Physical chemistry and geological principles against the possibility of explosive H₂ reservoirs beneath deep seafloors

Page 1 of 2 Physical chemistry and geological principles against the possibility of explosive H₂ reservoirs beneath deep seafloors Yaoling Niu * China University of Geosciences, Beijing 100083, China *[email protected] Submitted as a Perspective to National Science Review Abstract Recent discoveries of hydrogen-rich hydrothermal systems in the western Pacific, notably the Kunlun system, have highlighted the potential for significant hydrogen generation in deepocean environments. These findings underscore China's leadership in deep-sea research and oceanographic exploration. However, certain interpretations of these discoveries appear to conflict with established principles of chemistry, physics, and geology. Such discrepancies, if unaddressed, could undermine the scientific credibility of these achievements and the reputations of supporting institutions. This perspective provides a balanced analysis of the thermodynamic and geological factors influencing hydrogen generation during serpentinization of ultramafic rocks. By synthesizing current research, we aim to inform ongoing discussions and guide future investigations in this area. The objective is to promote rigorous scientific standards and contribute positively to the advancement of geoscience, particularly among emerging scientists. Main text Hydrogen generation beneath the seafloor has become a topic of extraordinary interest, both scientifically and societally. The recent discovery of a large hydrogen-rich hydrothermal field at over 4300 m depth in the western Pacific, named Kunlun hydrothermal field, is a notable milestone for marine geochemistry and energy exploration [1]. These studies have proposed that hydrogen explosions could explain brecciated seafloor “pipes” observed there, which resemble, in scale and form, the well-studied diamond-bearing kimberlite pipes in thick cratonic lithosphere [2]. These interpretations suggest the potential for colossal energy release, equivalent to billions of tons of TNT, and hint at the possibility of vast, naturally occurring hydrogen reservoirs beneath the seafloor. My long-standing research interest in the subject stems from studying the processes and consequences of oceanic upper mantle serpentinization, which is known to generate H₂ and other reduced compounds in hydrothermal systems [3-6]. The implications of these discoveries extend beyond pure science. If seafloor serpentinization produces hydrogen in sufficient quantities and if we develop methods to capture it, the world could quickly transition from gray or blue hydrogen to a truly green hydrogen economy, powered by naturally abundant molecular hydrogen. The Lost City and Rainbow hydrothermal fields discovered at the mid-Atlantic Ridge [5–6], has already provided a natural laboratory for studying serpentinization-driven hydrogen production, influencing our understanding of early life energy sources [7–9] and enabling quantitative thermodynamic modelling of H₂ generation [8,10]. Such discoveries motivate both continued scientific exploration and the long-term vision of sustainable deep-ocean hydrogen harvesting. However, careful interpretation is required: while these findings are exciting, the likelihood of explosive H₂ accumulation at the seafloor remains extremely low. This perspective aims to highlight both Page 2 of 3 the promise of hydrogen-rich hydrothermal systems and the constraints imposed by fundamental physical chemistry and geology, without diminishing the significance of the original discoveries [1]. Hydrogen combustion thermodynamics. The basic hydrogen combustion (or explosion) reaction is: 2H2+ O2→ 2H2O It represents the simplest H₂ oxidation reaction, producing water either as vapor (g) or liquid (l) depending on temperature. Key standard-state thermodynamic values for the reaction per mole of H2 are: reaction enthalpy ΔHᵣ ≈ −258.83 kJ (l), −241.83 kJ (g); Gibbs free energy ΔGᵣ ≈ −237.13 kJ (l), −228.63 kJ (g); and volume change ΔVᵣ ≈ −0.03717 m³ (l), −0.01240 m³ (g). These values are applicable to deep seafloor conditions. Because ΔGᵣ < 0, the reaction is thermodynamically spontaneous; ΔHᵣ < 0 indicates a strongly exothermic reaction, which is the basis for arguments about hydrogen explosions [2]. However, a thermodynamically favourable reaction is not equivalent to spontaneous ignition: activation energy is required, and under deep-sea conditions, no natural trigger exists. The volume contraction (ΔVᵣ < 0) further indicates that high pressures, such as the ~40 MPa at 4000 m depth, favour the reaction thermodynamically but do not cause explosive expansion. Stoichiometric and chemical constraints. Hydrogen combustion requires coexisting H₂ and O₂ molecules in proper stoichiometric ratios: O₂ ≥ 0.5 H₂. In air, where O₂ is ~21 vol%, combustion is only possible when H₂ is locally concentrated, with flammability and explosion limits of ~4–75 vol% and 13–65 vol%, respectively. By Henry’s law, the lower flammability limit (LFL) 4 vol% H₂ corresponds to ~0.0325 mM/L in surface water [10]. At 4000 m depth, accounting for hydrostatic pressure, the equivalent flammability threshold is ~13 mM/L and the lower explosion limit (LEL) is ~42 mM/L, assuming sufficient O₂. However, actual dissolved H₂ in deep seawater is extremely low, ~0.1–0.3 