Xavier MANGENOT

Exploring the clumped isotope composition of gases such as hydrogen (H₂) or methane (CH₄) opens new frontiers in gas resource development. This study presents pioneering measurements of the clumped isotope composition of molecular hydrogen from natural geological settings using the Thermo 253 Ultra high-resolution mass spectrometer. Our findings from submarine hydrothermal vents and a continental reservoir reveal significant insights into the biogeochemical cycling of hydrogen. Our research focuses on natural hydrogen samples collected from high and low-temperature submarine hydrothermal vents (Lost City, Rainbow, Ashadze) and an intracontinental reservoir in Mali. By measuring the clumped isotope composition (ΔDD) of H₂, we can infer the temperatures of fluid venting and long-term storage. The ΔDD values of these samples ranged from 225 to 72 ‰, corresponding to temperatures from 27 to 375 °C. These measurements closely align with known environmental temperatures (storage or venting), primarily due to rapid isotopic re-equilibrium between hydrogen and water in near surface condition. Our study also underscores the importance of clumped isotope analysis in understanding biogeochemical processes. Experiments with methanogen cultures revealed that metabolic 'back reaction' progressively drives the ΔDD values of residual H₂ towards equilibrium with environmental temperatures. This insight into microbial activity provides valuable information on the subsurface microbial consumption of hydrogen, impacting the isotopic signatures detected in geological settings. Furthermore, the applicability of clumped isotope analysis is beyond hydrogen. Clumped isotope measurements of co-associated gases like methane and nitrogen can similarly reveal the thermal and chemical processes affecting the gases mixture. For instance, methane's clumped isotope composition can indicate its formation temperature and distinguish between biogenic, abiotic and thermogenic sources. Coupling clumped isotope analysis across multiple gases enhances our ability to better trace complex geochemical processes and improve our understanding of Earth's subsurface complexe gas systems. Our preliminary results also pave the way for several promising avenues of future research. Detailed studies on the rates of isotope exchange between H₂ and H₂O, both intramolecularly and with other hydrogen-bearing compounds, are essential to refine our understanding on the formation condition of natural hydrogen (temperature, depth, timing). Investigating the clumped isotope composition of other gases associated with H₂ in various geological settings can provide comprehensive insights into their formation and transformation/alteration processes. Moreover, expanding the application of clumped isotope analysis to industrial processes, such as hydrogen storage and commercial production, could improve the efficiency and sustainability of these technologies. Clumped isotope may also serve as a new tracer of H2 origin, i.e., distinguishing between sources such as methane steam reforming and water electrolysis, and help in making gas certificat of origin.

Co-auhors: Hao Xie, Antoine Crémière, Thomas Giunta, Marvin Lilley, Olivier Sissmann, Victoria Orphan, Arndt Schimmelmann, Eric Gaucher, Jean-Pierre Girard, John Eiler