Evidence that the Aso-3 caldera-forming eruption (southwest Japan) marks the termination of Marine Isotope Stage (MIS) 6

The Aso-3 caldera-forming event of Aso volcano was one of the largest eruptions of the Quaternary period, blanketing vast regions of Japan and surrounding seas in ash. However, uncertainties surrounding the eruption age and geochemical variability have limited its utility as a robust time-stratigraphic marker. Distal occurrences previously attributed to Aso-3 span a broad temporal window (135–110 ka) and glass shards often lack compositional agreement with those of proximal datasets. Here, we re-evaluate the characteristics of Aso-3 using new stratigraphic and geochemical evidence from proximal and distal settings. In the Lake Suigetsu sediments, three Aso tephra layers are newly identified, including a 3 cm thick layer at ∼133 ka with glass shards that compositionally span the proximal Aso-3 range. Additionally, we identify a compositionally identical Aso-3 cryptotephra in the Sea of Japan (core U1427A). Combined stratigraphic, geochemical, and palaeoenvironmental evidence (pollen, diatom and benthic δ 18 O) from these records confirms that Aso-3 occurred prior to Marine Isotope Stage (MIS) 5e, during the termination of MIS 6. This establishes Aso-3 as a regional isochron, aiding synchronisation of paleoclimate records across the glacial–interglacial transition (Termination II). Our findings caution against correlating to Aso-3 based on partial geochemical matches, given that Aso experienced numerous explosive eruptions responsible for widespread ash dispersals throughout MIS 6 and 5.