Astrochronology of the Mediterranean Miocene: Linking palaeoenvironmental changes to gateway dynamics. Geologica Ultraiectina (295)

The semi-enclosed land-locked configuration makes the Mediterranean basin an ideal natural laboratory to study both, fundamental plate tectonic processes and astronomically induced variations in the climate and their influences on the marine and continental realms. The prime disruption of the Tethys seaway along the colliding African-Arabian and Eurasian plates disconnected the Mediterranean from the Indian Ocean. This disruption is believed to have triggered the Middle Miocene Climatic Transition. During the late Miocene, Mediterranean-Atlantic marine passage progressively closed leading to restricted conditions in the Mediterranean basins, and eventually to widespread deposition of thick evaporites. To accurate date discrete steps in the constriction of marine gateways and the associated environmental changes, high resolution time frameworks are required. In this PhD project a high resolution time framework was established by extending the astronomical polarity time scale into the middle Miocene in the Mediterranean. This time framework was then used to accurately date environmental changes in the Mediterranean basin, which are reflected in changes in the sediment colour, the internal build-up of basic sedimentary cycles and their patterns, and in geochemical and magnetic proxies. The astronomically dated environmental changes were correlated with geodynamic processes of the gateway closures in the eastern and western Mediterranean, global climatic changes and variations in Earth’s orbital configuration. During the late Miocene, discrete steps in environmental changes at 8.017, 7.616, 7.168 and 6.719 Ma occurred remarkably synchronous in the Mediterranean basin and can be directly linked to steps in the progressive constriction of the marine gateway through northern Morocco and southern Spain. Commonly, the Betic Seaway through southern Spain is thought to have closed in the late Tortonian at ~7.8 Ma, whereas the Rifian Corridor through northern Morocco persisted at least until ~6.8 Ma. Our new chronology for a Neogene basin in southern Spain indicates a stratigraphic hiatus of at least 2 Myr between fully marine conditions at ~7.8 Ma and the first continental deposits. This suggests that a Messinian gateway though southern Spain cannot be completely ruled out. During the middle Miocene, discrete steps in environmental changes in the Mediterranean are for the first time astronomically dated at 15.007 and ~14.485 Ma. They coincide with a time of global climate variability but cannot be linked to processes related to the closure of the Mediterranean-Indian Ocean gateway because it lacks any high resolution and accurate age control. To improve the chronology of this marine passage, Neogene basins in SE Turkey were studied. The new results indicate that basins north of the suture zone emerged during the late Oligocene (~23 Ma), which must have severely constricted this gateway. On the southern side of the suture, foreland basins formed no longer a deep marine connection after ~11 Ma, giving an upper limit to the closure age. Surprisingly, all astronomically dated discrete steps in the Miocene environment occurred during ~400-kyr eccentricity minima. This suggests that the influence of the ~400-eccentrcity cycle occurred superimposed on long-term climatic or directional tectonic trends, such as gateway closures, exerting an additional control on passing certain thresholds at discrete steps in time