Global stages, regional stages or no stages in the Plio/Pleistocene?

摘要

Over recent decades, with extended exploration of all the oceans, with the study of ice cores from Greenland and Antarctica, with all the high resolution methodologies now available for integrated stratigraphy, there has been a tendency in several milieux to abandon the use of stages for the Plio/ Pleistocene. We do not agree with this approach and consider the use of standard stages in the Plio/ Pleistocene as appropriate as for older parts of the Phanerozoic. In the hierarchy of chronostratigraphic units (erathem-system-series-stage), stage is the unit of lowest rank, and of widest and longest usage. Moreover, stage is the only chronostratigraphic unit defined on the basis of a typological criterion: units of higher rank are usually defined on the basis of the hierarchical principle. In the Plio/Pleistocene there are three formalized stages: Zanclean, Piacenzian and Gelasian (oldest to youngest), all defined in the Mediterranean area, a small geodynamically active ocean basin, where uplift of coastal areas has exposed continuous successions of deep marine Plio/Pleistocene strata. The Zanclean GSSP is an example of event stratigraphy: the Pliocene transgression marking the sudden invasion by Atlantic water masses of the desiccated Mediterranean, terminating the Messinian salinity crisis. The other two Plio/Pliocene GSSPs are defined using multiple criteria, but mainly by reversals of the magnetic field polarity. Following the recent IUGS ratification of the formerly Pliocene Gelasian Stage as the oldest stage of the Pleistocene, formalization of younger Pleistocene stages is in progress, starting from the Calabrian. With just one name of precise and well controlled time significance and multiple criteria for global correlation, one can identify a defined time interval in both terrestrial and marine realms, independent of latitude, bio-provincialism and historical evolution.