Identification and correlation of the Aso‐3 tephra in the Omaezaki area, central Japan: A valuable key stratum for the MIS 6/5 transition period

ABSTRACT

The Aso‐3 tephra is one of the most significant widespread marker layers from the Middle to Late Pleistocene, generated by a large caldera‐forming eruption at the Aso volcano in Kyushu, southwestern Japan. Despite its importance, a distal co‐ignimbrite ash correlative has yet to be clearly identified, primarily because although volcanic glass shows a broad chemical composition range, available refractive index and major‐element glass data remain limited. This issue has made it difficult to identify the distribution area of the widespread Aso‐3 tephra and utilize it as a marker layer. Consequently, the ability to synchronize terrestrial and marine records from Japan and the northwestern Pacific during the Marine Isotope Stage (MIS) 6/5 transition is limited. This study reports the identification of a previously unrecognized tephra layer (SKR tephra) within the basal part of the Furuya Formation in the Omaezaki area of central Japan as Aso‐3 tephra. Petrological and geochemical analyses reveal that the SKR tephra consists of two compositionally distinct glass groups, whose major element composition closely corresponds to that of the Aso‐3B unit within the Aso‐3 pyroclastic flow deposits. Furthermore, the glass composition of SKR tephra matched that of cryptotephra (MD26.70 tephra) contained within a sample taken from marine core MD012422 off the coast of Shikoku at a depth of approximately 26.7 mbsf. The MD26.70 tephra was deposited during the MIS 6/5 transition, demonstrating that the SKR tephra represents a valuable stratigraphic marker synchronizing terrestrial and marine records for this period. Consequently, the SKR tephra contributes to refining the regional tephra chronology of central Japan. It serves as an indicator layer enhancing the potential for high‐resolution paleoenvironmental reconstruction of the Japanese archipelago and the northwestern Pacific during the MIS 6/5 transition.