Around the start of this century, the ICS Subcommission on Quaternary Stratigraphy had intended to formally divide the Pleistocene Epoch into three subepochs/subseries: the Early/Lower Pleistocene, the Middle Pleistocene and the Late/Upper Pleistocene. In 2009, however, the ICS's governing body (the IUGS), overuled this approach, preferring instead to divide the Pleistocene Epoch into four ages/stages. Some GSSP candidate sites were selected and informal names based on these locations were proposed. This resulted in the 2009 proposal that the Middle Pleistocene subepoch/subseries should be replaced by the Ionian age/stage, the start/base of which would be defined at a location in Italy.
The Ionian stage includes all of the European Sicilian Stage and the first part of the Tyrrhenian Stage.
Suitable sections for defining the base of the Ionian Stage are located in southern Italy. During Lower and Middle Pleistocene the Adriatic-Ionian margin was characterized by high sedimentation rates, in response to intense differential tectonic subsidence and massive sedimentary yield. Specifically, candidate sections are the “Montalbano Jonico” Section (Bradanic Trough, Basilicata Region) and the “Valle di Manche Nord” Section (San Mauro Marchesato, Crotone Basin, Calabria Region). In the former, a spectacular succession of marls laid in the Apennine foredeep is exposed which, however, are unfortunately unsuitable for magnetostratigraphic analyses. The latter, which is represented by a shallowing-upward succession that developed in an outer-shelf environment, has the bonus of a sharp biomagnetostratigraphic record, which provides tighter age control.
Besides these exposures, a number of Ocean Drilling Program cores drilled either in the Ionian Sea (Site 964) and in the Sicily Channel-Capo Rossello area (Site 963) are available, which correlate well with the on-land sections mentioned above based on a significant number of criteria (magnetostratigraphy, biostratigraphy, sapropel stratigraphy, stable isotopes, Tephrochronology). The interval corresponding to the Ionian Stage is especially well represented in Site 963, where no turbidites and/or hiatuses are recognized. Presently, the Montalbano Ionico section is possibly the main candidate for defining the Lower - Middle Pleistocene boundary, when a reliable magnetostratigraphic record is found. The oldest known human's DNA dates to the Middle Pleistocene.
In November 2017, the Chibanian (defined at a site in Chiba Prefecture, Japan) replaced the Ionian as the Subcommission on Quaternary Stratigraphy's preferred GSSP proposal for the age/stage that should replace the Middle Pleistocene subepoch/subseries. Until the Chibanian is ratified by the IUGS, the Chibanian will remain an unofficial informal, proposed stratigraphic division and Middle Pleistocene remains the ICS's unofficial name for this time period and series of rocks.