The so-called Yayoi regression in Japan is known as a relative sea-level lowering to m below T.P. (Tokyo Peil; Mean Sea Level of Tokyo Bay) during -cal kyr BP. However, the regions where this event occurred in the Japanese Islands and its continuous sea-level index points from verified regions have yet to be clarified. Lake sediments beneath the Innermost Tonegawa Lowland, deposited in response to a sea-level rise during -cal kyr BP, indicate a paleo-water depth of ca. -m. The sea-level index points of the lake sediments indicate that the sea level dropped to . m T.P. at . cal kyr BP, while the paleo-water depth estimated from plant macrofossils in the sediments is m. The amount of sea-level lowering is greater than the sediment compaction effect; moreover, no large tectonic movements have affected the region over the past years. These results demonstrate that the Yayoi regression occurred in the Innermost Tonegawa Lowland and that isostatic movements triggered by sediment loading might have brought about the observed lowering of sea level.

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