Less impact of limestone bedrock on elemental concentrations in stream sediments

- Case study of Akiyoshi area -

Abstract

Abstract: Geological survey of Japan, National Institute of Advanced Industrial Science and Technology has created the nationwide geochemical maps of 53 elements using fine stream sediments (< 180 μm) in Japan. The spatial distribution patterns of elemental concentrations of stream sediments reflect faithfully the distribution of geology and mineral deposits. However, the exception is limestone bedrock, which insignificantly influences on elemental concentrations of stream sediments. To clarify the reason, we collected stream sediments from Akiyoshi-dai, where is underlain by the largest-scale limestone bedrock. Fine stream sediments (< 180 μm), whose drainage basins are occupied by limestones, have high CaO and Sr concentrations and intensive peak of calcite obtained by X-ray diffractometry. Examining variation of elemental concentrations against the particle size of sediments, the finer particle contains a higher proportion of calcite and has higher CaO concentration. However, CaO concentration (10–20 wt. %) in stream sediments is much lower than expected values (~50%); nevertheless limestone outcrops in more than 70 % in their watersheds. The contradictory finding is explained by less contribution of limestone clastics to river system because limestone bedrocks are easily dissolved by water (chemical weathering) but not susceptible to physical weathering and erosion process. In other words, the supply quantity of clastic materials from limestone bedrocks is much smaller than those of other rock types. In addition, Sr concentration in some samples does not correlate with either CaO concentration or the peak intensity of calcite; nevertheless Sr is expected to have similar chemical properties to CaO. The fact suggests that calcite formed from water oversaturated for calcium carbonate is supplied to river system; it has the different Sr concentrations from Akiyoshi Limestone.