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An active barrier system is present in Akkeshi Bay and a lagoonal area along the Pacific coast of eastern Hokkaido, Japan. This rare feature consists of a so-called lagoon (the Akkeshi-ko lagoon), a flood tidal delta, a barrier, and a tidal inlet that leads into the outer sea of Akkeshi Bay and the Pacific Ocean. Large oyster reefs were living on the flood tidal delta until their sudden extinction in the early 1980s. The processes involved in the formation of this barrier system during a postglacial transgression period are not well understood. Here, we use sedimentological, paleontological, and radiocarbon dating methods to analyze Holocene drillcore obtained from the Akkeshi fishing port in 2009 and borehole log data previously obtained from public works around the Akkeshi lowlands. The data obtained from both drillcore and borehole logs indicates that: (1) postglacial transgression occurred in this area some 11,400 years ago and reached 50 m below the present sea water level, with the barrier system in the study area established 8800 years ago. (2) The preservation of this barrier system is due to the stable sea level in this area. The sea level has been constant for the past 5500 years, and has led to the development of oyster reefs on the tidal flood delta. (3) The current topographically distinctive barrier system has been affected by sudden uplifts associated with giant earthquakes. These events occur every few hundred years, with the barrier undergoing prolonged but relatively rapid post-earthquake subsidence at about 1 cm/year, as observed after a giant 17th century earthquake.

Reconstructed Holocene sea level curve from the Akkeshi barrier system in eastern Hokkaido, Japan