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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 indicates concentrations measured at Site U1370 during KNOX-02RR site survey (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 corresponds to diffusive loss of H2 to both overlying ocean and underlying basement aquifer, red line corresponds to diffusive loss to the overlying ocean with basement 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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