High-precision zircon U-Pb geochronology of astronomically dated volcanic ash beds from the Mediterranean Miocene

- We present high-precision zircon U-Pb dates for astronomically dated ash beds.
- Detailed inter-comparison of two entirely independent geochronometers.
- Insights into U-Pb systematics of igneous zircon.
- Recognition of non-systematic bias between zircon U-Pb and astrochronology.
- Important implications for U-Pb geochronology as a tool for time scale calibration.

Several orbitally tuned Miocene sedimentary sequences around the Mediterranean contain abundant intercalated volcanic ash beds. These sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology. We present a large data set (N=16, n=166) of zircon U-Pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) techniques for ash beds from an almost continuous orbitally tuned Messinian to Langhian (6.2-15.4 Ma) sedimentary sequence exposed along the Adriatic coast south of Ancona, Italy. We use this unique data set to evaluate (1) the accuracy of zircon U-Pb dates, (2) the significance of initial intermediate daughter product disequilibria for zircon U-Pb geochronology of young rocks, (3) the effect of prolonged pre-eruption zircon crystallization and zircon recycling on U-Pb derived ash bed deposition ages, and (4) discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the Geologic Time Scale.