The strontium isotope value (Sr-87/Sr-86) is often used to identify the origins of agricultural products and the movements of ancient people. Maps delineating the distribution of Sr-87/Sr-86 values can therefore be of great use. Previous Sr-87/Sr-86 distribution maps of stream sediments covering small areas of Japan largely reflect the Sr-87/Sr-86 values of underlying bedrock. It remains unclear, however, whether the Sr-87/Sr-86 values of stream sediments may also reflect the Sr-87/Sr-86 values of the nearby vegetation and fauna. We sought to resolve this question by investigating the relationship between Sr-87/Sr-86 values in biological samples and in geological samples from the sites from which the biological samples grew. The geological samples consisted of soil samples from 2 paddy fields and 13 bed sediment samples (grain size < 180 mu m) from streams surrounding the paddy fields. All were collected from regions underlain by Inagawa granite bedrock, i.e., granite geology, around Toyota, Aichi Prefecture, Japan. The biological samples consisted of 4 rice plants and 1 wild boar bone, all collected in or near the same paddy fields. In the geological samples, the Sr-87/Sr-86 values of water-extracted Sr (water-soluble Sr) and ammonium-acetate- extracted Sr (exchangeable Sr) fractions in paddy soils were almost the same as each other (0.7098-0.7100) and slightly lower than the Sr-87/Sr-86 values of bulk Sr fractions (0.7099-0.7106). The 87Sr/86Sr values of biological samples (0.7098-0.7100) were also similar to those of the water-soluble Sr and exchangeable Sr fractions in paddy soils, though they varied among the parts of plants analyzed. This indicates that exchangeable Sr is absorbed by plants from soils through nutrients and water, and ingested by herbivorous animals from soils through plants and water. The Sr-87/Sr-86 values of bulk Sr in paddy soils ranged somewhat widely, while those from stream sediments collected around paddy fields were less variable (0.7099-0.7100). The difference in Sr-87/Sr-86 values between the paddy soils and stream sediments, samples similarly derived from Inagawa granite bedrock, may be related to the variable degree of weathering, and ultimately to the mineral composition, of the samples. This difference, however, was smaller than the range of Sr-87/Sr-86 values observed in the watershed geology. We therefore propose that in a silicate-dominated geology of the type typical of Japan, an Sr-87/Sr-86 spatial distribution map (using both exchangeable Sr and bulk Sr fractions in stream sediments) can provide useful guidance for identifying the geographic origins of food or for serving other related purposes.
