Using isotopes to determine the contribution of volcanic ash to Sr and Ca in stream waters and plants in a granite watershed, Mt. Tsukuba, central Japan

Abstract

Strontium (Sr) isotope ratios of soils, stream water, plants, and atmospheric precipitation were used to trace calcium (Ca) and to determine the influence of volcanic ash in soils on Ca cycling in forest ecosystems in a small forested watershed on Mt. Tsukuba, central Japan. The 87Sr/86Sr ratios of soils on steep slopes and the valley floor ranged from 0.7110 to 0.7139, which indicates that the major source of Sr was the granite substrate (87Sr/86Sr ratio 0.7120–0.7131). Soils on a ridge had lower 87Sr/86Sr ratios (0.7068–0.7072), indicating that they were mainly composed of volcanic ash materials released around 30,000 years ago from Mt. Akagi (0.7069). The 87Sr/86Sr ratio decreased and the concentrations of Sr (and Ca) increased in stream water with increased elevation from the valley bottom to the ridge, indicating that volcanic ash was the dominant source of both cations in the upstream area. The contributions of volcanic ash-derived Sr to upstream and downstream water were 50 and 0–1 %, respectively. The 87Sr/86Sr ratios of plants were between those of the soils in which the plants were growing and those of atmospheric precipitation (0.7100). More than 74 % of the Sr in plants on the ridge, but <17 % in plants in the valley bottom, was volcanic ash-derived. The origin of Sr (and thus Ca) in stream waters and plants varied depending on the volcanic ash content in soils, which was significantly influenced by the site elevation.