To the Editors:
The article "Dating Ancient Mortar" (March–April), by John Hale, Jan Heinemeier, Lynn Lancaster, Alf Lindroos and Åsa Ringbom, struck a chord. Long ago, when I was an active spelunker, it had occurred to me that radiocarbon techniques ought to be applicable to the dating of secondary deposits in limestone caves. I am not aware of any attempts along these lines.
The generally accepted theory of cave deposits is that atmospheric carbon dioxide dissolves in rainwater, forming a weak solution of carbonic acid. This weak acid seeps through openings in the limestone overburden, dissolving minute amounts of calcium carbonate as it goes. Upon entering a large opening, such as a cave (itself perhaps the result of such solution), the water has a chance to evaporate, leaving a deposit of solid calcium carbonate behind.
Half the carbon in this redeposited solid should be in equilibrium with the contemporary atmosphere, and should therefore have the appropriate concentration of carbon-14. The other half of the carbon, of course, will have been derived from the overburden rock, whose carbon-14 will have long since decayed. Speleothems (stalactites, stalagmites, flowstone) usually display a layered or banded structure in cross section, not unlike growth rings in a tree. Radiocarbon dating of samples from these layers might serve to establish growth rates and ages of deposits. There have been cases where archaeological artifacts or remains have been found under flowstone layers, a possible application for such dating techniques.
I realize, of course, that in the ideal case only half the carbon involved would be in equilibrium with the atmosphere at the time of deposit, and that therefore the maximum datable age would be restricted. I am also aware that speleothems are rarely pure calcium carbonate, but often contain large amounts of impurities such as soil or clay particles or surface organic material. Such contamination might well render the technique unworkable.
Charles E. Hendrix
To the Editors:
Pacific Palisades, California
Since archaeology is my hobby, I really enjoyed "Dating Ancient Mortar." I have two observations for the esteemed members of the research team that wrote it.
Have you considered dating the mortar found in Mayan ruins, such as Tuluum in the Yucatán peninsula? Because the Mayans used highly calcareous beach sand and honey in their mortar and concrete, this should put you to a real test. The chemistry of honey should pose all sorts of interesting problems!
Glenn G. Dahlem
Dr. Hale responds:
As these readers point out, the potential uses of our mortar-dating method go beyond the applications described in the article. Many ancient peoples manufactured lime mortar, including the Egyptians, the Mayans and the Islamic builders of the Alhambra. Mortar dating might also help pinpoint the age of an engineering wonder from ancient Greece: the kilometer-long tunnel that carries water through a mountain on the island of Samos. As for cave formations, it has been suggested that analysis of well-stratified deposits could extend the recalibration curve for atmospheric 14C well back beyond the age of the oldest surviving tree rings.