Archaeomagnetism in the Levant and Mesopotamia Reveals the Largest Changes in the Geomagnetic Field.

Abstract

Our understanding of geomagnetic field intensity prior to the era of direct instrumental measurements relies on paleointensity analysis of rocks and archaeological materials that serve as magnetic recorders. Only in rare cases are absolute paleointensity data sets continuous over millennial timescales, in sub-centennial resolution, and directly dated using radiocarbon. As a result, fundamental properties of the geomagnetic field, such as its maximum intensity and rate of change have remained a subject of lively discussion. Here, we place firm constraints on these two quantities using Bayesian modeling of well-dated archaeomagnetic intensity data from the Levant and Upper Mesopotamia. We report new data from 23 groups of pottery collected from 18 consecutive radiocarbon-dated archaeological strata from Tel Megiddo, Israel. In the Near East, the period of 1700-550BCE is represented by 84 groups of archaeological artifacts, 55 of which were dated using radiocarbon or a direct link to clear historically dated events, providing unprecedented sub-century resolution. Moreover, stratigraphic relationships between samples collected from multi-layered sites enable further refinement of the data ages. The Bayesian curve shows four geomagnetic spikes between 1050 and 600BCE, with virtual axial dipole moment (VADM) reaching values of 155-162ZAm2, much higher than any prediction from geomagnetic field models. Rates of change associated with the four spikes are ∼0.35-0.55μT/year (∼0.7-1.1ZAm2/year), at least twice the maximum rate inferred from direct observations spanning the past 180years. The increase from 1750 to 1030BCE (73-161ZAm2) depicts the Holocene's largest change in field intensity.