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