Uranium series dating of travertine from archaeological sites, Nahal Zin, Israel

AT present, absolute dates of occupation can readily be determined for prehistoric sites no older than about 40,000 yr BP using conventional 14C techniques. Ages of older sites can be obtained with the K–Ar method, but only where volcanic deposits are interstratified with the archaeological deposits. The majority of Middle and Lower Palaeolithic sites contain no volcanic deposits. However, many of these sites are associated with synchronously deposited beds of calcium carbonate, occurring either as tufa mounds left by now extinct springs, or as layers of travertine (speleothem) in the mouths of formerly inhabited caves. We show here that such calcareous deposits can be dated by the 230Th–234U method, as long as they are relatively impermeable and free of detritus; ages in the range 5,000–400,000 yr BP are obtainable with a precision of 5–10%. When the carbonate is deposited, it invariably contains traces of uranium but essentially no thorium. Its age can be obtained from the extent to which 230Th has grown into radioactive equilibrium with its parent 234U. The technique of radiochemical analysis is given in ref. 1. The 230Th–234U method has been found to be less reliable for the dating of molluscs2; although it has also been applied to the dating of bones3, the method has often yielded contradictory results, probably attributable to continuous diffusion of radionuclides through the bone during burial4. Some permeable, very fine-grained tufas have also given poor results in earlier studies5; however, careful choice of dense, impermeable, coarsely crystalline travertines generally yields dates that are mutually consistent and coherent with other geochronological tests6.