Periods of beach ridge range formation on the Kitakami River lowlands and late Holocene sea-level changes around the Sendai Bay, northeastern Japan

Abstract

The Kitakami River lowlands are located along the lower reaches of the Kitakami River and face Sendai Bay, northeastern Japan. This study discusses the periods of the formation of beach ridge ranges on the lowlands, using the analysis of borehole records, radiocarbon dating, grain size analysis, and interpretation of archeological data. The late Holocene sea-level change around Sendai Bay is discussed based on the periods of the formation of beach ridge ranges and buried shallow valleys and the examination of the previous radiocarbon data showing the past sea level in the alluvial lowlands facing Sendai Bay. Beach ridge ranges on the coastal part of these lowlands are divided into five groups. The beach ridge ranges, in order from the most inland, were formed in the periods of the early Jomon period (6,000 to 4,600 yr B. P.), the middle Jomon period (4,600 to 4,000 yr B. P), the late Jomon period (4,000 to 3,000 yr B. P.), the later Jomon period to the Yayoi period (3,000 to 1,600 yr B. P.), and before 1,000 yr B.P. to the present, respectively. The late Holocene sea-level fluctuations consist of five peaks and four troughs recognized from the study of the beach ridge ranges. Two peaks in 3,500 yr B. P. and 2,200 yr B.P. were clarified by the previous radiocarbon data, and two troughs, estimated from the study of buried shallow valleys, in 2,500 yr B.P. and 1,600 yr B.P. The range of fluctuations was considered to be within three meters above/below the present sea level.