Extensive decentralized hydrogen export from the Atlantis Massif

Abstract:

Hydrogen gas (H₂) is an important energy source for subsurface microbial communities, but its availability beyond the flow focused through hydrothermal chimneys is largely unknown. We report the widespread export of H₂ across the Atlantis Massif oceanic core complex and its associated detachment fault, which is distinct from the circulation system of the Lost City hydrothermal field situated on the southern wall of the massif. Reducing fluids exit the seafloor over a wide geographical area, including the summit of the massif, and along steep areas of mass wasting that occur along the southern ridge and to the east of the field. The fluid depths and H₂/CH₄ ratios of these fluids indicate they are supplied by a source separate from the Lost City hydrothermal field. We argue that the extensive fluid export is the natural consequence of fluid flow pathways that are strongly influenced by tectonic features and the volume and density changes that occur when ultramafic rocks react to form serpentinites, producing H₂ as a byproduct. The circulation of H₂-rich fluids through uplifted mantle rocks at moderate temperatures, as reported here, would have provided geographically expansive and stable environmental conditions for the early evolution of biochemical pathways. These results provide insight into the spatial extent of methane and hydrogen associated with serpentinization, independent of the focused flow of a vent field.