島根県益田市中世港湾遺跡の土壌多元素分析

A large-scale Medieval harbor site has recently been discovered at Nakazu Higashihara in Masuda City, Shimane Prefecture, Japan. Archaeological information indicates that the maritime trade network at this site was on an East Asian scale. The concentrations of 22 elements in 40 soil samples from the archaeological floor horizon and overlying topsoils in the Trench NKZ were determined by X-ray fluorescence spectroscopy, in order to identify the geochemical signatures due to human activity in the harbor site. The results show that chemical feature of soils from the archaeological floor was influenced by ancient anthropogenic activities, while that of the topsoil samples were not. Significant differences in chemical feature of soils occur between areas of differing activity. Strong relationships exist between As, Pb, Cu, Br, TS (total sulfur), MnO and P_2O_5 concentrations in the fire pit and charcoal area, and between Cr, Sr, Sc, F, I, TS, CaO, MnO and P_2O_5 in a building pillar area. Soils from an area of ancient metallurgical activity (blacksmithing) showed strong correlations between As, Pb, Zn, Cu, V, Sc, MnO, Fe_2O_3* and P_2O_5. Those elements can be potential discriminators in this Medieval harbor archaeological site. In contrast, Y, Nb, Zr, Th and TiO_2 are not key elements archaeologically, and are mainly of natural origin. However, these elements are useful for identify the element sources and their origins. Cross-plots against TiO_2 clearly illustrate that Zn, Cu, and P_2O_5 are significantly enriched above detrital background levels in the fire pit and charcoal area, as are As and Pb in the blacksmith area. Those from the building pillar area lie on background trends, and hence anthropogenic activity in that did not greatly affect soil chemistry.