Coupled CH4, CO2, and N2O cycling in a subsurface serpentinising system

Serpentinising systems are among the most plausible environments for life’s emergence, where reactions between water and ultramafic rocks generate hydrogen, methane, and simple organics that could have fuelled early metabolisms. These reactions create highly alkaline fluids and steep pH–redox gradients that persist today, sustaining diverse microbial processes that regulate greenhouse gas fluxes. Here, we examined subsurface fluids from the Samail Ophiolite (Oman), the world’s largest and best-exposed terrestrial serpentinising system, to characterise greenhouse gas dynamics and their interconnections across contrasting geochemical conditions. CH₄ concentrations increased markedly with pH and were highly supersaturated (up to 48,000× atmospheric equilibrium) in reduced, hyperalkaline fluids (pH > 11), indicating strong net production. In contrast, CO₂ concentrations decreased with pH, consistent with substantial CO₂ consumption and carbonate precipitation under hyperalkaline conditions, whereas CO₂ remained elevated in pH-neutral, oxidised fluids. N₂O concentrations were low (0.001–1.5 μM) and showed strong net consumption under hyperalkaline, reducing conditions. However, addition of CH₄ alongside ¹⁵N-nitrite stimulated N₂O production — up to 72-fold higher in hyperalkaline fluids, revealing a mechanistic link between CH₄ and N₂O cycling. Isotopic data (⁴⁵N₂O, ⁴⁶N₂O) further indicated depth- and pH-dependent shifts in dominant N₂O pathways. Our findings show that interactions between geological and microbial processes control the balance of greenhouse gas production and consumption in serpentinising systems. These insights illuminate how life and geochemistry interact under extreme conditions, with implications for modern CO₂ storage strategies and ancient Earth environments.