Hokkaido University, Japan

In this paper we propose the possible existence of a new biosphere called the "serpentinite biosphere." We also suggest that serpentinites act as a receptacle of the deep biosphere. Analyses of geology, geophysics, biology, and chemistry of these "serpentinite capsules" are useful for determining the deepest limits of the subsurface biosphere. We obtained bathymetry, sediments, geochemical, and microbiological data from the Chamorro Seamount, a serpentinite seamount consisting of a pile of serpentinite flows in the Mariana forearc. These data reveal that serpentinite flows are products of upper mantle peridotite altered by the addition of water from the subducting slab. Alteration of peridotite to serpentinite provides hydrogen gas and methane, which are the most important energy sources for extremophile life. It also induces a buoyant rise of serpentinite diapirs, which are likely to capture and transport portions of the deep biosphere during their ascent to the surface. The conditions and the characteristics of serpentinite seamounts indicate that the serpentinite diapir is a transported capsule, or a "postcard" from the deep subsurface biosphere, as if meteorites are packages from space and snow is a letter from heaven. We should read them carefully to obtain a broader understanding of the subsurface biosphere.