Investigation of the ingrowth of radioactive daughters of 238U in Mediterranean sapropels as a potential dating tool

Five different sapropels (S1, S3–S6) from two sites (2750 m and 3308 m water depth) in the eastern Mediterranean were analysed for Corg content and specific activities of 238U, 234U, 230Th and 232Th. Anoxia in the sapropel sediments during or shortly after deposition leads to U enrichment, and the observed excess of 234U over 238U activity is consistent with a seawater source for authigenic U. In certain cases, the U profile shape in and around the sappropels is interpreted to indicate that oxidation shortly after sapropel formation has caused a post-depositional downwards migration of both U isotopes. Systematic variability of the 234U/238U activity ratio across the sapropel units indicates that a different diagenetic process has also led to preferential relocation of 234U produced in situ from the 238U parent. The construction of quasi-isochrons in successively older sapropel units demonstrate that radioactive secular equilibrium is approached by a systematic ingrowth of 230Th over time from the marked initial disequilibrium between 230Th and the U isotopes. The effects of post-depositional diagenesis violate the isochron model boundary conditions, however, and lead to a deviation of the isochron ages from the expected stratigraphic ages. Following assessment of each data point, some improvements in the calculated ages were achieved in one case by exclusion from the calculations of those points most severely affected by open system behaviour.