島根県益田市中世港湾遺跡の土壌多元素分析

中世港湾遺跡が島根県益田市中須東原遺跡にて発見された.この遺跡の考古学的研究からここが東アジア規模の海上交易圏を形成していたとされている.遺跡に掘られたトレンチNKZに見られる考古学面とこれに重なる土壌についてこの港湾の土壌の地球化学的特性を明らかにするために,蛍光X線分析により40試料の土壌について22元素の分析を行った.その結果をもとに考古面の土壌について人間活動の影響を評価した.土壌の化学組成の有意な相違は人間活動の違いによって生じていると言える.被熱跡と炭の分布する場所ではAs,Pb,Cu,Br,TS(全イオウ),MnOやP_2O_5などの元素間の相関が高く,その一方で構造物の穴のある場所ではCr,Sr,Sc,F,I,TS,CaO,MnOやP_2O_5などの元素間の相関が高い.また,鍛冶炉跡ではAs,Pb,Zn,Cu,V,Sc,MnO,Fe_2O_3*やP_2O_5などの元素間の相関が高い.これらの元素の相関関係は中世の港湾遺跡における土地利用の識別に役立つ.Y,Nb,Zr,ThやTiO_2などの元素は考古学的関連よりも自然的由来に関連すると考えられる.一方,これらの元素はその由来や起源の特定に役立つ.TiO_2に対しての砕屑物の基本値との比較判別図では,ここに示した被熱跡と炭の分布する場所ではZn,CuやP_2O_5などの元素が有意に高くなり,鍛冶炉跡ではAsとPbが同じく高くなる.構造物の穴のある場所での土壌の化学組成は基本値の組成線の上に有り,ここでは人為的影響は土壌の化学組成には及んでいないことが分る.

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.