Archaeologists joke that nothing makes them happier than finding signs of destruction. Ancient sites that slowly declined or were simply abandoned are hard to date and often contain a confusing jumble of remains from different periods. But when an entire city gets suddenly buried by a volcanic eruption, demolished by an earthquake or burned down by a brutal invader, the tragedy freezes a single moment in time, giving experts a clearer picture of how people lived in that period, right up until disaster struck.
This archaeological schadenfreude is now taking on a new dimension, as researchers are increasingly using ancient ruins left over by violent conflagrations for an unexpected purpose: trying to understand the enigmatic and unpredictable behavior of Earth’s magnetic field.
In the latest effort, a team of researchers from Tel Aviv University, the Hebrew University in Jerusalem and the Israel Antiquities Authority has been able to collect data on our planet’s magnetic field using fragments of floor tiling found in an upscale Jerusalem house that was burned down after the city fell to the Babylonians in 586 B.C.E.
The study, published Friday in the journal PLOS ONE, shows that when King Nebuchadnezzar destroyed the First Temple and the capital of the Kingdom of Judah, more than 2,600 years ago, the intensity of the magnetic field in the region was almost double than today. This is part of a known, but still unexplained, magnetic anomaly that was centered on the Middle East during much of the period that we now call the Iron Age, and which saw the strength of the field spike to unprecedented levels.
The findings also add to evidence that our planet’s all-important magnetic shield against cosmic radiation has fluctuated wildly during the past without necessarily disappearing, as some fear it may be doing today.
One of the great mysteries of physics
Earth’s magnetosphere, which extends into space, protects us from solar radiation and other high-energy particles that are dangerous for living beings. It is also used for navigation by birds and other animals, including any human armed with a compass. This otherwise invisible shield lights up the sky when it interacts with solar wind particles at higher latitudes, near the poles, creating the spectacular phenomenon of auroras.
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Most scientists agree that Earth’s magnetic field originates in our planet’s outer core, which begins some 2900 kilometers underground and where the motion of liquid metal functions as an enormous electromagnetic dynamo. Less clear is why the field fluctuates in intensity and direction, and why its polarity sometimes flips, with the latest reversal happening some 780,000 years ago.
Albert Einstein once called the behavior of the magnetic field one of the great mysteries of physics, but understanding and possibly predicting its changes has taken on a new urgency for scientists. The field has lost around 10 percent of its strength since measurements began less than 200 years ago, leading some researchers to question whether we are on the way to a flip in polarity, which would be preceded by a loss of our precious shield against cosmic radiation.
One of the problems with building a model of the field’s behavior is the relatively short period of time for which we have records, says Yoav Vaknin, a PhD student in archaeology at Tel Aviv University who led the study published in PLOS ONE.
“In geological terms, two centuries are nothing, they are like a single moment in time,” Vaknin tells Haaretz. “Usually there are many sophisticated scientific methods that help archaeology, but in this case, the benefits flow in the other direction as well.”
This is where archaeomagnetism comes in – a technique through which researchers can recover data on the magnetic field from artifacts that are thousands of years older than our oldest records.
Many ancient artifacts, whether made of stone, brick or ceramic, contain trace amounts of ferromagnetic materials, such as iron oxides and other minerals, Vaknin explains. When heated to a certain temperature (which varies according to the substance), these materials cool down and become magnetized according to the direction and intensity of the surrounding magnetic field.
So if archaeologists know when an object was last heated to a high temperature, they can also pinpoint how the magnetic field looked at that time. The method also works for bricks, pots and other artifacts that were commonly heated in an oven, but it is less precise, because we can only estimate their age based on the pottery styles or by carbon-dating organic materials found in their vicinity. Sites that have been destroyed by violent fires, and which can be dated precisely, work best.
That was the case for the two-storied house that was uncovered recently near the so-called City of David, the oldest nucleus of Jerusalem in the First Temple period.
The large house, which likely belonged to a member of the elite in the Kingdom of Judah, was gutted by a massive fire that archaeologists have linked to the Babylonian destruction of Jerusalem. The fall of the city and the destruction of the First Temple are of course described in the Bible (2 Kings 25).
While the historicity of the holy text is generally a much-debated issue, few experts doubt that this particular event actually happened, says Yiftah Shalev, an archaeologist with the Israel Antiquities Authority who excavated the site.
Archaeological signs of the Babylonian destruction have been found across the ancient city, well before the upscale house was uncovered, Shalev notes. Based on the style of pottery remains and the names on seal impressions found among the ruins, the most likely interpretation is that the destruction of this building too should be attributed to Nebuchadnezzar’s fury, he says.
It was from the elegant floor tiles that once adorned to the building’s now collapsed second floor that Vaknin took samples for his analysis. The tests were conducted using very sensitive magnetometers at the paleomagnetism lab of the Hebrew University directed by Doctor Ron Shaar.
“This kind of work is crucial for understanding the magnetic field,” says Professor Lisa Tauxe, a geophysicist from the University of California, San Diego. The precise dating of the materials analyzed means there is “high confidence” that we now have a good picture of the magnetic field for that time and place, says Tauxe, who was not involved in the study.
A Levantine anomaly
While this is the first time that magnetic information from such a distant period has been dated with this level of precision, the high intensity of the field reported by the study was not a complete surprise. Previous research on artifacts dated to the Iron Age, mainly pottery, has already shown that in the Levant the magnetic field was unusually strong and variable during this period. This is what experts have termed the Levantine Iron Age Anomaly, which spanned from the 10th century B.C.E. until at least the fifth century B.C.E. in the Near East and parts of Europe.
Such anomalies are not uncommon and can be linked to anything from volcanic eruptions on the surface of the earth to activity deep within the planet’s core.
It is important to note that even when there is no major local anomaly in play, Earth’s magnetic field varies from region to region, says Erez Ben-Yosef, a professor of archaeology at Tel Aviv University who has taken a lead role in this research. So, information gathered about the field in Israel can only be considered valid for a radius of around 1,000 kilometers, Ben-Yosef says.
Because of this variability, there are teams of archaeologists at work all around the world now trying to glean regional magnetic records from burnt antiquities or other sources, he says. While the picture is far from complete, it does seem that concerns over the ongoing decline and potential disappearance of the magnetic field may be overblown.
“From our observations, this is not a rare phenomenon, the field went up and down very quickly in the past, it fluctuated a lot,” Ben-Yosef tells Haaretz.
In Israel, Ben-Yosef and other researchers are working on sites that range in time from the Neolithic to the Ottoman period. Fortunately for archaeologists (and unfortunately for pretty much everyone else), the Levant is riddled with the burnt-out husks of destroyed towns and cities. From the Egyptians, Assyrians and Babylonians to the Romans, Crusaders and Ottomans, many conquerors have left their mark on the region by leaving a trail of devastation. Experts have been able to date much of these destructions with some precision, making the Levant a fruitful target for collecting archaeomagnetic information.
The search for ancient magnetic records is not just of use to geophysicists, as it promises to provide archaeologists with a valuable new tool to date artifacts and sites that cannot be chronologically pinned down through other methods. Once they will have enough securely dated records as a reference, just like the Jerusalem house from 586 B.C.E., researchers will be able to compare the information from these ‘anchors’ in time to magnetic information coming out of newly discovered sites and understand where they fit in the chronology.
“The effort is now to build a database using artifacts whose date is already known,” Ben-Yosef says. “We are still far from being able to send a burnt sample to the lab and get back its age, but, it will happen.”