Authigenic Be as a tool to date river terrace sediments? – An example from a Late Miocene hominid locality in Bulgaria

A hominid tooth found in a river terrace layer from Azmaka (Bulgaria) has been biochronologically dated to ∼7 Ma. This age is younger than expected and indicates that hominids became extinct in Europe later than previously thought. In this study, authigenic 10Be/9Be is applied to potentially determine the absolute age of the Miocene river terrace layers containing the hominid tooth. Modern fluvial channel bed deposits provide the initial starting value for the geochronological clock. Homogeneity of modern fluvial channel bed deposits and closed-system behavior of the authigenic minerals in the river terrace layers are required. 10Be/9Be ratios analyzed for modern fluvial channel beds vary by a factor of two. The initial starting value for the geochronological clock is not well-constrained and age calculations uncertain. Furthermore, the 10Be/9Be ratios analyzed for different cycles of the terrace layers are variable within different layers and do not decrease with formation depth. The mean authigenic age based on the mean 10Be/9Be ratio from six terrace layers and using the mean initial ratio from measured modern fluvial channel bed samples is 5.80 ± 0.70 Ma. An authigenic 10Be/9Be age of 6.20 ± 0.41 Ma is calculated for the river terrace layer associated with the hominid tooth. Despite these first promising ages, the method of authigenic minerals to date river terrace sediments needs further and more detailed investigations.