The eastern Australian magnetic inclination record: Dating the recent past and re-assessing the historical geomagnetic archive

A new compilation of historical observations and archaeomagnetic measurements of magnetic inclination for the last 1000 years from eastern Australia (the eastern Australian Inclination Record [eAIR2012]) has revealed the existence of a well-defined inclination anomaly in the region. Evidence of this magnetic feature has been preserved in sedimentary records from across eastern Australia, though this has not previously been recognised. Analyses of additional sedimentary sequences have confirmed the incidence and timing of this feature, revealing its presence between the 13th and 18th centuries ad. The inclination of the field during this episode appears to have been steeper than at any time since the start of the Holocene. Lake sediment evidence suggests that the anomaly is a composite feature, displaying a distinct peak at cal. ad 1270-1386 (+/- 2 sigma uncertainty), reappearing after cal. AD 1431-1651 (+/- 2 sigma uncertainty) and disappearing before cal. ad 1822 +/- 46 (+/- 2 sigma uncertainty). The disappearance of the anomaly is tightly bracketed in the historical record between ad 1770 and 1777. The rapid shift in inclination during the 18th century ad offers considerable potential as a means of dating a critical period of Australian environmental history, an episode that currently lies beyond the reach of established dating methods. This information also provides a valuable constraint on models of regional geomagnetic field change over centennial and millennial timescales. Our examination of the historical record has revealed that the inclination measurements made by the 18th-century French explorer La Perouse are consistently erroneous. Since La Perouse's data make up 13% of the total body of pre-19th century inclination records, the inclusion of these measurements in global compendia of magnetic observations may seriously skew attempts to model the geomagnetic field. We advocate that La Perouse's inclination measurements should therefore be employed only with considerable caution.