Carbon-14 dating unlocks ancient Jerusalem’s water secret: monumental Siloam Dam was built in 800 BCE to face a climate crisis

More than 2,800 years ago, residents of Iron-age Jerusalem faced a climate change, marked by years of drought and sudden flash floods. The ruling establishment of Judah, probably led by King Jehoash or his successor Amaziah, came up with a large-scale engineering solution for the unpredictable weather conditions and water shortage: fortifying the city’s primary water source, the Gihon Spring, and redirecting its waters into an artificial reservoir, the Siloam Pool, which also served for catching rainwater. The reservoir was created by the construction of the monumental Siloam Dam.

This story emerges from a new study conducted by the Weizmann Institute of Science’s Scientific Archaeology Unit in collaboration with the Israel Antiquities Authority, published today in the Proceedings of the of the National Academy of Sciences (PNAS). Using advanced microarchaeological methods and precise radiocarbon dating of samples that included microscopic, uncharred straw and charred twigs embedded in the mortar of the Siloam Dam during construction, the team – led by Weizmann’s Dr. Johanna Regev and Prof. Elisabetta Boaretto, who worked together with archaeologists Dr. Nahshon Szanton, Dr. Filip Vukosavović and Itamar Berko of the Israel Antiquities Authority – revealed that the dam was built in the years 805-795 BCE: an unusually precise range, which constitutes an exceptional achievement in the dating of ancient findings.

To complete the climatic picture of the period, the Weizmann Institute researchers combined the precise dating with existing climate data from Dead Sea drill cores, from Soreq Cave stalagmites and from records of solar activity traced through its influence on the formation of radioactive cosmogenic isotopes. This integration of data led to the conclusion that the imposing Siloam Dam was built as part of a comprehensive water system designed to cope with the climate challenges of the time. “Our findings point to sweeping urban planning for managing Jerusalem’s water system as early as the 9th century BCE – evidence of the city’s power and strength,” the researchers say. 

Science Numbers

The dam dated in the study was the oldest and largest ever discovered in Israel, measuring at least 14.9 by 14.4 by 6.3 meters.

Prof. Elisabetta Boaretto is the incumbent of the Dangoor Chair of Archaeological Sciences and head of the Helen and Martin Kimmel Center for Archaeological Science. Her research is supported by the Dangoor Research Accelerator Mass Spectrometry Laboratory.