Breakthrough technique detects magnetic field history in prehistoric stone tools

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Among the things we take for granted is Earth’s magnetic field, which among other things protects us from harmful radiation. But it is a fickle beast. It fluctuates, it intensifies and weakens, the poles move – and let’s not even discuss their tendency to flip. Since the magnetic field protects life-forms from hazardous solar radiation, the more we know about the magnetic field’s vagaries, the better.

Now a breakthrough study led by Prof. Erez Ben-Yosef at Tel Aviv University has discovered details on the magnetic field in the Middle East 10,000 to 8,000 years ago – using not only pottery, which is the usual way to study the magnetic field in the Neolithic period, but burned flints. The study was published in PNAS.

The pottery and burned flints in question, 129 of them all told, were from archaeological sites in Wadi Feynan, Jordan.

“Wadi Feynan is extremely rich in well-dated sites from this period, so we could test the entire Neolithic in a single region (pre-pottery to Pottery Neolithic),” Ben-Yosef explains.

In the Middle East, the “pottery Neolithic” goes back about 8,500 years, though in eastern Asia it goes back as much as 20,000 years (which indicates that hunter-gatherers did use pottery after alll). In any case, the beauty of this new technique is that it may help us study the magnetic field very precisely going back hundreds of thousands of years, to the dawn of early humans tossing flints into the fire to work them more easily, Ben-Yosef says.

How do clay pots in fact preserve information about the magnetic field? Much the same way basalt ejected in volcanic eruptions does. The rock, or clay, contains iron minerals. When heated, whether in the bowels of the Earth or a kiln, the electrons in the ferromagnetic minerals align with the magnetic field at the moment. Think what happens when you throw iron chips at a magnet: they align.

Prof. Erez Ben-Yosef.Photo courtesy of Yoram Reshef

So, the alignment of the ferromagnetic minerals in volcanic rock, or pottery, or burned flints, can show where the poles were at the moment of firing. What about the magnetic field’s intensity? Scientists deduce that based on a series of experiments – which take several weeks, Ben-Yosef points out – in which they gradually replace the magnetic signal in the artifact (be it clay or a burned stone ax) with signals acquired in known magnetic fields in the lab. Mathematics is involved.

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The upshot of their study shows that like pottery, when the flint is burned, it records the intensity of the magnetic field at the time of burning, the professor explains. “Flint was deliberately heated in order to produce tools. Say, 50,000 years ago somebody used fire to make tools; and the flint acquired the signal of the magnetic field of the time.”

Theoretically, this technique should be applicable up to about 400,000 years ago, which is when, some archaeologists are starting to think, control of fire was achieved and began to spread.

The technique of detecting the magnetic field at the time a flint was burned is the brainchild of Ben-Yosef with Anita Di Chiara of Rome’s National Institute of Geophysics and Volcanology, Lisa Tauxe, Thomas Levy, Mohammad Najjar and Fabio Florindo.

In previous work, Ben-Yosef and his colleagues worked on magnetic field “signals” in clay pots around 3,000 years old that had been made in and around Jerusalem. Their work demonstrated that pottery can be dated using its magnetic field “signal.” Now his work shows that when archaeologists are studying a site that features burned flints, they will be able to elucidate their magnetic intensity in the lab and compare it with a database – assuming there is a database – and know when in prehistory this tool was made, he says.

For the purposes of research on magnetic fluctuations and flips, archaeology has a huge advantage over geology: the time resolution, the researchers point out. Dating in geology is on the scale of thousands of years at best. Archaeological artifacts can usually be dated at a resolution of centuries or even tens of years, or even, sometimes, given an exact date. The fact that pottery was only invented 8,500 years ago (in the Middle East at least) was a constraint; now, the discovery that burned flints can also serve the purpose of detecting magnetic field mayhem enormously widens potential research.

Burnt flints and ceramics used to reconstruct the strength of the ancient geomagnetic field.Anita Di Chiara, Lisa Tauxe, Thomas Levy, Mohammad Najjar, Fabio Florindo, Erez Ben-Yosef

Worry not about the magnetic field

If the magnetic field flips on us again, all bets are off – separate research suggests that field flips have been associated with climatic chaos and extinctions. For instance, about 42,000 years ago, the planetary magnetic field weakened and then, within about a thousand years, it flipped. During this time the ozone layer in the atmosphere was depleted and altogether the events drove global climatic and hence environmental changes. But the fact that it’s been demonstrably weakening of late need not cost you sleep, Ben-Yosef’s research on the prehistory of the magnetic field indicates.

Why the magnetic field fluctuates and flips is not clear. “The magnetic field is generated by processes that take place below a depth of approximately 3,000 kilometers [1,865 miles] beneath the surface of the planet (for the sake of comparison, the deepest human drilling has reached a depth of only 20 kilometers),” Ben-Yosef says.

The magnetic field protects the planet from the continued bombardment by cosmic radiation and thus allows life as we know it to exist; it is volatile and its strength and direction are constantly shifting, and it is connected to various phenomena in the atmosphere and the planet’s ecological system, including affecting the climate in ways we don’t reallly understand yet. Nevertheless, the magnetic field’s essence and origins have remained largely unresolved. “In our research, we sought to open a peephole into this great riddle,” the researchers say.

And what do we see through that peephole? Not much yet, but they can say that about 7,600 years ago, during the Neolithic, the field spiked downward, as it were, marking the weakest values ever recorded in the last 10,000 years. Happily, it quickly rebounded.

In fact, in his previous work, Ben-Yosef demonstrated using the Jerusalem pottery that there was a massive, anomalous spike about 2,800 years ago, after which the magnetic field lost 27 percent of its strength over 30 years. Then it gently waned from the sixth to the second century B.C.E., spanning the Iron Age through the Hellenistic era in Judea.

The view west of the 1999 excavations at Wadi Feynan.Photo courtesy of Thomas E. Levy

And now, in our time, since measurements began less than 200 years ago, we have seen a continuous decrease in the field’s strength, Prof. Tauxe says. “This fact gives rise to a concern that we could completely lose the magnetic field that protects us against cosmic radiation and, therefore, is essential to the existence of life on Earth,” she said. “The findings of our study can be reassuring: This has already happened in the past. Approximately 7,600 years ago, the strength of the magnetic field was even lower than today, but within approximately 600 years, it gained strength and again rose to high levels.”