Global changes in intensity of the Earth's magnetic field during the past 800 kyr

Recent advances in palaeomagnetic and dating techniques have led to increas ingly precise records of the relative intensity of the Earth's past magneti c field at numerous field sites. The compilation and analysis of these reco rds can provide important constraints on changes in global magnetic field i ntensity and therefore on the Earth's geodynamo itself. A previous compilat ion for the past 200 kyr integrated 17 marine records into a composite curv e(1), with the geomagnetic origin of the signal supported by an independent analysis of Be-10 production made on different cores(2). The persistence o f long-term features in the Earth's magnetic intensity or the existence of long-term periodic changes cannot, however, be resolved in this relatively short time span. Here we present the integration of 33 records of relative palaeointensity(3-19) into a composite curve spanning the past 800 kyr. We find that the intensity of the Earth's dipole field has experienced large-a mplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction, reflecting the emergen ce of non-dipole components. No stable periodicity was found in our composi te record and therefore our data set does not support the hypothesis that t he Earth's orbital parameters have a direct and strong influence on the geo dynamo.