Astronomical tuning of the La Vedova section between 16.3 and 15.0 Ma. Implications for the origin of megabeds and the Langhian GSSP

The early-middle Miocene, marked by the Middle Miocene Climatic Optimum (MMCO) followed by the Middle Miocene Climate Transition (MMCT) towards cooler temperatures, represents a crucial period in Earth's climate evolution. To understand this episode and reconstruct its origin and the regional impact of the observed global changes, it is critical that high-resolution astronomical age models are developed for climate sensitive regions around the world. One of these areas undoubtedly is the Mediterranean, but so far no such an age model has been established for the interval of the MMCO. Nevertheless, this interval is well exposed in the coastal cliffs along the Adriatic Sea near Ancona (Italy), where it is characterized by the occurrence of 7 conspicuous limestone beds, termed megabeds, alternating with marl intervals. Here, we use the Lower La Vedova Beach section to construct an astronomical time scale for the younger part of the MMCO in the Mediterranean. The tuning to ~ 100-kyr eccentricity seems robust, but is less certain for precession in some intervals, as a consequence of the less clearly developed internal structure of the basic precession related cycles and uncertainties in the phase relation with climatic precession and insolation and in the astronomical solution in terms of tidal dissipation and dynamical ellipticity values. The tuning nevertheless provides astronomical ages for calcareous plankton events and magnetic reversals for the interval between 16.3 and 15.0 Ma. Individual megabeds are related to the ~ 100-kyr eccentricity cycle corresponding to eccentricity minima and the megabed interval itself is partly controlled by the 405-kyr cycle, as it marks two successive minima and the maximum in between. However, no relation with very long period eccentricity cycles (2.4 and 1 myr) is evident, and a link to regional tectonic processes (a major orogenic phase at the base of the Langhian and the likely associated Langhian transgression) seems more plausible. The higher sedimentation rate in the megabeds can be explained by the additional preservation of biogenic silica, which may also account for the diluted planktonic foraminiferal assemblages. With the integrated magnetobiostratigraphy and the tuning to eccentricity and to precession/insolation, the Lower La Vedova Beach section meets key requirements for defining the Langhian GSSP.