Geomagnetic field intensity: How high can it get? How fast can it change? Constraints from Iron Age copper slag

Ron Shaar*, Erez Ben-Yosef, Hagai Ron, Lisa Tauxe, Amotz Agnon, Ronit Kessel

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

102 Scopus citations

Abstract

The intensity of the geomagnetic field varies over different time scales. Yet, constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record. The purpose of this study is to place firm limits on these fundamental properties by constructing a high-resolution archaeointensity record of the Levant from the 11th century to the early 9th century BCE, a period over which the geomagnetic field reached its maximum intensity in Eurasia over the past 50,000years. We investigate a 14C-dated sequence of ten layers of slag material, which accumulated within an ancient industrial waste mound of an Iron Age copper-smelting site in southern Israel. Depositional stratigraphy constrains relative ages of samples analyzed for paleointensity, and 14C dates from different horizons of the mound constrain the age of the whole sequence. The analysis yielded 35 paleointenisty data points with accuracy better than 94% and precision better than 6%, covering a period of less than 350years, most probably 200years. We construct a new high-resolution quasi-continuous archaeointensity curve of the Levant that displays two dramatic spikes in geomagnetic intensity, each corresponding to virtual axial dipole moment (VADM) in excess of 200 ZAm2. The geomagnetic spikes rise and fall over a period of less than 30years and are associated with VADM fluctuations of at least 70 ZAm2. Thus, the Levantine archaeomagnetic record places new constraints on maximum geomagnetic intensity as well as for its rate of change. Yet, it is not clear whether the geomagnetic spikes are local non-dipolar features or a geomagnetic dipolar phenomenon.

Original languageAmerican English
Pages (from-to)297-306
Number of pages10
JournalEarth and Planetary Science Letters
Volume301
Issue number1-2
DOIs
StatePublished - 3 Jan 2011

Bibliographical note

Funding Information:
The authors wish to thank the management and staff of the Timna Park and the Israeli Antiquities Authority for their support of the excavation (license #G-38/2009 to E.B.-Y.). We thank Jason Steindorf for helping with the measurements, and also to Hai Ashkenazi and Uri Davidovich for their help in the field. We thank Y. Ritov and C. Constable for fruitful discussions. We thank Joshua Feinberg and Yehoshua Kolodny for comments and suggestions that substantially improved the manuscript. We thank Yehuda Enzel for his assistance and useful discussion. The manuscript was greatly improved by the constructive comments of the editor P. DeMenocal and by the comments of Andrew Biggin and an anonymous reviewer. This work was partially funded by the US–Israel Binational Science Foundation grant 85739A (H.R. and L.T.), and NSF grants EAR0636051 and EAR0944137 to LT.

Keywords

  • 14C
  • Archaeomagnetism
  • Archaeometallurgy
  • Iron Age
  • Paleointensity
  • Paleomagnetism
  • Radiocarbon
  • Secular variations
  • Slag
  • Thellier
  • Timna

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