The most important three issues in ocean & environmental sciences may be “climate change, ecosystem change, and pollution” which are driven mainly by humans. These issues are tightly linked to the fluxes of terrestrial materials to the ocean and their biogeochemical cycles in the ocean. We study (1) fluxes of chemical species via the atmosphere, groundwater, and river to the ocean, and (2) biogeochemical reactions, cycling, and sedimentation of chemical species in oceans (see the illustration). We take advantages of various radionuclides for tracing these environmental processes. Under this broad research scope, our main research fields can be divided into three specific areas: (1) Radionuclides – applications to key environmental studies; (2) Carbon- nitrogen-phosphorus cycling and ecological consequences; (3) Trace elements and rare earth elements – pollution and behaviors.