Abstract
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 BCE was punctuated with two geomagnetic spikes with rates of change of intensity exceeding 1 μT/yr, 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 no less than six spikes, each with an approximate duration of between 30 and 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 at the fragment level within reasonable levels of 3–6 μT and the data age uncertainty up to 50 years. Of the six spikes found, the most resilient when increasing the error budget was dated at ∼970 BCE. However, we show that even this spike sensitively depends on the age model proposed for data from the Levant archeological site Timna-30 and disappears when considering a single Gaussian age prior distribution for these data and a moderate minimum intensity error. 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. An error of 6 μT at the fragment level produces a spikeless model with strong similarity with the reconstruction from the SHAWQ-Iron Age global model with rates of change of ∼0.2–0.3 μT/yr.
Highlights
Anomalous behaviour of the past geomagnetic field, captured by materials carrying an ancient magnetization, can have profound con sequences for our understanding of the Earth’s magnetic field and how it is generated
We have illustrated that the observation and very existence of geomagnetic spikes in the Near East, as based on the set of arche ointensity data currently available, is crucially dependent on the defi nition of the data that are used, as well as the error budget allocated to these data
The analysis presented in this work was made possible by adopting the transdimensional Bayesian method recently developed by Livermore et al (2018), which is well adapted to the detection of rapid and extreme intensity variations
Summary
Anomalous behaviour of the past geomagnetic field, captured by materials carrying an ancient magnetization, can have profound con sequences for our understanding of the Earth’s magnetic field and how it is generated. We apply the recently developed transdimensional Bayesian method (Livermore et al, 2018) for the calculations of regional geomagnetic intensity variation curves, which relies on minimal a priori information and innately takes account of uncertainties in both intensity and age, and importantly here makes no assumption on inherent time-scales through any form of explicit regularisation. This method is well suited to examine the extreme fluctuations in the geomagnetic field proposed in the Near East. It should be noted that other geomagnetic spikes have been proposed in other regions of the world (e.g. Texas, China, New Zealand: Bourne et al (2016); Cai et al (2014, 2017); Turner et al (2020), respectively), some at dates different from the 1st millennium BCE, but they remain beyond the scope of this study
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call