Beating the Plateau

Jerusalem has been inhabited continuously for thousands of years, serving as both a center of religious significance and a seat of power for kingdoms, yet despite the vast number of historical texts about the city, there are still gaps in its absolute chronology. Researchers from the Weizmann Institute of Science, in collaboration with a team of archaeologists from the City of David archaeological site in Jerusalem, Israel Antiquities Authority and Tel Aviv University, have now managed to put together a detailed chronology of Iron Age Jerusalem, when the city would have served as the capital of the biblical Kingdom of Judah. The findings of this study are being published in the journal Proceedings of the National Academy of Sciences, USA (PNAS).

Despite how much has been written about Jerusalem, studying its Iron Age has proven challenging in terms of absolute chronology, which involves determining the exact dates or time periods to which archaeological evidence belongs, as opposed to a relative chronology, which establishes the order of events based on similarities to architecture or ceramic evidence at other sites. Part of the challenge is a phenomenon known as the Hallstatt plateau, which stems from a particular interaction of cosmic rays with Earth's atmosphere at the time in question and interferes with the use of radiocarbon dating, the gold standard of determining how old something is. The plateau means that during the Iron Age, radioactive dating, rather than pointing to an object's specific age, generates a graph with a flat area: the range between the 8th and 5th centuries BCE. Therefore, radiocarbon dating generally isn't accurate during this time period, making overcoming the Hallstatt plateau one of the biggest struggles in archaeological studies of the later part of the Iron Age.

As a result, archaeologists exploring Iron Age Jerusalem relied more on biblical and historic texts and studying pottery rather than using radiocarbon dating. Moreover, the mix of architecture and continuous habitation over more than 4,000 years has led to Jerusalem being an amalgamation of construction from different time periods; it is a city that has seen numerous wars, destructions and reconstructions, turning into sprawling and complex urban areas built on top of the ruins of what came before.

"We have to be able not only to collect material like seeds, bones or charcoal from the site, but to identify the context, such as where the seeds were burnt"

All these combined to create gaps in establishing an absolute chronology of Iron Age Jerusalem. Filling these gaps would require successfully dealing with the Hallstatt plateau problem. Luckily, the Weizmann researchers were able to do just that by using microarchaeology, a relatively new field within archaeological sciences that they had developed. This approach focuses on carefully examining pieces of evidence left behind at sites, using scientific instruments with an almost forensic-like level of care and attention.

"It's a question of thoroughly understanding the connection between the materials to be dated and the layers with evidence of human occupation or construction material - and that's how we were able to apply the microarchaeology method," said Prof. Elisabetta Boaretto, director of Weizmann's Scientific Archeology Unit.

Digging into Jerusalem's history

Developed in the 1940s, radiocarbon dating works by measuring radiocarbon (carbon-14, or 14C) in a given object. Radiocarbon is constantly being produced in the atmosphere and becomes part of the carbon cycle. These atoms are absorbed into the tissue of organic matter such as plants, animals and people - but when that living organism dies, it stops absorbing radiocarbon. The 14C undergoes radioactive decay, turning into nitrogen-14. Since radiocarbon has a known rate of decay, researchers can use the number of remaining 14C atoms to determine something's age.

Going to the excavation sites in Jerusalem, Boaretto, with Dr. Johanna Regev, was able to carry out more than 100 radiocarbon measurements on organic material, mostly charred seeds.

"We have to be able not only to collect material like seeds, bones or charcoal from the site, but to identify the context, such as where the seeds were burnt," Boaretto says. "We achieve this with the methods we have developed over the years, using analytical instruments that we have at Weizmann and also bring with us to the field. In this manner, we can go beyond the standard archaeological analysis of the site."

After that, the researchers separated the original material from contaminants and carried out multiple radiocarbon measurements at Weizmann's Dangoor Research Accelerator Mass Spectrometry (D-REAMS) Laboratory in order to get the highest level of accuracy and precision in dating.

"We have an understanding of how the site was formed, so when we collect seeds or mortar samples related to the site, we can be confident that these were there when the site was built, which means we can date the site itself from that," she explains.

Overcoming the Hallstatt plateau was also made possible with the help of 100 calendar-dated tree rings obtained from well-known archives. Tree-ring dating, also known as dendrochronology, is built on the fact that a tree will grow a ring every year until its death. The more rings a tree has, the older it is. Combining this with the radiocarbon method, researchers were able to obtain a more precise and detailed determination of the radiocarbon concentration in the atmosphere during the period of interest, which also helped create an absolute chronology. This study was made possible by an experiment set up by Dr. Lior Regev at D-REAMS, Weizmann's dedicated accelerator for research.

The existence of two historical events that occurred at well-established dates - the 586 BCE destruction of Jerusalem by the Babylonians, and the 8th century BCE earthquake and subsequent widespread reconstruction efforts - helped provide further insights into the radiocarbon behavior in the atmosphere. The researchers noticed differences between the radiocarbon in the material in the region compared to the measured concentration in European and American tree rings from the same time. These differences - when the radiocarbon data don't match what we know they should be thanks to the tree rings - are known as "offsets," and understanding them can be of fundamental importance for scientists studying the climate and atmosphere, as well as for archaeological chronologies.

A promising archaeological approach

The study's biggest accomplishment was its success in creating an absolute chronology, with unprecedented detail and fidelity, for a continuously inhabited city.

In particular, the researchers were able to provide concrete evidence of widespread presence of human habitation in Jerusalem as far back as the 12th century BCE. A westward expansion of the city was precisely dated back to the 9th century BCE by determining the timing of the construction of a large ancient building. Establishing the dates of a major shakeup to the urban planning made it possible to attribute it to a devastating earthquake and further development up until 586 BCE. Notably, while previous research had credited the post-earthquake redevelopment to King Hezekiah, the radiocarbon dating and chronology show that it likely occurred during the reign of King Uzziah.

"Jerusalem is a living city; it's not like a tel site that's built as a sequence of layers," Boaretto says. "This is a city that has been constantly rebuilt all this time, and the archaeological evidence is scattered. But despite these challenges, layers and layers of construction and the Hallstatt plateau, we were able to put together its absolute chronology during the Iron Age."

The methods developed in the study could have an impact beyond Jerusalem, since problems with using radiocarbon dating at Iron Age sites are a global issue. The team's microarchaeology approach can be used at many of these other sites, helping fill in the gaps in this pivotal period of human development and history. Not such a "small" feat for something called microarchaeology.

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