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Figure F6. Models of radiolytic H2 concentrations with depth. If H2 is microbially oxidized, its concentration will be below our detection limit (pink line = concentrations measured at Site U1370 during Cruise KNOX-02RR [below detection limit of 3.5 × 10–8 M]. Other lines indicate predicted concentrations if the H2 is not oxidized (e.g., if sediment is sterile), but radioactivity and porosity are constant throughout the sediment (blue line = diffusive loss of H2 to both overlying ocean and underlying basement aquifer, red line = diffusive loss to the overlying ocean but basement is impermeable to chemical exchange). The constancy of radioactivity and porosity with depth and the concentrations of dissolved H2 at depth can only be tested by drilling to basement.

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