Revised age and distribution of ca. 87 ka Aso-4 tephra based on new evidence from the northwest Pacific Ocean

Abstract

The major-element compositions of glass shards of Aso-4 tephra can be distinguished from those of other widespread tephras in and around the Japanese islands. By analysing 325 tephras in 27 sediment cores collected from the northwest Pacific Ocean and the Sea of Okhotsk, Aso-4 tephra was identified at 14 locations on the basis of the glass major-element composition and refractive indices of glass shards and phenocrysts. Similarity coefficients between Aso-4 (type) and its correlatives ranged between 0.92 and 0.99, whereas comparisons between Aso-4 tephra and 92 other widespread Quaternary tephras were all <0.85. CaO–FeO plots clearly show the bimodal composition of glass shards in Aso-4 tephra. Aso-4 tephra reached the northern part of the Shatsky Rise, 2910 km from its source Caldera. Its thickness and grain size suggest that the axis of fall-out zone is oriented to the northeast of the Aso Caldera. A detailed δ 1 8O isotopic stratigraphy embracing the last 144 ka has been determined for core MD01-2421, collected off central Japan. Aso-4 tephra is present in this core and occurs in zone MIS 5b, more precisely at 86.8–87.3 ka. Using this datum, the sedimentation rate in the northwest Pacific Basin to the north of the 37.5° N latitude is greater than that to the south of 35° N latitude. This new information on the distribution and age of Aso-4 tephra improves our understanding on the magnitude of this eruption and provides a precise chronostratigraphic marker in the late Quaternary pelagic sediment of the northwest Pacific Ocean.