The behavior of the ancient geomagnetic field's strength, “paleointensity” for short, has widespread implications for a variety of disciplines including geophysics, geochronology, climatology and archaeology. Unfortunately, accurate data that are linked to precise age determinations are very difficult to obtain, mainly because magnetic materials that are suitable for paleointensity research are rare. Most difficulties can be overcome by using copper slag material, a remnant of the ancient copper industry that flourished in Israel and Jordan for the past six millennia. Slag is potentially an ideal material since it was cooled fast enough to form ferromagnetic particles in size of single-domain, optimal for the recording of the absolute ambient field strength. The abundance of slag in large quantity in Timna valley (southern Israel) allows for a unique high-resolution sampling with unprecedented accuracy.

Here we present a paleomagnetic study of a stratified sequence of Iron-age slag pile in Timna, aiming at providing answers to the most fundamental questions in paleomagnetim such as “How strong can the field get?” and “How fast can the field change?”. The pile contains a stratified sequence of ten distinguishable layers, rich in charcoals and organic matter. Radiocarbon analyses of short-lived seeds constrain the age of the pile from the middle 12th to early 9th centuries BCE. High-resolution paleomagnetic analyses of 34 slag samples display a unique geomagnetic episode with an extremely high field values and a rapid rate of change, confirming a recently published record of Jordanian slag deposits. All together, the two data sets demonstrate unrecognized features of the geomagnetic field, exhibiting high values of field strength, namely geomagnetic spikes, and short time-scale variations of the order of tens of years.