A major discovery has been made in the depths of the western Pacific. A unique hydrothermal system reveals hydrogen emissions of unprecedented scale.
This finding, achieved by the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), opens a new window on abyssal geochemical processes. The Kunlun hydrothermal field, explored using the manned submersible Fendouzhe, presents exceptional geological characteristics that challenge established models.
Illustration of the mechanism of hydrothermal activity.
Seawater sinks into the lithosphere through deep faults in the curved parts of the subducting Caroline Plate, west of the Mussau Trench. It reacts there with hot rocks, forming serpentinite and hydrogen. Alkaline fluids rich in hydrogen, magnesium, and iron, generated by serpentinization, rise along fractures and continuously dissolve calcium from surrounding rocks, leading to calcium enrichment in hydrothermal fluids. After rising, these fluids mix with shallow seawater circulation and are released at the ocean floor as hydrothermal discharge.
Near the vent, under high temperatures, calcium and magnesium ions from hydrothermal fluids combine with carbon dioxide from seawater to form dolomite. Further from the vent, under the cooling effect of seawater, authigenic carbonate rocks, mainly composed of calcite, form under normal seafloor temperature conditions. These hydrogen-rich hydrothermal fluids accumulate hydrogen in the vent after being released at its base, thus promoting a chemolithoautotrophic ecosystem. A little-known giant of the abyss
The site consists of twenty large circular depressions. Some of these underwater craters exceed one kilometer (0.62 miles) in diameter.
These structures form a vast network on the Caroline Plate. Their morphology resembles natural chimneys channeling fluids from the depths of the Earth.
The entire area covers 11.1 square kilometers (4.3 square miles). This expanse is a hundred times larger than that of the famous Lost City site in the Atlantic Ocean.
A hydrogen-generating process
The phenomenon of serpentinization is at the heart of this activity. It involves a chemical reaction between seawater and mantle rocks rich in olivine.
This process generates serpentine minerals and releases significant amounts of molecular hydrogen. Raman spectroscopy measured concentrations reaching 6.8 millimoles per kilogram in the fluids.
The annual hydrogen flux is estimated at 480 billion moles. This figure represents at least five percent of global submarine abiotic production, an impressive contribution from a single system.
To go further: What is a hydrothermal field?
A hydrothermal field is an active area of the ocean floor where water heated at depth escapes. This water comes from the infiltration of seawater into fractures in the Earth's crust. It heats up upon contact with magma and becomes enriched with mineral elements. These charged fluids then rise to the surface.
These systems form vents or mineral chimneys with sometimes complex shapes. Fluid temperatures can range from a few degrees to over 400 °C (752 °F) for black smokers. They deposit metal sulfides and other minerals by precipitating upon contact with cold water. These structures constitute unique geochemical archives.
These abyssal oases host ecosystems based on chemosynthesis. They harbor specialized microbial and animal life, independent of sunlight. Their study sheds light on the limits of life on Earth and potentially elsewhere. They play an important role in the chemical composition of the oceans.
Article author: Cédric DEPOND