Clay bricks from the building projects of ancient kings recorded a historical “map” of changes in the Earth’s magnetic field, a new study has found.
About 3,000 years ago, for reasons that aren’t well understood, the Earth’s magnetic field suddenly spiked in power above what is now modern Iraq.
And when it did, that change was recorded, researchers revealed Monday — a history literally baked into the bricks of cities throughout Iron Age Babylonia.
The findings published in Proceedings of the National Academy of Sciences (PNAS) provide a rare ancient record of transient changes in the Earth’s magnetic field — and a new guide to a pivotal period in Western history: the development of ancient Mesopotamia.
In the territories anchored by the fertile land between the flood-prone Tigris and Euphrates rivers, the West’s earliest empires rose — states such as Uruk, Babylon and Assyria.
From their urban centers built of mud-brick, surrounded by high walls of masonry, these polities helped invent the machinery of statehood — a means to “channel resources and human effort on a scale that other forms of political and social organization struggle to match,” as historian Patrick Wyman has written.
For example, the signature artifact of the earliest regional cities, Wyman argued, was a kind of mass-produced ceramic bowl found all over the city of Uruk, which bespoke “a deeply unequal and centralized way of organizing the world, with superiors and inferiors existing in complex status hierarchies.”
But for all their importance, the ruins of those cities present archeologists with a serious problem: one of timing. The most reliable means for determining the age of an organic artifact — from cloth to wood to bones — is by tracking the clockwork-like decay of radioactive carbon atoms contained within.
Such carbon dating — which effectively counts from the point of death — provides a very accurate means of determining how long ago a person buried in a tomb died, or when the tree used to make a spear- or hoe-handle was cut down.
But it is useless for dating the characteristic objects of the region, said co-author Mark Altaweel, a professor at University College London.
“Some of the most common cultural remains, such as bricks and ceramics, cannot typically be easily dated because they don’t contain organic material.”
And even where it works, radiocarbon dating can only place an object within a few hundred years — useful for dating very ancient artifacts, but less helpful the deeper one gets into recorded history.
The results released Monday, however, offer an alternate means of dating ancient bricks and ceramics: a means of correlating the reign of ancient kings to fluctuations in the Earth’s magnetic field whose timing can be more accurately measured.
Such artifacts are rare. Most bricks in these cities were dried in the sun, not fired, while fired bricks were reserved for particularly prestigious buildings or important public structures such as dikes, drains or paving floors, where erosion was a particular concern.
The fact of their having been fired also gives them particular scientific significance when it comes to dating them: The clays that made those bricks contained iron oxide — which recorded the strength of the Earth’s magnetic field at the time the raw bricks were put in the oven.
To assemble a new chronology of the area’s history, scientists chipped small pieces off of 32 kiln-fired bricks from ancient cities across the region — such as the Assyrian capital of Ashur — each decorated with an inscription heralding the king at that time.
By comparing the known chronology of the kings on the bricks to the magnetic strengths found within, scientists were able to create a time-map of the region that allows them to correlate unmarked bricks from other sites — which bear their own magnetic stamp — to the reigns of specific rulers.
In doing this, the scientists confirmed two long-suspected pieces of information: that the power of the magnetic field over Mesopotamia surged between 1050 and 550 B.C., and that sometimes these spikes were quite erratic.
For example, the divergent magnetic readings from five bricks fired during the reign of the king Nebuchadnezzar II — famed in biblical history for his destruction of Jerusalem and the exile of its elites in the Babylonian Captivity — found that “rapid spikes in [the] intensity” of the Earth’s magnetic field are possible.
But the findings also hold a much more practical implication for archeologists. By correlating specific magnetic readings to specific kings, archaeologists can now place ceramic artifacts within years or decades of their creation, creating “an important dating baseline that allows others to benefit from absolute dating using archaeomagnetism,” Altaweel said.
Lead author Matthew Howland of Wichita State University added that this would allow scientists to “estimate the dates of any artifacts that were heated up in ancient times.”