Archeomagnetic intensity variations during the era of geomagnetic spikes in the Near East

Observational records of rapidly varying magnetic fields strongly constrain our understanding of core flow dynamics and Earth's dynamo. Archeomagnetic analyses of densely sampled artefacts from the Near-East have suggested that the intensity variation during the first millennium BC was punctuated with two geomagnetic spikes with rates of change of intensity exceeding 1 μT/y, whose extreme behaviour is challenging to explain from a geodynamo perspective. By applying a new transdimensional Bayesian method designed to capture variations on both long and short timescales, we show that the data considered only at the fragment (thermal-unit) level require a complex intensity variation with six spikes, each with a duration between ~30-100 years. However, the nature of the inferred intensity evolution and the number of spikes detected are fragile and highly dependent on the specific treatment of the archeomagnetic data. No spikes are observed when the data are considered only at the level of a group of fragments from the same archeological context, with a minimum of three different artefacts per context. Furthermore, the number of spikes decreases to zero when increasing the error budget for the intensity within reasonable levels of 3-6 μT and the data age uncertainty up to 50 years. Thus, depending on the choices made, the Near-Eastern data are compatible with a broad range of time-dependence, from six spikes at one extreme to zero spikes on the other, the latter associated with much more modest rates of change of ~0.2-0.3 μT/y, comparable to secular variation at other periods and in other regions.