Physical and geological constraints on natural hydrogen accumulation beneath deep seafloors

Natural hydrogen generated by serpentinization of ultramafic rocks has attracted increasing attention as a potential clean energy resource. Recent studies of hydrogen-rich hydrothermal systems beneath deep seafloors, including the Kunlun hydrothermal field in the western Pacific, have suggested that some seafloor depressions and sedimentary conduits may represent explosive hydrogen-release structures associated with large subseafloor hydrogen reservoirs. Here we evaluate the physical and geological feasibility of such interpretations using first-principles thermodynamic, kinetic, hydrological, and geological constraints. Hydrogen generation by serpentinization is thermodynamically favorable over a broad range of crustal conditions and may occur widely wherever water interacts with mantle peridotites. However, hydrogen generation alone does not imply hydrogen accumulation. Molecular hydrogen is highly mobile in aqueous systems and is strongly affected by diffusion, fluid leakage, dilution, and limited oxidant availability under deep-seafloor conditions. Effective long-term preservation additionally requires thick, compacted, low-permeability sedimentary seals, conditions generally absent in many deep-ocean hydrothermal environments characterized by thin, porous, water-rich sediments. Quantitative evaluation indicates that the simultaneous physical and geological conditions required for large explosive hydrogen reservoirs beneath deep seafloors are highly restrictive and are not satisfied in typical soft-sediment hydrothermal settings. Nevertheless, hydrogen accumulation may occur in geological settings where serpentinized mantle is overlain by thick compacted low-permeability sedimentary successions capable of functioning as effective seals. Potential examples include sedimented passive margins, continent–ocean transition domains, and the Bohai–Yellow Sea system, where long-term burial of Yellow River-derived sediments may provide favorable preservation conditions. These results highlight the fundamental distinction between hydrogen generation and hydrogen preservation in subseafloor natural hydrogen systems.