Funds from this grant support a field project that aims to separate and quantify the sources, pathways, and fates of carbon that originated as mantle rock or as sedimentary biomass. The location of the fieldwork is west of Costa Rica, where the seafloor sinks or “subducts” beneath the Caribbean plate. A team led by Peter Barry of the University of Oxford will look closely at this subduction zone to see to what extent the burial of microbes (organic material rich in carbon) on the slab during oceanic sedimentation is a one-way road to death. Prior research estimates that 85 percent of the subducted carbon sinks under the tremendous pressure of gravity and the overlying plate into the deep, lifeless mantle, but recent measurements have detected unusually high carbon dioxide degassing from the zone. This opens the possibility that quite a lot of “biotic” carbon in deep seafloor mud might recycle as surface life. The project is a continuation of ongoing work by Deep Carbon Observatory scientists to probe the limits to life and accurately characterize the outer bound of pressures and temperatures that are nonlethal to some environmental microorganisms.