Tephra layers were identified from three boring cores in Aizubange Town, middle west part of the Aizu Basin, one of the tectonic basins in the south part of Northeast Japan. Ages of seven horizons were determined by radiocarbon dating. These results clarified the tephrostrati-graphy, chronology, and accumulation rates of subsurface sediments in the study area. Sedi-ments with a depth less than ca. 100m are mainly composed of silt, peat, and sand with several intercalations of gravel beds and tephra layers. These tephra layers can be correlated with Nm-NM (5.4ka), AT (30ka), DKP (55-66ka), Nm-KN, Ag-OK (<85.1ka), TG (129ka), and Sn-MT (180-260ka). Estimated accumulation rates determined for the longest core are 0.46-0.55m/kyrs after the deposition of DKP to present, whereas they are 0.19-0.23m/kyr at the depth between the horizons of DKP and TG.

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The Hamamatsu strand plain of the eastern Nankai Trough region is typical of the strand plains in Japan. In the western part of the plain, six series of beach ridges (BRs I to VI) have developed from land to sea in the past 7,000 years. The formation processes of BRs I to IV were reconstructed mainly from salinity changes in the back marshes, which indicate enhanced or reduced seawater invasion, which in turn reflects the degree to which the beach ridges were developed and thereby protected the back marshes from the sea. Salinity changes were estimated from analyses of fossil diatom assemblages in sediment cores. The ages of the deposits were estimated based on AMS ¹⁴C ages and tephrochronology of the core samples.
In the Higashi-Kandagawa lowland, an inner bay environment developed at ca. 8,000-7,000 calBP. Water salinity in the lowland showed repeated fluctuations before finally arriving at stable freshwater marsh conditions at ca. 3,200 calBP. The inter-ridge marsh between BR I and II was a brackish lagoon environment after 7,000 calBP. The lowland showed an increase in water salinity at ca. 3,200 cal BP and thereafter was a fresh-water marsh. The inter-ridge marsh between BR III and IV was a brackish lagoon at ca. 4,000calBP. Following an increase in water salinity, the lowland became a fresh-water environment at ca. 2,000calBP.
A reconstruction of environmental changes in the lowlands indicates that the formation of BRs I to IV started at ca. 8,000-7,000, ca. 7,000, ca. 7,000-4,000, and ca. 4,000calBP, respectively. The BRs were fully developed by ca. 3,400-3,000calBP (BRs I to III) or ca. 2,000 cal BP (BR IV).
In the study area, BR I is a primary barrier that formed during a period of sea level rise, and BRs II to IV are secondary barriers that formed after the Holocene high stand. It is a unique and important feature of the geomorphological development of the study area that the closure of each back marsh by beach ridges did not occur via a simple development. Instead, the salinity-increase events in the back marshes indicate the repeated rise and fall of the beach ridges. Breaching of the beach ridges was probably in response to erosion by storm and tsunami waves or seismic-related coastal subsidence.

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