Comparing the 87Sr/86Sr of the bulk and exchangeable fractions in stream sediments: Implications for 87Sr/86Sr mapping in provenance studies

Abstract

We are preparing a nationwide distribution map of strontium isotope ratio (87Sr/86Sr) in Japan, using stream sediment to obtain basic 87Sr/86Sr data for the provenance analysis of food production and archaeological substances. To clarify the effect of particle size on 87Sr/86Sr in stream sediments, we analyzed stream sediment from the Shigenobu River system in Matsuyama, Ehime Prefecture, Japan, where a variety of silicate-dominated lithologies, that is, several bedrocks of andesitic, granitic, and siliciclastic sedimentary rocks, are distributed. This paper reports elemental concentrations and 87Sr/86Sr in stream sediments for six particle-size fractions (1000–500, 500–300, 300–180, 180–125, 125–75, and <75 μm). The results from stream sediments were compared with results from bedrock units and stream water over the catchment. The elemental concentrations in stream sediment tended to increase with decreasing particle size in all lithologies; however, Sr concentrations varied less than other elements across particle sizes. For most of the samples, 87Sr/86Sr varied by less than 0.001 among the six particle-size fractions, which was less than the variation among the different lithologies. Therefore, 87Sr/86Sr in the <180 μm particle-size fraction, which is normally used in Japanese nationwide geochemical mapping, should be a reliable proxy for bedrock 87Sr/86Sr. The 87Sr/86Sr values in water samples from the Shigenobu River system were lower and less variable than 87Sr/86Sr in the stream sediments, and they did not faithfully correspond to the watershed geology. The inconsistency may reflect selective dissolution of Sr from plagioclase. Interestingly, 87Sr/86Sr values of the exchangeable fraction of stream sediment in the <180 μm fraction were strongly correlated with 87Sr/86Sr of stream water samples. Because 87Sr/86Sr in plant and animal bodies reflects that of their water sources, the exchangeable fraction of stream sediment may be a useful proxy for geochemical provenance in Japan instead of stream water.