AMS Radiocarbon Dates of Wood, Mammalian Bone and Shell Fossils Collected from the Same Horizons of a Shell Mound Excavated at Awazu Submarine Archeological Site, Shiga Prefecture


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  • 滋賀県粟津湖底遺跡第3貝塚の同一層から出土した木片,哺乳類骨片,セタシジミ貝殻化石の放射性炭素年代の比較

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The Awazu submarine archeological site, 2-3 meters below the water surface, is located at southern basin of Lake Biwa, near the mouth of Seta River flowing out of the Lake Biwa. A shell mound was excavated during the 1990-1991 year survey of the site. Seven sets of wood, mammalian bone, and shell fossil samples collected from the same layers of the shell mound were dated with the AMS radiocarbon method. The ^<14>C dates for each of the three kinds of samples did not show big difference between the seven layers. This tendency is consistent with the fact that the shell mound was formed within ca. 100 years, because pottery fragments of the Funamoto-I type, which corresponds to early stage of the middle Jomon, were preferentially discovered from this shell mound. However, ^<14>C dates were systematically different among three types of samples : shell fossil samples showed the oldest dates from 4800-5080yr BP, except for a very young date (4630+/-80yrBP); wood samples did the middle ones (4570-4760yrBP); and bone fragment samples did the youngest ones (4090-4430yrBP). The reasons for the difference of ^<14>C dates among the three kinds of samples collected from the same horizons are not clarified yet. Shell carbonate originates from dissolved inorganic carbon in the lake water, which carbon was produced partly in the dissociation of old organic materials in the lake sediment and was also added with dead carbon from limestone rock surrounding Lake Biwa. In addition, younger carbon contaminated the bone samples when the samples were in the sediment may not have been removed completely during chemical preparation procedures of the samples.


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