Seafloor hydrothermal systems have played an important role in the Earth’s evolution at sites where interaction between lithospere and hydrosphere occurs using Earth’s internal energy as a heat source. Reaction between oceanic crustal rock and fluid that is derived from the seawater under high temperature and pressure conditions results in efficient transport of thermal energy and in significant changes in chemical composition. Alkaline hydrothermal systems seem to be more favorable conditions for the occurrence and evolution of Earth’s early life, because of the higher thermal stability of amino acids. Boron isotope can be a new useful proxy of fluid-rock interaction in hydrothermal systems. Combination of boron isotope with existing conventional proxies can elucidate sub-seafloor fluid-rock reactions in detail, such as hydrothermal alteration of lower oceanic crust and multi-stage alterations related to sulfide ore deposit formation.

To elucidate the role of Himalayan river systems in the global carbon cycle, I have conducted sampling surveys and studied carbonate systems in these rivers. On modern timescales of 1–100 years, I showed that abundant soil-derived CO₂ was released from the surface water to the atmosphere. On the geological timescales of more than 10,000 years, I re-evaluated chemical weathering of both silicate and carbonate rocks and the atmospheric CO₂ consumed in the basins. I also investigated the dynamics of radiocesium (¹³⁷Cs) released by the Fukushima Dai-ichi Nuclear Power Plant accident in 2011 and its decadal trends, in Fukushima forest soils. I showed that most of the radiocesium is now retained on the surface of the mineral soil horizon, and that the distributions of radiocesium in each forest compartment (e.g., tree leaves/needles and forest soils) are entering the “quasi-equilibrium steady-state”. In addition, I highlighted the potential importance of organic matter in forest soils as a temporary reservoir of radiocesium and as a source for radiocesium cycling in forest ecosystems.