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 rely on paleointensity analysis of rocks and archaeological materials that serve as magnetic recorders. Only in rare cases absolute paleointensity datasets are continuous over millennial timescales, provide sub-centennial resolution, and are directly dated using radiocarbon. As a result, fundamental properties of the geomagnetic field, such as its maximum intensity and maximum rate of change have remained a subject of lively discussion. Here, we place firm constraints on these two quantities using Bayesian modelling 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 between 1700-550 BCE is now represented by 87 groups of archaeological artifacts, 57 of which dated using radiocarbon and/or direct association to clear historically-dated events, providing an unprecedented sub-century resolution. Moreover, stratigraphic relation between samples collected from multi-layered sited enable further refinement of the archaeomagnetic ages. The Bayesian curve shows four geomagnetic spikes between 1050 and 600 BCE, with virtual axial dipole moment (VADM) reaching values of 155-162 ZAm 2 -much higher than any prediction from geomagnetic field models. Rates of change associated with the four spikes are ˜0.35-0.55 μT/year (˜0.7-1.1 ZAm 2 /year) -at least twice the maximum rate inferred from direct observations spanning the past 190 years. Moreover, the increase from 1750 BCE to the first spike depicts the Holocene largest change in field intensity.