[H-TT16] Development and application of environmental traceability methods

[HTT16-06] Volcanic ash in soil as a source of Sr in stream waters on Mount Amamaki

*Masami Kanao Koshikawa¹, Mirai Watanabe¹, Tomoyoshi Murata¹, Takejiro Takamatsu¹, Shingo Miura¹, Ki-Cheol Shin², Takanori Nakano² (1.National Institute for Environmental Studies, 2.Research Institute for Humanity and Nature)

Keywords:Sr isotopes, Volcanic ash, Chert-bedrock forests, Stream water

Although acid deposition is widespread in Japan, acidification is rare in forest ecosystems, largely because acid is neutralized by ion exchange with cations in soil. Cations are mainly supplied to forest ecosystems through atmospheric deposition and bedrock weathering. However, volcanic ash that has been deposited widely during volcanic eruptions in the past may be another important source of cations in forest ecosystems in volcanic areas, including Japan. Our previous study showed that the Sr isotope ratios of stream waters indicated a Sr contribution from volcanic ash in a granite area in Tsukuba, Japan (Koshikawa et al., 2016). To assess the larger contribution of volcanic-ash-derived Sr to Sr in stream water, we conducted a similar study in the forest on Mount Amamaki. This study area contains more volcanic ash in soil than the Tsukuba area and is partly underlain by chert bedrock that is highly resistant to chemical weathering (Koshikawa et al., 2019). In this presentation, we will discuss the difficulties of estimating atmospheric-derived Sr in stream water and determining the ⁸⁷Sr/⁸⁶Sr ratio of the bedrock-weathering endmember. Measurements of ⁸⁷Sr/⁸⁶Sr ratios in environmental samples were conducted in two chert-bedrock catchments and in two sandstone-bedrock catchments for comparison. We found that the contribution of volcanic-ash-derived Sr is higher than that of bedrock-derived Sr and atmospheric-derived Sr in one chert-bedrock catchment. For the other three catchments, volcanic-ash-derived Sr is a non-negligible source of Sr in stream water. The contribution of bedrock-derived Sr was higher than the contribution of volcanic-ash-derived Sr in sandstone-bedrock catchments.

References
Koshikawa, M.K. et al. (2016) Environmental Earth Sciences, 75 (501)
Koshikawa, M.K. et al. (2019) Japan Geoscience Union Meeting 2019