桜島における縄文人の生活と火山災害-桜島・武貝塚の発掘調査-

Abstract

We excavated from 1988 to 1990 Take shellmound which was famous of the Jomon site in active volcanic island. Take shellmound about about 4, 000 years ago is located near the coast of Sakurajima Volcano in the rim of Aira caldera, southwestern Japan. We revealed the pit dwelling, the little stone circle and shellmound of late Jomon period in this excavation.Results of Arcaeological, tephrochronological and geophysical studies are shown below.(1) Archaeological studies① Excavated house site is overlain by lahar deposit, and many pieces of pottery are scattered around them. The occurrence suggests that the dwelling was destoroyed by lahar. No pit dwelling is found in the overlying beds, suggesting that the people of those days moved to another safety places after the diserster.②The shellmound is constructed many thin layers. We could separate many layers, because each layer had each other main shellfishes. Three layers of crushed shellfishes were clearly found in the shellmound and between each them the alternate pattern of shellfish species was recognized. One cycle of this pattern consisted of 6 units of shellfish species and more than three cycles existed in the shellmound. It suggests that the cycle shows an year and the unit shows season. The layer of crushed shellfishes included the most volcanic ash and over it there were many micro land shellfishes. Presently at Take site volcanic ash fall in summer. And generally shellfishes are gathered without in summer. So that layer might be made just before summer and while the surface of it had been earth's surface, in all of summer, many shellfishes was crushed, volcanic ash fell and micro land shellfishes lived on it.③ About Animals, Japanese macaques were excavated in Take shellmound three times more than in other shellmouds in Kyusyu. It may depend on the enviroment of active volcano Sakurajima.(2) Holocene sea level changes and ground deformation around Kagoshima Bay Beach sand bed about 4, 000 y.B.P. was found at Take shellmound on August, 1989, During the excavation of the shellmound at Take, a beach sand bed was discovered under the layer which in-cluded remains such as shells and pieces of earthen vessels. This beach sand bed was estimated to be piled in the late Jomon age about 4, 000 y.B.P. As the upper surface of this beach sand was 8.4 m-8.6 m above the present mean sea level, ancient sea level at Take about 4, 000 y.B.P. was estimated to exist at 6 m-7 m higher than the present sea level.Assuming that upheaval movement of the ground with the rate of 1.3 mm/year have been continuing since Jomon age, an attempt was made to estimate the height of Holocene sea levels around Kagoshima Bay, subtracting the vertical displacements of the ground since Holocene. As a result of this study, sea levels around Kagoshima Bay at 7, 000 y.B.P., 6, 000 y.B.P. and 4, 000 y.B.P.were estimated at the present heights of -3 m, 7 m and 1 m-2 m, respectively.(3) Tephrochrnological studyTake shellmound is found in the volcanic sand deposit which was formed by intermittent Vulcanian eruptions of Minamidake Volcano. Thin ash layers are also found intebedded with shell layers of the mound. Two cases are considered for the cause of alternation; one is that the ash and shell were accumulated alternatively, and the other is that the ash layers were formed by the sinking of ash after the formation of shellmound. In the former case, it seems to suggest that the ash fall by intermittent eruptions had little effect of people, animal and vegetation.In order to clarify the cause of alternation of ash and shell layer, petrographica study had been made. However, exact solution to the cause could not be obtained.