[H-QR05] Quaternary, Diachronic dynamics of human-environment interactions

[HQR05-03] Revisiting the beach-ridge set hypothesis in Japan based on luminescence dates in the Akita coastal plain

*Toru Tamura^1,2, Kyoko Kataoka³, Hirona Homma^4,1 (1.Geological Survey of Japan, AIST, 2.Graduate School of Frontier Science, The University of Tokyo, 3.Research Institute for Natural Hazards and Disaster Recovery, Niigata University, 4.Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University)

Keywords:Coast, Luminescence dating

Beach ridges are ubiquitous landforms formed when foredunes or berms are abandoned inland by the shoreline progradation owing to coastal sediment supply. With their accurate chronology, they provide information of the past shoreline positions and long-term fluctuations in coastal sediment supply. Strand plains in the Japanese archipelago have been supposed to have three or four beach-ridge sets with inter-regional synchroneity assumed. This view was also followed in the characterization of strand plains in NE Japan, such as the Sendai, Ishinomaki, and Akita, but only substantiated indirectly by radiocarbon dating of inter-ridge swales. This weak chronology has allowed for a further speculative hypothesis that the ‘synchronous’ beach-ridge sets reflect sporadically intensified fluvial sediment supply following millennial-scale large earthquakes and resultant landslides along the Japan Trench. To validate the beach-ridge set hypothesis, we determined feldspar luminescence (pIRIR) ages of beach ridges in the Akita coastal plain. Three Ridge sets, I to III, from the land to sea, are identified in the plain. Ridge sets I and II are c. +10 m in elevation while Ridge set III is +15–20 m. As the height of the ridges exceed the swash limit on the beach face, the upper part of the ridges consists of a thick aeolian layer. Ridge set III partly exhibits features of parabolic dunes, suggesting the dominance of landward aeolian sand transport. Seven pIRIR ages from Ridge sets I and II range from 3700 to 6000 years. These ages are much older than Ridge sets I and II in the Sendai plain, and thus do not support the synchroneity between the two plains. Eight pIRIR ages at a site on a parabolic dune in Ridge set III ranged 420 to 820 years and suggest rapid accumulations of aeolian sand between 500 and 700 years ago. The pIRIR chronology also indicates that the half of the Akita plain was completed before 4000 years ago, followed by a period of decelerated shoreline progradation. The decelerated progradation is relevant with the completion of the decreased accommodation of the embayment between the Oga Peninsula and Akita and promoted the higher accumulation of the coastal dunes in Ridge set III. pIRIR ages of the parabolic dunes in Ridge set III correspond to the period of enhanced deposition of coastal dunes in the Wakayama plain. Thus, the parabolic dunes possibly reflect the intensified winter monsoon.