Open system model for uranium - Series dating of pleistocene samples

One radioactive dating method is to use 230Th/234U and 23U/238U ratios in Pleistocene shell samples. In previous studies on uranium-series dating, the system was assumed to have been closed to the addition or removal of 238U, 234U, 230Th and 226Ra after the original incorporation of uranium in the shells. One stringent criterion for a closed system, not included in previous studies, is the determination of 231Pa and a demonstration that its growth from 235U gives an age concordant with that calculated for the growth of 230Th from 234U. Protactinium-231 has been determined in five aragonitic mollusk shells collected from elevated marine terraces in Southern California; however, none gave concordant ages for 231Pa/235U and 230Th/234U. Commonly, in mollusk shells greater than 100 000 years of age, 231Pa occurs in excess of the amount required for radioactive equilibrium with 235U. Based on a closed system, no age can be calculated from the 231Pa/235U activity ratios that have a value greater than one. Thus, samples containing excess 231Pa indicate that a closed system did not exist in the shells and that, in permeating solutions, more uranium was available to the shell than was actually assimilated by the shell. An open-system model describing the migration of uranium in Pleistocene mollusk shells and in the surrounding environment is developed for uranium-series dating. Analyses of 238U, 234U, 231Pa and 230Th are required for mathematical solution of the model to determine radiometric ages extending to about 300 000 years in the past. Ages based on an open system, compared to the apparent ages that would be obtained assuming a closed system for uranium migration, indicate that aragonitic shells which do not meet the rigid criteria demanded of a closed system can be dated using an open-system model. (author)