Hydrogen is the most abundant element in the solar system. As a source of clean energy, hydrogen is well-suited for sustainable development, and Earth is a natural hydrogen factory. However, most hydrogen vents reported to date are small, and the geological processes responsible for hydrogen formation—as well as the quantities that can be preserved in geological settings—remain unclear.
To better understand the availability of geological hydrogen, researchers from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) and their collaborators discovered and analyzed a large pipe swarm—a cluster of cylindrical geological structures—with remnants of hydrogen hydrothermal activity on the east Caroline Plate, west of the Mussau Trench. The Mussau Trench is a fossil trench, meaning it is an ancient (started about 25 million years ago) and now inactive trench that ceased tectonic activity. The newly discovered pipe swarm, named "Kunlun," consists of pipes with diameters ranging from 450 to 1,800 meters.
The study was published in Science Advances on September 5.
Hydrothermal fluids—a mixture of heated water and dissolved minerals—spray out through small tubes, ranging in diameter from centimeters to sub-centimeters, along the sides of pockmarks (small, crater-like depressions) within large pipes, or through gaps or cracks in breccia piles (accumulations of angular rock fragments). Most of the breccias in the hydrothermal cracks are partially yellowish, likely due to microbial mats (layers of microorgAanisms).
Similar to other hydrothermal fields, hydrothermal biotas (communities of living organisms) are also found in the Kunlun pipe swarm. The scorpionfish—the ecosystem's apex predator—is commonly found in the Kunlun pipe swarm. Since the biomass of the apex predator should be far less than that of its prey, researchers expect a large amount of microbial mat to be found within piles of breccia at the bottom of the pipe swarm.
Additionally, more than 800 short-duration seismic events—small earthquakes—were detected over a period of 28 days along a 150-kilometer profile across the trench, indicating ongoing widespread active gas leakage across the entire Mussau Trench. Clumped nitrogen isotope analysis (a method for tracing gas origins) of a hydrothermal fluid sample revealed a dominant atmospheric gas component.
Previously reported hydrogen hydrothermal activity has been located near active plate margins, e.g., spreading ridges, or near active transform faults that expose mantle peridotite, such as the Lost City. In contrast, the large hydrogen-rich Kunlun hydrothermal fields are located about 80 kilometers from active plate margins.
These hydrothermal pipes have steep walls, with abundant breccias and several generations of smaller bowl-shaped pockmarks on the bottom, similar to those of kimberlite, indicating multiple generations of explosions. Using empirically derived blast energy estimates, the formation of such large pipes would require millions of tons of TNT.
The most likely source of energy for the formation of such large pipes is hydrogen. Compressed hydrogen can release a huge amount of energy. For example, one ton of hydrogen expanding adiabatically from 1500 bar to 400 bar—the pressure at the water depths of the Kunlun pipe swarm—can release the same amount of energy as 0.21 ton TNT. To form such pipes, a large amount of hydrogen would be needed. Alternatively, a mixture of hydrogen and oxygen would be highly explosive. One ton of hydrogen reacting with oxygen releases 143 GJ of heat, which is 150 times more energy than the amount released by physical expansion.
According to Prof. XIAO Yuanyuan, first author of the study, the results suggest a potentially huge amount of hydrogen may have been formed deep in the ocean lithospheric mantle. "It could be economically mineable in the future," said Prof. XIAO.
