Many vent systems with elevated hydrogen have been linked to serpentinization, a process in which hot, mineral-rich fluids react with ultramafic rocks in the crust to generate hydrogen, methane, and other reduced species. This mechanism has been considered the main explanation for hydrogen rich fluids in many parts of the deep ocean.
An international team led by Dr. Alexander Diehl of MARUM - Center for Marine Environmental Sciences and the Faculty of Geosciences at the University of Bremen has now identified a different source of hydrogen in sediment-hosted hydrothermal systems. The work focuses on a site where fluids circulate through sediments at an ultraslow spreading mid-ocean ridge.
The study area is the Jotul Hydrothermal Field on the Knipovich Ridge in the Norwegian Sea off Spitsbergen, where the North American and Eurasian plates meet. The field occupies the flank of a rift valley on an ultraslow spreading ridge that is covered by sediments from the adjacent continental slope, and the region hosts multiple seeps and vents.
The Jotul field was first discovered in 2022 during expedition MSM131 of the research vessel MARIA S. MERIAN, operated by MARUM. During that cruise, the remotely operated vehicle MARUM-QUEST 4000 collected initial hydrothermal fluid samples from the vents.
Those early samples, however, did not allow accurate gas measurements because gases escaped during recovery. Diehl compares the process to opening a pressurized soda bottle, where the gas rapidly leaves the liquid and cannot be quantified precisely in the laboratory.
To obtain reliable gas data, the team returned to the Knipovich Ridge in 2024 with gas-tight sampling containers designed to keep fluids at in situ pressure. These tools preserve dissolved gases during ascent, enabling detailed analyses of hydrogen and other volatiles.
The Jotul Hydrothermal Field is also distinguished by its depth of about 3,000 meters, which exerts strong pressure on both sampling operations and subsurface processes. Chief scientist Prof. Dr. Gerhard Bohrmann notes that such high pressures, combined with sediment cover, affect the geological and chemical reactions that shape the vent fluids.
In the laboratory, the researchers measured major components, dissolved gases, and isotopic signatures of the fluids. They then applied thermodynamic modeling to investigate how the hot fluids interact with surrounding rocks and sediments in the subsurface.
The models indicate that under the high pressures and temperatures beneath the vents, organic matter in the sediments decomposes under supercritical conditions and releases hydrogen molecules. This reaction provides a second pathway to generate hydrogen in deep hydrothermal systems in addition to serpentinization.
Diehl reports that the results demonstrate that serpentinization alone cannot account for the elevated hydrogen observed in all deep-sea vent fields. The findings suggest that interactions between hydrothermal fluids and organic-rich sediments at sediment-dominated vent sites may represent a more significant source of dissolved hydrogen to the ocean than previously assumed.
The research was carried out within the Cluster of Excellence The Ocean Floor - Earth's Uncharted Interface, which investigates processes at and beneath the seafloor. In the second funding phase of the cluster, starting in early 2026, additional expeditions will return to the Knipovich Ridge to extend measurements and comparisons.
Prof. Dr. Wolfgang Bach of MARUM and the University of Bremen states that upcoming cruises will focus on refining knowledge of vent structures and fluid compositions at Jotul. Comparing this field with other hydrothermal systems will help determine how widespread sediment-driven hydrogen production is along slow and ultraslow spreading ridges.
Research Report: High H2 production in sediment-hosted hydrothermal fluids at an ultraslow spreading mid-ocean ridge
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