Holocene sea-level change and hydro-isostasy along the west coast of Kyushu, Japan

Mid- to late-Holocene sea-level variations have been obtained at sites along the west coast of Kyushu, Japan. Sea-level observations for this relatively stable area are very important for evaluating the crustal tilting associated with the hydro-isostatic adjustment due to the last deglaciation, and for examining the cause of underwater Jomon sites (submerged archeological sites during the Jomon period of mid-Holocene) typically observed in this region. For these purposes, we conducted systematic boring samplings at sites from Goto Island to Tamana along the latitude of about 33°N. Then we performed diatom assemblage and pyrite-sulfur analyses of these sediment samples and 14C datings for intertidal shells in order to get the sea-level variations for these sites. Observations at Tsumizu faced to Oomura Bay with tidal range of 0.9 m show a sea-level high-stand of 1 m at about 5500 yr B.P. (years before present) followed by smoothly falling to the present level. Evidences for sea-level oscillation of greater than 50 cm have not been found here. Relative sea-levels at about 5000 yr B.P. are, however, −2 m at Goto Island (125 km west of Tsumizu) and 2 m at Tamana (50 km east of Tsumizu). Thus observations at sites along the traverse from Goto Island to Tamana are indicative of crustal tilting of about ∼3–4 m for the distance of 175 km, which seems to be consistent with the prediction caused by the hydro-isostatic adjustment.