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Carbon Jerusalem: extensive and accurate dating of the city during the Kingdom of Judah

Weizmann Institute of Science scientists overcame a cosmic phenomenon that makes it difficult for archaeologists around the world and presented for the first time an absolute dating of Jerusalem in the Iron Age

Excavation site in the City of David showing signs of the earthquake that occurred in 750 BC (photo: Yohana Regev)
Excavation site in the City of David showing signs of the earthquake that occurred in 750 BC (photo: Yohana Regev)

Jerusalem has been inhabited, non-stop, for thousands of years. The city served as a religious center and capital of more than one kingdom, but despite an inexhaustible amount of historical documents available, there are still holes in the dating of various historical events that took place in the city. The scientists of the Weizmann Institute of Science, together with a team of archaeologists from the excavations of the City of David in Jerusalem, the Antiquities Authority and Tel Aviv University, were able to compile for the first time a detailed chronology of the city during the Iron Age - in the days when it served as the capital of the biblical Kingdom of Judah. Their findings are published in the "Records of the National Academy of Sciences of the USA" (PNAS).

Much has been written about the history of Jerusalem, but dating the human settlement in the city during the Iron Age was and remains a particularly challenging task. The challenge is related, among other things, to a cosmic matter: the interaction between the Earth's atmosphere and cosmic rays outside the Earth at the same time disrupts the ability to carry out carbon-14 dating - the accepted and most accurate scientific method for determining the age of an archaeological relic. As a result, when you try to date any sample from the end of this period using carbon-14, instead of getting an exact age, you get a flat graph that does not make it possible to distinguish between remains from the 8th century BC and those from the 5th century BC. This phenomenon, called the "Hallstatt Plain" - after the Hallstatt culture that dominated Central Europe at the time - is one of the main challenges in the archaeological research of this formative period in human development.

As a result, archaeologists studying Jerusalem in the Iron Age usually rely on historical documents, biblical texts and pottery - methods that allow establishing a relative dating of "what came before what", but do not allow establishing an absolute dating based on carbon-14.

But Jerusalem poses additional challenges besides the "Hallstatt plain". The architectural mix from different periods, the countless wars it has gone through, a host of disasters and ceaseless reconstruction, make the city a complex and complicated urban space. "Archaeological research of urban fabric is much more complicated than dating carried out in other archaeological environments. Not least when it comes to an urban center that has been continuously inhabited for more than 4,000 years, the buildings in it have been in use for hundreds of years, and parts of them may have been destroyed and rebuilt," points out Prof. Elisabetta Boerto, head of the scientific archeology unit at the institute.

In order to overcome these many challenges that left significant holes in the chronology of Jerusalem, the research team led by Prof. Boerto and Dr. Yohanna Regev harnessed the toolbox of microarchaeology - a relatively new field in the archeological sciences that was developed, among other things, at the Weizmann Institute of Science and allows for the accurate dating of archaeological finds through Analysis of organic remains, even the tiniest ones. The microarchaeological approach focuses on the careful sampling of evidence left behind at archaeological sites - spermatozoa, bones, remains of cement or coal - and examining them using innovative scientific equipment.

For the purpose of the meticulous dating work, Prof. Boerto and Dr. Regev visited several archaeological sites in Jerusalem and collected thousands of samples of organic material. "Sampling the organic materials from the archaeological sites is really not enough, you have to understand the context", explains Prof. Boarto of the work process. "We need to understand how the archaeological site was created, so that when we sample in the field, we have confidence that the materials we collected were there when the site was built and that we can use them to date the site itself. For example, when we found a large amount of charred spermatozoa - we first had to understand where exactly they were burned. We do this using methods we have developed over the years, using advanced laboratory equipment that also comes with us to the field."

After collecting the samples and examining them in the laboratory, over 100 of them were selected for dating. The researchers isolated them from various pollutants and ran a series of carbon-14 measurements in the Dengor Mass Spectrometer Accelerator Laboratory (D-REAMS) at the institute. Carbon-14 is formed in the Earth's atmosphere, and is assimilated into the tissues of plants, animals and humans. After they die, the tissues stop absorbing the radioactive carbon, and the remaining carbon gradually decays into nitrogen-14. Since the rate of radioactive decay of carbon-14 is known, the exact age of the object can be determined by counting the number of carbon-14 atoms remaining in it. Although dating using carbon-14 was developed in the XNUMXs, in the institute's laboratory the measurement is carried out with the help of an accelerator, so that the results obtained are extremely accurate even when it comes to tiny samples, for example of a single sperm weighing a few milligrams.

Trees from Europe, seeds from the Levant

To deal with the challenge posed by the Hallstatt Plain, the research team led by Dr. Lior Regev also used 100 tree rings previously dated to the same period. Dating using tree rings - dendrochronology - is based on the fact that every year one ring is added to a tree trunk. By combining dendrochronology with carbon-14 dating, the researchers were able to obtain a more complete and accurate picture of the concentration of radioactive carbon in the atmosphere during the period under consideration.

In addition to the tree rings, two historical events whose date of occurrence is known and established served as anchor points and provided insights into the behavior of radioactive carbon in the atmosphere: the destruction of the First Temple and the Babylonian exile in 586 BC, and the earthquake that struck the city in the 8th century BC. Using this approach, the research group was able to identify gaps between the concentration of radioactive carbon in the Jerusalem samples, and its concentration in European and American tree rings from the same period. Understanding these gaps can help not only in establishing archaeological chronology, but also in the study of climate and atmosphere.

Remains of charred straw on an urn from the time of the destruction of the First Temple, 586 BC (photo: Yohana Regev)
Remains of charred straw on an urn from the time of the destruction of the First Temple, 586 BC (photo: Yohana Regev)

A city without a break

The combination of the methods led to the creation of a Jerusalem chronology at an unprecedented level of detail in a city that never stopped being inhabited for a moment. In particular, the researchers were able to establish evidence for large-scale human settlement in the city as early as the 12th century BC, and date the city's westward expansion to the 9th century BC; This dating was achieved by establishing the date of construction of an ancient building located in this part of the ancient city. Also, while previous studies attributed the extensive urban development after the earthquake to King Hezekiah, the dating revealed that the rebuilding apparently took place already during the reign of King Uzziah.

"Jerusalem is a city that never stopped living," says Prof. Boerto. "Archaeological research in the city is fundamentally different from working in an archaeological mound, where layers are piled on top of each other. It is a city that has been constantly changing over thousands of years, and the archaeological evidence is scattered in it. But despite these challenges and despite the Hallstatt plain, we managed to put together its absolute chronology in the Iron Age."

The methods applied in the research have implications beyond Jerusalem and could also be applied to other archaeological sites in the world, since carbon-14 dating of remains from the Iron Age is a worldwide archaeological issue. The researchers hope that this method can be used to reduce the gaps in human knowledge about this crucial period in human development.

The research was led by Prof. Elisabetta Boerto and Dr. Johanna Regev in collaboration with Dr. Lior Regev and Evgenia Mintz, all from the scientific archeology unit at the institute. The research was carried out in collaboration with archaeologists Dr. Joe Uziel from the Antiquities Authority, Prof. Yuval Gadot from Tel Aviv University and their colleagues. The research is also signed by Ortal Kalaf and Dr. Yiftach Shalev from the Antiquities Authority; Helena Roth, Dr. Nitzan Shalom and Dr. Nachshon Zanton from Tel Aviv University; Efrat Butzer from Bar-Ilan University; Prof. Charlotte L. Pearson from the University of Arizona, United States, and David M. Brown from Queen's University in Belfast, Northern Ireland.

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