by 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 are not well understood, the Earth’s magnetic field suddenly increased in power over what is now Iraq.
And when he did, that change was recorded, researchers revealed Monday – a history literally baked into the bricks of cities across Iron Age Babylonia.
The findings published in the 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 clue to a crucial 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 earliest empires of the West rose—states such as Uruk, Babylon, and Assyria.
From their urban centers built of mud-brick, surrounded by high masonry walls, these polities helped shape the machinery of statehood — a means of “directing human resources and effort on a scale that other forms of political and social organization struggle to achieve.” match,” wrote historian Patrick Wyman.
For example, the signature artifact of the earliest regional cities, Wyman argued, was a type of mass-produced ceramic bowl found throughout the city of Uruk, which called for “a very unequal and centralized way of organizing the world, with leaders and inferiors. is in a complex status hierarchy.”
But for all their importance, the ruins of these cities pose a serious problem for archaeologists: one of time. The most reliable method of determining the age of organic artifacts – from cloth to wood to bones – is to track the decay of radioactive carbon atoms within the clockwork.
Such carbon dating – effectively counting from the point of death – provides a very accurate way of finding out how long it has been since a person buried in a tomb died, or when the tree used for a spear or gallows was cut down. do.
But it is useless to date the specific objectives of the region, said co-author Mark Altaweel, professor at University College London.
“Some of the more common cultural remains, such as bricks and pottery, cannot usually be dated because they do not contain organic matter.”
And even when it works, radiocarbon dating can only place an object within a few hundred years—useful for dating very ancient artifacts, but not as helpful for going deeper into recorded history.
The results released Monday, however, offer an alternative method of dating ancient bricks and ceramics: a way to correlate the reigns of ancient kings with fluctuations in the Earth’s magnetic field whose timing can be measured more precisely.
Such artifacts are rare. Most of the bricks in these cities were dried in the sun, not fired, and burnt bricks were reserved for prestigious buildings or important public structures such as dikes, drains or paving floors, where corrosion was a particular concern.
The fact that they were fired also gives them particular scientific importance in dating them: The clays that made those bricks contained iron oxide — which recorded the strength of the Earth’s magnetic field when the raw bricks were placed in the oven. .
To piece together a new chronology of the area’s history, the scientists chipped small pieces from 32 kiln-fired bricks from ancient cities across the region – such as the Assyrian capital Ashur – each decorated with an inscription that the king had proclaimed at the that time
By comparing the known chronology of the kings on the bricks with the magnetic strengths found within, the scientists were able to create a time map of the region allowing them to unmark bricks from other sites — which have their own magnetic stamp — to be correlated with the administration of specific rulers.
In doing so, the scientists confirmed two long-suspected pieces of information: that the strength of the magnetic field over Mesopotamia increased between 1050 and 550 BC, and that these spikes were occasional.
For example, the various magnetic readings from five bricks fired during the reign of king Nebuchadnezzar II — famous in the Bible for the destruction of Jerusalem and the exile of its elite in Babylonian captivity — found that there were “quick spikes in the Babylonian captivity . [the] intensity” of the Earth’s magnetic field possible.
But the findings also have a much more practical implication for archaeologists. By correlating specific magnetic readings with 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 the overall dating using archaeology,” Altaweel said.
Lead author Matthew Howland of Wichita State University added that this would allow scientists to “estimate the dates of any artifacts heated in antiquity.”
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