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The original definition of the Miocene and Pliocene goes back to Lyell (1833) when he proposed to subdivide the Tertiary into four groups (Eocene, Miocene, older Pliocene, and newer Pliocene) based on the proportion of recent to extinct molluscan species. The (older) Pliocene with the Subapennine formations as type was distinguished as having more than 50 percent of living species while the Miocene had 8 percent or more.
The Miocene-Pliocene (M-P) boundary was easily recognised in the Mediterranean area. However, its identification on a global scale was long clouded by a basic miscorrelation when it became associated with Pontian faunas and the arrival of Hipparion. With the downfall of the older Pliocene, interest focussed on stages, several of which competed to become the standard stage for the Lower Pliocene. At the end the Zanclean was chosen but restricted to the lower unit of the marnes blanches à Foraminifères (Trubi marls) in the original definition of Sequenza (1868). This definition is fully consistent with Lyell's (1835) observation of the marked difference between his Miocene and Pliocene macrofaunas in southern Europe, which he used to distinguish the epochs. These differences can now be ascribed to the annihilation of Miocene communities during the Messinian Salinity Crisis (Van Couvering et al., 2000).
After an intense debate between stratigraphers who agreed that the Zanclean definition embodied the historical Pliocene, unchanged since Lyell's time, and those who advocated an extra-Mediterranean definition, the M-P boundary and Zanclean GSSP were defined at the base of the Trubi at Eraclea Minoa on Sicily where it abruptly overlies evaporitic sediments of the underlying Messinian. This conspicuous level marks the abrupt flooding of the Mediterranean at the base Pliocene following the termination of the salinity crisis and the restoration of the connection with the Atlantic.
Eraclea Minoa is the lower partial section of the Rossello Composite Section (RSC), which covers the entire Lower and Middle (Piacenzian) Pliocene in a continuous deep marine succession. The section provided an excellent integrated stratigraphy including cyclostratigraphy that was used to tune the section and provide astronomical ages for all the events. As such the RCS underlies the age calibration of - this interval of - the standard astronomically tuned Neogene Time Scale. The RSC can further be considered as the prime example of the renewed concept of the Unit Stratotype, in this case for the Zanclean and Piacenzian Stages, while the Milankovitch cycles present may be considered as chronozones, i.e. non-hierarchical formal chronostratigraphic units of lower rank. The latter would increase the accuracy and resolution of the standard chronostratigraphic scale. The importance of this fundamental next step in chronostratigraphic classification is illustrated by unraveling the origin of the M-P boundary and the Pliocene flooding of the Mediterranean.
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