Long-period orbital control on middle Miocene global cooling: Integrated stratigraphy and astronomical tuning of the Blue Clay Formation on Malta

Application of an astronomical age model to a bulk carbonate oxygen isotope record in the Ras il Pellegrin section on Malta indicates that the major step in the middle Miocene global cooling (13.82 Ma +/- 0.03) coincides with minimum eccentricity values associated with the 400-kyr cycle and minimum obliquity amplitudes associated with the 1.2-Myr cycle. This orbital configuration is very similar to that found for comparable oxygen isotope enrichment events in the late Paleogene and Neogene. The stepwise character of the middle Miocene cooling event appears to be controlled by the combined influence of the 100-kyr eccentricity cycle and the 172-kyr cycle in obliquity amplitude. The integrated stratigraphy further allows extension of the astronomical polarity timescale to the top of chron C5ACn. The boundary between the Globigerina Limestone and the Blue Clay Formation coincides with the major step in middle Miocene global cooling and provides a level suitable for placing the physical reference point for the Langhian/Serravallian boundary.