Influence of Akiyoshi limestone bedrock on elemental concentrations and Sr isotopic ratio in stream sediments

Abstract

Chemical compositions and Sr isotopic ratio (⁸⁷Sr/⁸⁶Sr) of stream sediments collected from Akiyoshi-dai, where is underlain by a large-scale limestone bedrock, were measured to investigate the influence of limestone bedrocks on geochemical maps. The sediments were sieved with eight screen sizes, and their bulk and acetic acid soluble fractions were both analyzed. 50–100% of the total Ca was extracted by acid leaching from stream sediments in the catchment of Akiyoshi limestone, indicating that Ca exists mainly in carbonate materials. In contrast, only 5–10% and 15–45% of the total Sr were extracted from their coarse (>125 μm) and fine particles (<125 μm),respectively. Therefore, Sr exists dominantly in aluminosilicate minerals even in stream sediments originated from limestone bedrock. Because the acid extraction percentages of Ca and Sr increase exponentially below 125 μm of grain size, carbonate materials would be supplied by fine-grained particles from limestone bedrock. ⁸⁷Sr/⁸⁶Sr ratios in the acid soluble fractions were systematically lower than those of the bulk fractions and comparable to ⁸⁷Sr/⁸⁶Sr of Akiyoshi limestone. Although limestone bedrock apparently scarcely influence on Sr concentration and ⁸⁷Sr/⁸⁶Sr of stream sediments, ⁸⁷Sr/⁸⁶Sr of carbonate materials is successfully extracted from the bulk sediments using acetic acid-based extraction method.