Hydrogen bond reorganization activates water radical chemistry for CO2-to-methane conversion under geologically relevant pressures.

The origin of abiotic methane in Earth's shallow crust remains unresolved, as existing theories require extreme conditions or catalytic surfaces. Here we show that hydrogen bond reorganization in pressurized bicarbonate aqueous solution (3 MPa, 25°C) activates radical chemistry for CO2-to-methane conversion (0.6 μM for 14 days). Through electron paramagnetic resonance (EPR) and in situ Raman spectroscopy, we demonstrate that pressure-induced restructuring of water's hydrogen bond (HB) network generates hydrated electrons (e-aq) and hydrogen radicals (•H), which drive CO2 reduction via a solution-chemistry mechanism independent of mineral catalysts. This water-mediated radical process provides experimental evidence for abiotic methanogenesis under geologically relevant conditions, fundamentally redefining water's role from solvent to redox-active participant in subsurface carbon cycling. The findings open alternative avenues for understanding natural hydrocarbon reservoirs and engineered carbon sequestration.