News Notes
Biblical tunnel timing

The Siloam Tunnel in Jerusalem, Israel, was actually built when the Bible says it was, according to new geological dating. The first well-identified biblical structure to be radiometrically dated, the man-made tunnel is around 2,700 years old and has carried water into old Jerusalem near-continuously since 700 B.C., says Amos Frumkin of Hebrew University of Jerusalem, lead researcher on the project.

Dating of biblical structures has historically been difficult, and the Siloam Tunnel proved no exception. The tunnel had been cleared of debris and artifacts by a French treasure-hunting team around the turn of the 20th century. Though the team kept exceptional records of their finds in the tunnel, no archaeological clues remained. In fact, the only artifact to be found is a broken inscription from the tunnel that now resides in Turkey and cannot be dated.

Artesian spring water exits the ancient Siloam Tunnel in the Siloam Pool in old Jerusalem. Photo by Jerry Barach, Hebrew University of Jerusalem.

Disregarding the partial inscription and historical record, a team of geoscientists turned to coring instead: They drilled the tunnel walls, ceiling and floor and obtained core samples. Reporting in the Sept. 11 Nature, Frumkin, Jeff Rosenbaum (a geochemist with Geochemical Solutions, Ltd.) and Aryeh Shimron (of the Geological Survey of Israel) sent the recovered cores to various laboratories for dating. Fine plant and wood fragments, well-preserved in the oldest plaster, were sent to Oxford University in England for radiocarbon dating. Cores from speleothems collected from the tunnel ceiling were sent to Open University in England for uranium-thorium dating. Petrographic, geochemical, scanning electron microscopy, x-ray diffraction and isotopic analyses of the cores took place at the Geological Survey of Israel and the University of Reading in the United Kingdom.

“All of these indicators — the radiometric dating, the paleogeography and the historical record — converged on a date around 700 B.C.,” Rosenbaum says. According to the Bible (2 Kings 20:20 and 2 Chronicles 32: 3,4, 30), King Hezekiah, who reigned from 727 B.C. to 698 B.C., commissioned the building of the tunnel to protect Jerusalem’s water supply during an Assyrian siege.

The 533-meter long Siloam Tunnel carries water from the Gihon syphon well about 20 meters outside the ancient city walls into what was then the heart of the city. The Gihon spring is the only fresh water source for tens of miles in any direction, Rosenbaum says. And for thousands of years, it was the primary source of drinking water for residents (they also had cisterns around the city collecting rainwater), though it now provides only agricultural water.

The tunnel bedrock is limestone and dolomite cut with ancient tools and was originally cemented with plaster that covered the floor, ceiling and walls. The plaster sealed the naturally occurring fissures in the bedrock to prevent leakage from the tunnel.

The plaster varies in thickness from 1 to 20 centimeters. The oldest plaster is a fine hydraulic lime plaster composed of a recarbonated lime binder with a filler of soil, chips of marl, organic materials, bone materials, charcoal and ash. A low firing temperature, absence of pottery shards, and a relatively high content of clays and phosphorus distinguishes the ancient plaster from younger plasters also in the tunnel, the researchers wrote.

The lack of evidence of a sedimentary level between the plaster and bedrock in the floor indicates that there was not a natural water conduit present along the bottom of the tunnel before it was built; it also indicates that the plaster was applied quickly after initial construction, Rosenbaum and colleagues concluded.

The researchers were also able to show that this tunnel was constructed completely underground. They found that the nature of the bedrock in the area is quite solid and transmits sound extremely well; engineers on the outside of the tunnel could instruct those on the inside with just rock hammer taps which way to go, Rosenbaum says. “And they did quite a good job. The sections of the tunnel that are straight are very straight. They meet in the center within fractions of an inch, which is just amazing.”

Still, Frumkin says that the greatest significance of their research is that Siloam is the first biblical structure to be conclusively dated. Rosenbaum agrees, saying: “We were able to pretty much put an end to the arguments in the biblical/archaeological literature about the age of the Siloam Tunnel, whether or not it could be attributed to King Hezekiah, and the argument over whether the tunnel is natural or man-made.”

Megan Sever

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