The Ideale section at Montalbano Jonico (southern Italy): a Standard Auxiliary Boundary Stratotype (SABS) for the Middle Pleistocene Subseries of the Quaternary System

The Ideale section (IS) at Montalbano Jonico, Italy, has been approved as a Standard Auxiliary Boundary Stratotype (SABS) for the Global boundary Stratotype Section and Point (GSSP) of the Middle Pleistocene Subseries/ Subepoch and Chibanian Stage/Age at the Chiba section, Japan. The proposal was submitted to the voting members of the International Commission on Stratigraphy’s Subcommission on Quaternary Stratigraphy (SQS) on May 4, 2023, and following discussions was approved on July 8, 2023. The 74 m thick IS continuously spans the Marine Isotope Stage (MIS) 20–18 interval, and is part of the longer Montalbano Jonico succession (Basilicata, southern Italy in the Mediterranean region) encompassing MIS 37 to early MIS 16. The IS provides a detailed record based on multiple chronologically well constrained marine and terrestrial proxies, which are particularly useful for outlining the paleoclimatic evolution through the Lower–Middle Pleistocene transition. The high-resolution carbon and oxygen isotope stratigraphy outlines glacial–interglacial and stadial– interstadial phases as well as the sub-millennialscale features of Termination IX and the onset of MIS 19c. The sapropel layer equivalent to insolation cycle 74 (784 ka) occurs in early MIS 19c. A prominent peak in the 10Be/9Be record at the MIS 19c–19b transition identifies the low geomagnetic dipole moment associated with the Matuyama– Brunhes boundary interval. Two tephra layers (V3 and V4) relevant to boundary interval are 40Ar/39Ar dated. The V4 layer, occurring at the MIS 19c–b transition and in the middle of the 10Be/9Be peak, has an age of 774.1±0.9 ka, corresponding to the age of the Middle Pleistocene GSSP. A high-resolution alkenone sea-surface temperature and several paleobiological records complement the rich chronological and paleoenvironmental dataset from MIS 20 to the inception of MIS 18. The GSSP boundary interval in the IS is represented from 35.50 to 39.50 m, which corresponds to the interval of the highest values of the 10Be/9Be ratio (~776.35–771.87 ka) and includes the V4 tephra layer and the MIS 19c–MIS 19b transition. This SABS extends the correlation potential of the Middle Pleistocene Subseries/Subepoch GSSP interval to the Mediterranean region.