENSITY

More than 20 years ago, on the basis of data from a Pacific sediment c ore, it was suggested that geomagnetic field intensity may vary with t he Earth's orbital obliquity (centred on a period of similar to 41 kyr ) as a result of the effect of obliquity on precessional forces in the Earth's core(1). It had also been proposed that precession plays an i mportant role in the energy budget of the Earth's geodynamo(2). But su bsequent analyses indicated that the energy available from precession is at least an order of magnitude less than that required to drive the geodynamo(3). Here, however, we report a spectral analysis of sedimen tary records of relative geomagnetic palaeointensity from two North At lantic sites which shows significant power both at orbital eccentricit y (similar to 100 kyr) and obliquity (41 kyr). The eccentricity power is also present in bulk magnetic properties (such as susceptibility) a nd is therefore attributable to lithological variations controlled by eccentricity-driven climate change. The obliquity power, however, is n ot 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 held intensity.