Natural H₂ and Sulfate Production via Radiolysis in Low Porosity and Permeability Crystalline Rocks

Abstract Natural hydrogen (H 2 ) is a candidate low‐carbon energy source for society and sustains microbial ecosystems when coupled with oxidants (e.g., sulfate). In crystalline rock settings, water‐rock interactions, including radiolysis, generate both H 2 and sulfate, but quantifying their production rates remains challenging when porosity and permeability are low. This study employs a Monte Carlo approach to estimate H 2 and sulfate production via radiolysis in the Revell batholith (Ontario, Canada), a well characterized site which is representative of hydrogeologically tight conditions. Results are then compared to estimates from a similarly isolated, but more fractured site at Kidd Creek (Ontario, Canada) to examine the spectrum of elemental cycling and habitability that may exist in these globally widespread settings. At Revell, production rates for both H 2 and sulfate are substantially lower than for Kidd Creek. Revell's most probable H 2 production rate is estimated at 1.6 nmol m −3 rock yr −1 , approximately half that of Kidd Creek, and Revell's sulfate production is 10 2 –10 6 ‐fold lower. These differences are primarily driven by Revell's lower porosity and sulfide concentrations, respectively. While statistical analyses suggest a small theoretical probability of low levels of biological activity, Revell's potential for sulfate‐supported habitability is calculated to be among the lowest observed in subsurface settings globally. This study demonstrates a framework for quantifying potential H 2 and sulfate production in settings where direct sampling of biomass, gas, and fluid is not feasible. It can be applied to investigations of the Earth's deep biosphere, geologic repositories, economic H 2 exploration, and habitability for other planets and moons.