Beneath the tranquil surface of the western Pacific Ocean lies a geologically dynamic and chemically intriguing phenomenon that promises to reshape our understanding of deep-sea hydrothermal activity and Earth’s subterranean hydrogen cycle. Researchers from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) have uncovered an expansive hydrogen-rich hydrothermal system named the Kunlun hydrothermal field. This discovery, positioned some 80 kilometers west of the Mussau Trench on the Caroline Plate, reveals a striking example of serpentinization processes occurring far from traditional mid-ocean ridge environments, challenging previous assumptions about where hydrogen-generating hydrothermal systems can develop.
The Kunlun hydrothermal field distinguishes itself through an assemblage of approximately 20 sizeable seafloor depressions, some extending beyond one kilometer in diameter. These formations group together in patterns evocative of “pipe swarms,” which are steeply inclined, vertical cylindrical geological structures recognized for channeling fluids and gases from Earth’s interior towards the ocean floor. Such pipe swarms are often associated with kimberlite volcanism and gas-driven eruptions in terrestrial settings, but their submarine analogs remain less understood. The unusual geomorphology and scale of the Kunlun system underscore a complex interplay of tectonic forces and fluid dynamics operating beneath the seafloor.
Application of cutting-edge technologies has greatly advanced the exploration of this hydrothermal site. Using the crewed submersible Fendouzhe, scientists conducted in situ examinations that allowed precise sampling and direct observation of the hydrothermal fluids and mineral deposits. Crucially, seafloor Raman spectroscopy measurements revealed molecular hydrogen concentrations ranging between 5.9 and 6.8 millimoles per kilogram in diffuse hydrothermal fluids venting from the seafloor. These elevated hydrogen levels, coupled with extensive carbonate mineralization observed below the carbonate compensation depth, mark the Kunlun field as a prolific source of abiotic hydrogen.
Despite the measured fluid temperatures being relatively moderate, below 40 degrees Celsius, geochemical evidence suggests that conditions deeper in the geological substrate reach significantly higher temperatures. These temperatures are sufficient to drive the formation of dolomite and other complex carbonate phases, indicating that intense fluid-rock reactions transpire far beneath the ocean floor. Such high subsurface thermal regimes coupled with rock alteration reactions are pivotal for sustaining serpentinization, the chemical process by which ultramafic rocks enriched in iron and magnesium interact with seawater to produce hydrogen gas and serpentine minerals.
The quantification of hydrogen flux from this system further underscores its significance. Based on detailed mapping of fluid discharge areas and velocity assessments, the Kunlun field’s annual hydrogen emission is approximated at 4.8 × 10¹¹ moles per year. To contextualize this magnitude, it represents around 5% of the global abiotic hydrogen output from all known submarine sources combined. This remarkable emission rate from a single site reveals an underexplored dimension of seafloor geochemical processes that may have far-reaching implications for global hydrogen budgets and geochemical cycles.
Geological scrutiny of the region has identified features resembling terrestrial kimberlite pipes, including steep-walled craters and explosive breccia deposits. These morphological markers signify a dynamic evolutionary history characterized initially by gas-driven eruptive events that created the pipe-like conduits. Subsequently, prolonged episodes of hydrothermal fluid circulation facilitated extensive mineral deposition and carbonate buildup within these structures, shaping the current landscape observed on the seafloor.
Beyond its geological and geochemical importance, the Kunlun hydrothermal system presents a unique biological niche. The research team documented a diverse assemblage of deep-sea organisms thriving in close association with the hydrothermal vent fields. Species such as shrimp, squat lobsters, anemones, and various tubeworms inhabit this ecosystem, likely relying on hydrogen-fueled chemosynthetic pathways for energy production. This ecological complexity highlights the integral role of submarine hydrogen sources in sustaining unique biological communities in otherwise nutrient-poor deep-ocean environments.
Scientifically, the implications of this discovery extend well beyond contemporary oceanography. The alkaline, hydrogen-rich fluids circulating at Kunlun are hypothesized to resemble the chemical conditions of early Earth’s oceans before the advent of widespread oxygenation. Studying such systems offers a natural laboratory for reconstructing primordial metabolic processes that might have spurred the emergence of life. Understanding hydrogen dynamics in these settings could illuminate critical stages in Earth’s biogeochemical evolution and the niche conditions that fostered abiogenesis.
The Kunlun hydrothermal system’s remarkable hydrogen flux and geochemical uniqueness also open exciting prospects for future exploration of submarine hydrogen resources. While terrestrial and industrial hydrogen production have garnered attention due to emerging energy demands, natural, untapped submarine hydrogen reservoirs represent a potential resource that is only beginning to be appreciated. The findings call for intensified scientific efforts to identify and characterize similar systems that could contribute to sustainable hydrogen resource portfolios.
This discovery further refines the global narrative of hydrothermal systems, moving beyond the well-studied mid-ocean ridges. It demonstrates that tectonically active regions on subducting plates, with their complex fracture networks and serpentinizing ultramafic rocks, are important contributors to Earth’s hydrogen cycle. These insights enrich models of deep Earth processes, geochemical fluxes, and their surface expressions in ocean chemistry and biology.
Taken together, the emitted hydrogen from the Kunlun field not only plays a pivotal role in local geochemistry and ecology but may also impact large-scale ocean chemistry. The interaction of hydrogen-rich fluids with seawater alters redox states and carbonate equilibria, influencing mineral precipitation and potentially affecting carbon sequestration mechanisms in the deep ocean. These processes warrant continued interdisciplinary research involving geologists, chemists, and biologists to fully elucidate their mechanics and consequences.
In conclusion, the identification and characterization of the Kunlun hydrothermal field represent a major milestone in marine geology and geochemistry. By revealing a massive, active, hydrogen-producing hydrothermal system with significant ecological and geochemical ramifications, the discovery challenges existing paradigms and inspires a reevaluation of Earth’s deep-sea hydrogen landscape. As investigations continue to probe its complex evolution, subsurface dynamics, and ecological interactions, the Kunlun hydrothermal system stands as a vital window into Earth’s past, present, and future subterranean processes.
Subject of Research: Hydrothermal hydrogen generation and deep-sea serpentinization on the subducting plate near the Mussau Trench
Article Title: Discovery of a Massive Hydrogen-Rich Hydrothermal System on the Western Pacific Seafloor
News Publication Date: August 8, 2023
Web References: http://dx.doi.org/10.1126/sciadv.adx3202
Image Credits: Image by IOCAS
Keywords: Marine geology, Ocean chemistry, Hydrothermal systems, Deep-sea serpentinization, Abiotic hydrogen, Submarine geology
