Calibrating the Devonian Time Scale: A synthesis of U–Pb ID–TIMS ages and conodont stratigraphy

The recalibrated Devonian time scale represents an important improvement of Devonian chronology regarding two aspects. Firstly, a data set of 13 biostratigraphically well-bracketed U–Pb ID–TIMS zircon and monazite ages constitutes the framework of the scale. Secondly, approximately time-linear biostratigraphic scales have been used for the interpolation between the isotopic ages. The new construction thus represents a ‘biochronometric’ time scale, which allows the assignment of numeric ages not only to stage boundaries but also to each biozone boundary. The method of interpolation applied also enables the projection of the geochronological error onto the time scale. According to the newly calibrated scale, the Devonian lasted 57.4 ± 5.7 Ma from 418.1 ± 3.0 to 360.7 ± 2.7 Ma. This is, compared to previously published time scales, the longest time span ever calculated for this period. The age interpolations for the eight stage boundaries are: ration of the Middle Devonian is calibrated here as quite short with 8.2 Ma and the Emsian and the Famennian are the longest stages with interpolated durations of 17.2 and 15.4 Ma, respectively. Together, all Devonian stages contain 57 conodont zones (including subzones, according to the standard conodont zonation) giving a mean duration of about 1 Ma. In the highly resolved part of the time scale (mid-Eifelian to the Devonian–Carboniferous boundary), zonal resolution averages 0.6 Ma. In contrast, the lowest resolution is shown in the early Emsian to mid-Eifelian interval with zonal durations of up to 5.5 Ma (serotinus Zone).