Development of hydrothermal and frictional experimental systems to simulate sub-seafloor water–rock–microbe interactions

Since the discovery in 1977 of deep-sea hydrothermal vents, they have been shown to host unique but diverse biological communities, despite the dark, barren ocean-floor settings in which they exist. Recent research has indicated that the production by fault systems of abundant reducing agents such as hydrogen possibly sustains the microbial communities in these chemoautotrophic ecosystems. High-pressure and high-temperature hydrothermal experiments, and friction experiments, have resulted in the development of important new experimental apparatuses. A batch-type (closed) experimental system that creates equilibrium conditions has contributed greatly to our understanding of sub-seafloor hydrothermal reactions. Flow-type experimental systems have allowed investigation of natural systems under non-equilibrium conditions. Friction experiments have recently been developed to better understand generation of the hydrogen that makes fault systems habitable by primary producers. These experiments suggest that microbial ecosystems sustained by chemical energy derived from fault systems might be widely distributed within oceanic crust. Moreover, flow-type systems that can be used to simulate natural hydrothermal environments that include crustal aquifers might provide insights into the ecological significance of microorganisms and their global contribution to biogeochemical cycles in the ocean and crust. Here we describe hydrothermal and friction experiment systems that we developed during our Trans-crustal Advection and In-situ biogeochemical processes of Global subseafloor Aquifer (TAIGA) project, and consider the application of some of them to explore the interactions among rocks, fluids, and microbes. For this purpose, our original data obtained in the experiment of interaction between basalt and water in the flow-type system was also included.