On the possible occurrence of ‘archaeomagnetic jerks’ in the geomagnetic field over the past three millennia

Abstract

Archaeomagnetism can provide a high-resolution full-vector description of the Earth’s magnetic field for the past several thousand years. We analyse the bulk of archaeomagnetic data (both direction and intensity) obtained recently in Western Europe and the Eastern Mediterranean covering the past three millennia. We demonstrate a remarkable coincidence between sharp cusps in geomagnetic field direction and intensity maxima (two clear ones at ∼AD 200 and 1400; two presently less well constrained at ∼800 BC and AD 800). These sharp changes may constitute a new feature of geomagnetic secular variation (‘archaeomagnetic jerks’) with time characteristics intermediate between ‘geomagnetic jerks’ and ‘magnetic excursions’.

Introduction

A major interest of archaeomagnetism is to provide a high-resolution full-vector description of the Earth’s magnetic field over the past several thousand years, including the temporal behaviour of its dipole and non-dipole components [1], [2], [3]. This clearly cannot be expected from palaeomagnetic studies of series of lava flows, because of the episodic nature of lava emplacement and radiometric age uncertainties. Continuous high-resolution sedimentary records may appear much more promising (e.g. [4], [5]), but we still understand insufficiently the link between sedimentation processes and magnetic acquisition. Archaeomagnetism, i.e. the study of remanent magnetisation of historically dated artefacts, remains our prime source of information over these millennial and centennial time scales. Unfortunately, no complete archaeomagnetic description of geomagnetic secular variation (SV) satisfying currently required standards has yet been obtained for any single region at the millennium scale. This is even the case in regions boasting a long and rich cultural past, such as the Middle East, China, Central and western South America or Western Europe. In some cases, only directional data exist in sufficient amount (and quality) to provide a relatively continuous record over the past few millennia [6]; in others, only (or mostly) archaeointensity results are available (e.g. China, Middle East, South America [7], [8], [9]). Moreover, most archaeointensity data sets from different regions display a large dispersion, which often prevents a consistent description of intensity variations [10]. This scatter likely reflects differences in experimental procedure and the insufficient reliability of many archaeointensity data, particularly those obtained more than 20 years ago [10], [11], [12].

In this study, we analyse the bulk of archaeomagnetic data (both direction and intensity) covering the last three millennia obtained recently in Western Europe and the Eastern Mediterranean [10], [11], [13], [14], which show evidence for a remarkable coincidence between sharp cusps in geomagnetic field direction and intensity maxima: two clear ones at ∼AD 200 and AD 1400 and two presently less well constrained at ∼800 BC and AD 800. These rather sharp changes may constitute a new feature of geomagnetic SV (‘archaeomagnetic jerks’) with time characteristics intermediate between ‘geomagnetic jerks’ and ‘magnetic excursions’.