A Potential GSSP for the Upper Pleistocene Subseries in an Antarctic Ice Core Based on the Sharp Methane Rise at ~128.5 Ma

In January 2020, the IUGS Executve Committee ratified the Lower Pleistocene Subseries comprising the Gelasian and Calabrian stages, the Middle Pleistocene Subseries comprising the Chibanian Stage, and the term Lower Pleistocene at the rank of subseries with a base then undefined but provisionally dated at ~129 ka. Defining the the Lower Pleistocene and its corresponding stage remains an important priority for the ICS Subcommission on Quaternary Stratigraphy. During the Second International Conference of the Association pour l’etude du Quaternaire européen (a precursor of the International Quaternary Association [INQUA] and its congresses) in Leningrad in 1932, it was decided that the base of the Upper Pleistocene should coincide with that of the last interglacial (the Eemian regional Stage in Europe), and at the 12th INQUA Congress in Ottawa in 1987 a proposal was approved to use the base of Marine Isotope Stage (MIS) 5 (termination II) to define the boundary. However, the base of MIS 5 is now known to be about 6 kyr older than the base of the Eemian pollen stage. Moreover, isotope stratigraphy gives Atlantic over Pacific leads of several thousand years for the past six terminations, and North Atlantic high-latitude temperatures lag southern hemisphere records by several thousand years. A further limitation is that Greenland ice core records do not extend beyond 129 ka. Isostatic rise at the beginning of the Last Interglacial appears to have been insufficiently rapid to be useful for narrow definitional purposes. Given these challenges, an Antarctic ice core should be considered to host the Upper Pleistocene GSSP, with an abrupt methane rise at Termination II potentially serving as the primary guide. This methane rise aligns with maxima for CO2 and δD in the EPICA Dome C ice core, and represents a distinctive global event closely related to rising temperatures in the northern latitudes. To provide a more recent comparison, methane lagged temperature rise by less than 30–70 years for the last glacial interval in Greenland, and its abrupt increase during Termination II in Antarctica is thought to reflect essentially synchronous abrupt warming of the air above Greenland. Termination II has a gas orbital age of 132.4 ka at its midpoint, with a subsequent steep methane increase at 128.51±1.72 ka in the EPICA Dome C core. This methane event appears synchronous with an abrupt increase in δ18O in Chinese speleothem records reflecting a rapid intensification of the Asian summer monsoon. North Atlantic responses suggest that the abrupt northern hemisphere warming signalled by the methane rise in Antarctica at 128.5 ka has left stratigraphic markers that would allow recognition of the base of the Upper Pleistocene across both hemispheres and in both marine and terrestrial records. An ice core is no longer an unconventional choice for a Quaternary GSSP: the Lower and Middle Holocene subseries GSSPs are both defined in Greenland ice cores.