Introduction

Direct measurements of the Earth's magnetic field have been collected for several centuries (Merrill et al., 1996). Even from such short historical records it is clear that the magnetic field is not static but rather changes continuously on a variety of time scales. This variability is termed secular variation (SV). Palaeomagnetists, over the last several decades, have taken advantage of the natural remanent magnetization (NRM) recorded in rocks to extend our knowledge of the geomagnetic field secular variation backward into prehistoric time. Studies of palaeomagnetic secular variation provide a long-term view of geomagnetic field variability and are useful for aiding our understanding of the dynamo process which generates the Earth's magnetic field (e.g. Merrill et al., 1996) and for chronostratigraphic correlations (e.g. Creer et al., 1990).

It is well known from historical measurements that the Earth's magnetic field varies in both intensity and direction. But, since it is easier to recover high quality palaeomagnetic field directions than it is to arrive at reliable estimates of past field intensity (e.g. Tauxe, 1993), many palaeomagnetic studies have reported directional data only, even though intensity is an intrinsic component of secular variation. Reliable measurements of past intensity fluctuations should, however, yield added valuable information in the quest for a better understanding of the core dynamo process as well as provide an important additional chronostratigraphic correlation tool. For these reasons there has been considerable recent interest in estimating past field intensity changes (e.g. Mejia et al., 1996; Lehman et al., 1996).