Revealing the timing and dispersal of large explosive eruptions at Aso volcano (Japan) by integrating proximal and distal tephra records over the last 130 kyrs

Aso caldera in central Kyushu, SW Japan, is one of the largest and most active volcanoes in the world. The Aso system has experienced four caldera-forming eruptions (Aso-1 to −4) and inter-caldera activity from multiple central cones. This study provides detailed glass geochemistry of previously uncharacterised near-source, predominantly silicic, tephra units so that they can be correlated to ashfall layers preserved in distally located sedimentary records. Here the near-source glass data from twenty eruption deposits are integrated with the distal tephra fall deposits recorded in the high-resolution Lake Suigetsu record (situated approximately 525 km to the north-east), to better constrain the eruption timing, frequency, and ash dispersal of pre-30 ka Aso eruptions. The glass chemistry of these large Aso eruptions typically straddles the trachy-dacitic to rhyolitic compositional boundary. While some units share overlapping or similar glass chemistries, many can be distinguished from each other using major (SiO2, K2O, CaO and FeOt) and trace element (Sr) contents. The newly available near-source volcanic glass dataset has enabled the identification and correlation of an additional six tephra and cryptotephra deposits within the Lake Suigetsu record to known Aso eruptions. Consequently, nine of the twenty deposits in the Lake Suigetsu record (SG06/SG14 cores) displaying an Aso glass composition have now been correlated to near-source eruption units; these include, from youngest to oldest: Aso-Kpfa, Upper ACP4, Lower ACP4, ACP6, YmP5, Aso-4, Aso-ABCD, Aso-EF and Aso-HI. Significantly, the identification of these tephra deposits in Lake Suigetsu provides improved age estimates for the eruptions, and helps constrain the repose periods between eruptions; for instance ~400 years are resolved between two eruptions associated with the ACP4 Plinian activities. Our integrated proximal-distal record also indicates a higher frequency of activity in the 10 kyrs leading up to Aso-4 caldera-forming eruption. Eleven Aso-derived eruption deposits in the Lake Suigetsu record have not yet been recognised in the exposed near-vent sequences, perhaps suggesting that Aso caldera has been responsible for more silicic high-intensity eruptions and widespread ashfall events than previously thought. This research demonstrates the merit of integrating proximal-distal records to better constrain eruption timing, frequencies and ash dispersals of pre-historic events.