Defining the Upper Pleistocene: The Quest for a GSSP

The formal chronostratigraphic subdivision of the Quaternary System, which includes the Holocene and Pleistocene Series, remains partly incomplete, particularly for its uppermost interval. In particular, the Upper Pleistocene Subseries, traditionally considered to begin with the onset of the Last Interglacial, has not yet been formally defined through the establishment of a Global boundary Stratotype Section and Point (GSSP), and the corresponding stage is still unnamed. Previous attempts to formalize this boundary have not led to formal ratification. A proposal based on the Amsterdam Terminal borehole (the Netherlands), which placed the GSSP at the base of the Eemian, was not ratified by the International Union of Geological Sciences (IUGS). A subsequent effort focused on the Fronte section (Taranto, Italy), which provides a continuous marine record of the MIS 5e plateau; however, this section does not capture Termination II, i.e. the transition into MIS 5. An alternative proposal has considered the use of an Antarctic ice core, where Termination II is clearly expressed by a sharp rise in atmospheric methane concentrations, as a potential candidate for the GSSP. Despite these efforts progress towards formal definition has remained limited over the past decade. To address this issue and advance toward the identification of a suitable stratotype, the “Working Group on the Upper Pleistocene GSSP” was established in May 2025, under the Subcommission on Quaternary Stratigraphy (SQS). In defining the Upper Pleistocene GSSP, several key issues must be carefully evaluated. For example, the MIS 6/5 transition does not strictly coincide with the onset of the Last Interglacial. In addition, the diachroneity of isotope stratigraphy across major oceanic basins may limit the use of single proxies as globally correlatable markers and underscores the need for integrated, multiproxy approaches. Stratigraphic continuity is an essential requirement for defining a GSSP and a sequence-stratigraphic approach may allow for the identification of stratigraphic unconformities, depositional hiatuses, and condensed sections. Eustatic sea-level changes represent a globally synchronous signal with strong correlation potential across different depositional environments. This signal is clearly recognizable in deltaic, coastal, and shallow-marine successions; however, these are often affected by stratigraphic discontinuities associated with episodes of sea-level fall at interglacial-glacial transitions. Finally, the dating methods applied to potential GSSP candidates vary significantly in terms of accuracy and precision, ranging from millennial to centennial scales. This variability must be carefully considered when evaluating candidate sections and establishing a robust chronological framework. While some exposed terrestrial successions may provide relatively continuous records, stratigraphic continuity is likely best preserved in sedimentary archives such as deep-sea and lacustrine cores, speleothems, and ice cores, where primary and secondary stratigraphic markers can be identified with very high chronological resolution. Any future proposal for the Upper Pleistocene GSSP will need to address all these challenges in a comprehensive and integrated manner.