nM/L [11-12], roughly eight to nine orders of magnitude below the required threshold. Therefore, free gas bubbles cannot form, and explosive reactions are physically impossible under these conditions. Even considering only the LFL limit, not the stricter LEL, H₂ concentrations are far too low for combustion. Sources of seafloor H₂. Serpentinization of mantle peridotite is a well-established mechanism for H₂ generation [3–9]. The Lost City and Rainbow hydrothermal fields, discovered at the mid-Atlantic Ridge in the early 2000s [5-6], provides extensive field evidence for this process and its role in supporting microbial life [5–9]. Thermodynamic models further quantify how serpentinization produces H₂ [7,9]. Reported H₂ concentrations in hydrothermal fluids range from ~0.07 to 26.5 mM/L [1]. While these concentrations meet the flammability threshold at the mid-ocean ridge vents (e.g., Lost City, Rainbow), they remain insufficient for large-scale gas accumulation at >4000 m depth, where H₂ solubility in cold, high-pressure seawater reaches ~300 mM/L [11-12], causing rapid dissolution and diffusion. Globally, serpentinization-driven H₂ occurs across diverse tectonic settings, including slowand ultraslow spreading ridges, transforms, forearcs, and trench systems. The Kunlun field [1] adds to this catalogue, yet local concentrations sufficient for explosive H₂ remain rare and transient. Oxygen availability. Even if H₂ concentrations were sufficient, molecular O₂ is extremely limited at depth. At 4000 m, ~6.5 mM/L O₂ is needed to reach the flammability threshold with ~13 mM/L H₂, but measured O₂ concentrations below 2000 m depth are <0.3 mM/L [13], about 20 times too low for combustion and 70 times too low for explosion. No seawater below ~500 m globally meets these requirements [13]. Without a co-located, highly concentrated source of molecular O₂, explosive hydrogen combustion is impossible. Even thermodynamically favourable reactions (ΔGᵣ < 0, ΔHᵣ < 0) cannot occur spontaneously in the absence of ignition energy; the autoignition temperature for H₂–O₂ mixtures is ~570 °C, far above the 2–4 °C deepsea environment. Geological constraints on hydrogen storage. Large-scale H₂ explosions would require trapped, concentrated gas volumes beneath the seafloor. At hydrothermal vents, steep thermal and chemical gradients cause rapid cooling and dilution within meters of the vent, preventing accumulation. Effective trapping would require impermeable lithologies, such as deeply buried Page 3 of 4 shales, which do not exist at Kunlun. Mud and carbonate, proposed as seals [2], are highly porous and permeable, allowing H₂ and water molecules to diffuse freely. Even oversaturated fluids cannot form gas bubbles under ~40 MPa pressure at 4000 m depth. DSDP observations. Drilling at DSDP Leg 7 Site 63 in the Kunlun hydrothermal field reached 561 m of sediments before basalt, consisting of chalk, marl, and calcareous clay with porosity 82–35% (surface to depth), averaging ~52%, and bulk density 1412–2166 kg/m³ [14]. The sediment pile exerts only ~4.6 MPa effective stress, far below the >22 MPa required to reduce porosity to ≤15% and >27 MPa for ≤10%. Formation of an effective shale-like seal would require >2.7–3.3 km burial, which is absent at Kunlun. Consequently, the watery soft mud sediments cannot trap large H₂ volumes. Reinterpreting “breccia pipes.” The reported 30–150 m deep, 450–1800 m wide “pipes” [2] are shallow features within the 561 m mud pile, consistent with dewatering structures seen in sedimentary basins worldwide [15]. Compaction of calcareous clay during sediment consolidation naturally forms breccias at and within dewatering conduits, and these features are unrelated to explosive H₂ release. Over-interpreting them as kimberlite-like diatremes leads to an overestimation of potential hydrogen storage and the associated explosive energy. Energy considerations. While hydrogen combustion is highly exothermic (ΔHᵣ < 0), releasing hundreds of kJ per mole of H₂, the absence of free O₂, insufficient H₂ concentration, and lack of ignition sources at depth render any explosive scenario physically impossible. Treating shallow dewatering pits in watery soft mud piles as kimberlite-like pipes to estimate energy release therefore misrepresents both the magnitude of stored hydrogen and the geological context [2]. Perspective and outlook. The Kunlun hydrothermal field and related studies [1,2] underscore the global importance of hydrogen-generating serpentinization systems and their potential role in the emerging green hydrogen economy. While explosive hydrogen reservoirs at the deep seafloor are unrealistic under known physical, chemical and geological constraints, the widespread production of H₂ in hydrothermal systems presents opportunities for research, monitoring, and technology development. Continued global seafloor exploration, combined with international collaboration and advances in subsea hydrogen collection technologies, could enable sustainable, naturally sourced hydrogen production in the future. These efforts should be carefully guided to avoid over-interpretation of structural features while maximizing scientific insight and potential societal benefit. Acknowledgements. This contribution is supported by NSFC grant (91958215) and the Ministry of Education111 Project (B18048). 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