Stratigraphy and wiggle-matching-based age-depth model of late Holocene marine sediments in Beppu Bay, southwest Japan

Abstract

We analyzed the lithology, magnetic susceptibility, bulk density, and X-ray radiographs of 14 sediment cores (1–9m long) from Beppu Bay in the western Seto Inland Sea, Japan, to establish the late Holocene stratigraphy in the deepest part of the bay and to develop an age–depth model for the sediments there. The cores contained 18 thick (major event) high-density layers (16 turbidites and two volcanic ash; >1cm thick), and both lithological observations and density variations in the hemipelagic mud that is dominant in the cores revealed a further 55 thin (minor event) high-density layers (<1cm thick). Analyses of color properties and opal and sand contents of the hemipelagic mud defined nine lithological units. After stratigraphic correlation of the event layers among cores, we projected 14C dates onto a single composite core. Forty-two AMS 14C dates from bivalve mollusk shells were used to construct a wiggle-matching-based age–depth model for the late Holocene sequence and to determine the local reservoir effect (ΔR). The age–depth model showed a sedimentation rate of 0.23–0.30cm/yr for a 7.8m-long composite core and an age of ∼2800calyr BP at the base. Wiggle-matching provided ΔR values of 115–155yr for late Holocene bivalve samples from Beppu Bay, which is consistent with previous estimates reported from coastal areas near the Kuroshio Front. Comparison of wiggle-matching-derived ages of thick turbidites with the ages of historical earthquakes showed differences within ±25yr. Our study demonstrated that wiggle matching with optimal fitting based on either the weighted least-squares or maximum likelihood method can minimize the effect of scatter of age data due to reworking and burrowing of bivalves and thus improve the accuracy of age–depth models.