<14>^C ages and vertical distribution of benthic for aminiferal assemblage of the log drilled on the sea bottom in Naze Bay, Amami-Oshima Island, Ryukyu Island Arc, Japan


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  • 奄美大島,名瀬湾の海底ボーリングコアの<14>^C年代と有孔虫群集の垂直変化

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In 1993,five logs were drilled on the sea bottom in Naze Bay for the construction of wharf. At two localities no. 7 and no. 10,the following geological column was observed; basement rocks (Nase Formation), sand and gravel, mud and sandy mud, and muddy sand with bioclastics, in ascending order. Radiocarbon (^<14>C) ages of molluscan shell fossils in two samples collected from the lower part of mud and sandy mud facies at localities no. 7 and 10,were measured by a Tandetron Accelerator Mass Spectrometer of the Dating and Materials Research Center, Nagoya University. These horizons are 55m (T7-17) and 53m (T10-13) below the sea level. As a result of measurement, radiocarbon age of the former is 9,350±80 years B.P. and the later is 9,370±110 years B.P. On the other hand, 16 samples of locality no. 10 were treated for study about the assemblage analysis of foraminifera. In samples of the lower horizons (T10-16 to T10-13), Ammonia tepida, an index species of an inner and shallow bay, reaches as high as 20.5-37.3% in frequency. These sediments of the horizons were deposited in an inner and shallow bay area, may be less than 15m in depth. In samples of the middle horizons (T10-8 to T10-5), the frequency of Ammonia tepida is comparatively low (0-0.3%) and the ratio of planktonic foraminifera, an index of open sea area, is comparatively high (12.8-16.9%). These sediments of the horizons were deposited in the deeper bottom compared with the lower horizons, under the influence of the open sea water mass. Zheng et al. (1994) mentioned that the sea level was at -30〜-40m in 10,000yrs B.P. and reached to -25m in 9,000yrs B.P. on the basis of the information from sediment stratigraphy, geomorphology, pollen assemblages, microfauna and prehistorical archeology in the Southern Yangtze Delta, China. These data suggest that the sea level was at -30〜35m in about 9,350 years B.P. In that time, the water depth was less than 15m at the area that T7 and T10 logs were obtained, and therefore the bottom surface was estimated to be less than 50m below the Resent sea level. From the information and data, it may be infrerred that the Nase Bay area has a tendency to have subsided for the past 9,350 years.



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