Reconstruction of Holocene relative sea-level change and residual uplift in the Lake Inba area, Japan

Abstract

We collected and analyzed fossil diatoms and volcanic ash and determined 14C ages in core samples from the lowlands around Lake Inba in the eastern Kanto Plain, central Japan, an area where late Quaternary sea-level change, fluvial erosion, and tectonic uplift have affected a region of archeological interest. We inferred Holocene paleoenvironmental changes and relative sea levels on the basis of indicators, such as high tide levels, derived from fossil diatom assemblages as well as the stratigraphic positions of 14C-dated samples and the K–Ah tephra. Our results lead to estimates of the upper half tidal range during 6800–7000cal yr BP of 1.7m. The Holocene highstand took place at approximately 6400–6500cal yr BP, and mean sea level reached an elevation of 1.9m. However, the timing of this sea-level rise is earlier in the Lake Inba area than documented in previous studies, and it is suggested that the timing of the Holocene highstand may appear at least 1000years earlier as a result of Holocene residual uplift. After this, sea-level fell abruptly around 4000cal yr BP. We also recognized a sea-level fall corresponding to the Yayoi regression at about 2600cal yr BP, during which the mean relative sea level may have been as low as −2.5m after compensating for uplift, subsidence or geomorphologic effects in this area.