Relative sea-level change in and around the Younger Dryas inferred from late Quaternary incised-valley fills along the Japan Sea

Abstract

The lastdeglacial sea-level change is characterized by episodes of rapid sea-level rise such as meltwater pulses (MWPs) and periods of slower sea-level rise such as the Younger Dryas (YD) period. However, the existence of MWP 1B, which occurred right after the YD at around 11,000 cal BP, has been debated for the last decade. The Echigo Plain, along the Japan Sea, is governed by an active fault along its western margin, and the latest Pleistocene to Holocene sedimentary fills in the buried incised valley of the Shinano River comprise intertidal salt marsh sediments aggraded during the last transgression. The present tidal range along the Japan Sea is less than 0.3 m. Therefore, eustatic sea-level trends and events such as MWP 1B are detectable from the incised-valley fills if the history of subsidence along the fault is constrained. In this study, the relative sea-level change during 13,000–9000 cal BP is reconstructed on the basis of radiocarbon dates and the depth of salt marsh sediments identified in two sediment cores obtained from the Echigo Plain. If it is assumed that the interseismic subsidence rate with sediment compaction effect of 2.6–3.7 mm/yr, calculated from the age and depth of sediments in the cores, was constant during the valley-fill deposition, five segments of rapid sea-level rise and five segments of slow sea-level rise during 13,000–9000 cal BP are detectable. Comparing relative sea-level curves (corrected for tectonic movements and sediment compaction effects) from the Echigo Plain, Barbados, and Tahiti, a rapid sea-level rise just before the YD and slow sea-level rise during the YD are found as common features of the three sites. However, three episodes of rapid sea-level rise at 11,000–10,900, 10,600–10,400 and 9900–9700 cal BP are found only for the Echigo Plain; these provide evidence of coseismic subsidence along the fault. The rapid sea-level rise during 11,400–11,300 cal BP on the Echigo Plain is consistent with MWP 1B in Barbados, but it can just as readily be attributed to coseismic subsidence, and if MWP 1B is real, its magnitude is only less than 4 m in the Japan Sea.