Abstract
The Middle Miocene Climate Transition (~15-13.7 Ma) is one of the major steps in Cenozoic climate evolution. The rapid expansion of the East Antarctic Ice Sheet at ~13.9 – 13.7 Ma caused important climate changes on a global scale. The aim of this PhD research has been to study the
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effects of the Middle Miocene Climate Transition in the Mediterranean area and to improve the age models for this crucial time interval. Sediments from the Adriatic coast near Ancona (Italy) and from the Maltese Islands (Malta and Gozo) record the Middle Miocene Climate Transition in the Mediterranean. Bulk and foraminiferal carbonate isotope records have been constructed to record the major ?18O isotope event and the ?13C carbon excursions associated with the Monterey Carbon Excursion (CM-events). The benthic and bulk carbonate isotope records show a shift in ?18O between ~13.92 and 13.76 Ma which can be related to the Mi3b oxygen isotope event. The ?13C carbon excursions CM5a, CM5b and CM6 are observed in the benthic and planktic records. A divergence between the planktic and benthic isotope signals points to an enhanced contrast between surface and deep water possibly related to increased run-off and decreased ventilation. Geochemical records show precession controlled enrichments in Ti/Al and Zr/Al, which suggests eolian dust input. Dust input becomes more significant during and after the MMCT, which may evidence aridification of North Africa. From 13.8 Ma onwards, distinct quadruplet cycles containing sapropelitic sediments are observed, suggesting a causal connection between the main Middle Miocene cooling step, and sedimentation and circulation in the Mediterranean. Alternation between sediments enriched in eolian dust and the occurrence of sapropels suggest that the African monsoon system was already operative in the middle Miocene. Benthic assemblages from the Maltese Islands point to a mesotrophic and well ventilated environment before 13.8 Ma. The rapid increase in terrestrial clay influx at ~13.76 Ma, related to enhanced river runoff, caused an increase in food availability and lower bottom water oxygen levels. The benthic assemblage responded by an increase in infaunal taxa. Ongoing environmental changes, related to restriction in circulation, after the MMCT are inferred from the successional increase in infaunal species and the occurrence of the first sapropelitic sediments. Comparing the astronomically tuned age models of the Mediterranean sections and Open Ocean records revealed several discrepancies. This is partly the consequence of a poor magnetobiostratigraphic framework causing misinterpretation of orbitally forced cycles in sedimentary records. The transition from the Globigerina Limestone Formation to the Blue Clay Formation on Malta is possibly not continuous and might contain a hiatus. Nevertheless, the major step in the climate transition remains concomitant with a 1.2 Myr minimum in obliquity amplitudes and a ~400 kyr minimum in eccentricity, which confirms the proposition that this orbital configuration triggered extensive ice sheet growth.
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