In a groundbreaking study that could reshape our understanding of submarine carbon cycling, researchers have identified expansive carbon dioxide (CO2) seeps and associated hydrate formations directly on the seafloor off the coast of Mayotte Island. These findings, revealed through a combination of advanced acoustic surveying and direct geochemical and optical investigations using remotely operated vehicles (ROVs), unveil a previously unobserved dynamic: the formation of CO2 hydrate mounds atop a vigorous liquid CO2 vent field. Such processes not only expand our knowledge of underwater carbon reservoirs but also expose significant environmental risks posed by geologically activated greenhouse gas emissions.
At the heart of this discovery is a phenomenon triggered by intensified volcanic activity in 2018, which appears to have initiated substantial subaqueous CO2 venting. This venting differs fundamentally from the well-studied methane hydrates embedded within marine sediments; here, pure liquid CO2 escapes the subsurface and crystallizes in situ into solid hydrates right on the seabed. This direct deposition of CO2 hydrate mounds is unprecedented in marine geology and challenges existing paradigms regarding the stability and distribution of gas hydrates in oceanic settings.
By utilizing sophisticated acoustic survey techniques, the research team mapped the seafloor to identify zones of anomalous reflectivity indicative of gas and fluid venting. These acoustic signatures, combined with meticulous geochemical sampling, confirmed the presence of high fluxes of CO2 emanating from discrete venting locations. Visual confirmation via ROVs equipped with optical imaging systems corroborated these findings, capturing vivid imagery of hydrate mounds forming conspicuously on the sediment-water interface, painting a detailed portrait of this rare underwater phenomenon.
The formation of these CO2 hydrates holds profound implications for benthic ecosystems. Unlike methane hydrates, which primarily modulate methane fluxes, these CO2 hydrate mounds mediate the flux of carbon dioxide into bottom waters. The direct release and subsequent sequestration of CO2 in hydrate form create a sharp gradient in local chemistry, severely acidifying the surrounding waters. This localized acidification detrimentally impacts benthic biota, including vital coral communities, whose calcareous structures dissolve or weaken under lowered pH conditions. Such dramatic shifts in geochemical environment threaten biodiversity and habitat stability along the affected seafloor regions.
Volcanic activity beneath the seafloor acts as the primary driver for this sudden and large-scale CO2 seepage. The 2018 eruptions injected voluminous quantities of CO2-rich fluids into the subsurface aquifers and sediments, elevating pore-fluid pressures and gas saturation levels until venting was triggered at the seafloor. This dynamic coupling between magmatic processes and oceanic gas emissions highlights the role of submarine volcanism as a potent, albeit episodic, source of CO2 to the ocean-atmosphere system. Continuous monitoring is thus essential to understand temporal variability and long-term impacts.
The geochemical analyses conducted provide compelling evidence that the vent fluids are characteristically enriched in CO2, differentiating them from methane-dominated gas seeps common in many continental margins. Furthermore, isotopic measurements reveal volcanic signatures, confirming the magmatic origin of these emissions. The coexistence of liquid CO2 and solid hydrate phases is thermodynamically stable under the measured pressure-temperature conditions at the Mayotte vent field, marking a natural laboratory for studying phase equilibria of CO2 in marine environments.
Notably, this discovery challenges traditional models of marine carbon cycling, which often assume that hydrates form within sediments rather than directly atop them. The existence of hydrate mounds exposed on the seafloor adds complexity to carbon flux pathways, as they interact directly with ocean currents and local ecosystems rather than remaining insulated within sediment pore structures. This exposure could also render these hydrates more susceptible to destabilization due to ocean warming or chemical perturbations, potentially accelerating CO2 release in response to climate change.
The impact on corals observed in proximity to these vents is profound, with researchers documenting marked decline and degradation of coral colonies. Acidification reduces calcification rates and compromises skeletal integrity, exposing corals to heightened vulnerability from biological and physical stressors. The vent field thus acts as a natural experiment illustrating the cascading effects of CO2 leakage on marine calcifiers and highlights potential future scenarios where ocean acidification linked to elevated atmospheric CO2 levels could jeopardize coral reef ecosystems globally.
Hydrate mounds also have potential implications for carbon sequestration strategies. Their formation directly atop the seabed suggests that CO2 could be naturally captured and stored in solid form, albeit temporarily. Understanding the stability and longevity of these hydrates under varying oceanic conditions may inform geoengineering approaches aimed at mitigating anthropogenic CO2 emissions. However, the environmental trade-offs, particularly the ecological damage from acidification, must be thoroughly evaluated before considering any such interventions inspired by this natural analog.
The integration of multi-disciplinary approaches comprising acoustic mapping, in situ optical observations, and comprehensive geochemical sampling underscores the necessity of holistic methodologies to unravel complex subseafloor phenomena. The deployment of ROVs facilitated unprecedented access to these vent sites, enabling researchers to observe and quantify hydrate formation dynamics and assess their ecological ramifications with precision. This multi-modal exploration sets a benchmark for future studies investigating submarine carbon fluxes, particularly those linked to active geological processes.
Geographically, the Mayotte vent field represents a unique setting where volcanic forces intersect with marine sediment dynamics, producing conditions optimal for CO2 hydrate genesis. The regional tectonic activity and magmatic-hydrothermal systems likely sustain the prolonged release of CO2, suggesting that similar systems may exist in other tectonically active submarine volcanic regions. Thus, this study opens avenues for global surveys aimed at identifying previously overlooked carbon-rich venting systems and their associated hydrate deposits in the world’s oceans.
The environmental consequences extend beyond localized coral communities, potentially influencing biogeochemical cycles at broader scales. Enhanced CO2 fluxes into bottom waters can perturb nutrient cycles, microbial activity, and carbonate chemistry, impacting a broad spectrum of benthic and pelagic organisms. Moreover, as these emissions contribute to ocean acidification regionally, understanding their interplay with global climate change processes becomes vital. The complex feedbacks between geological activity, ocean chemistry, and ecosystem health emphasize the intertwined nature of Earth systems.
This newly identified interaction between submarine volcanism, CO2 emissions, and hydrate formations also carries implications for carbon budgets and climate models. Quantifying natural CO2 fluxes from such vent systems is critical for accurately assessing regional and global carbon sources and sinks. The presence of exposed CO2 hydrates complicates flux estimations as transient dissolution and reformation cycles may either sequester or release gas episodically. Incorporating data from diverse vent systems like Mayotte into climate frameworks will enhance predictive capabilities regarding ocean-atmosphere carbon exchanges.
Future research efforts necessitate longitudinal studies to monitor the temporal evolution of the vent field and hydrate structures. Observations over multiple years will elucidate whether the venting remains stable, intensifies, or wanes, and how these trends affect hydrate stability and ecological health. Advances in underwater sensing technology combined with molecular biological techniques can further unravel microbial adaptation and geochemical transformations occurring in these extreme environments, offering insights into life’s resilience and biogeochemical functioning at CO2-rich interfaces.
In conclusion, the discovery of extensive CO2 seeps and their associated hydrate mounds on the seafloor near Mayotte Island represents a transformative leap in marine geosciences. It unveils a new natural mechanism regulating carbon dynamics at the ocean floor and underscores the significant environmental impacts induced by geological CO2 emissions. As ocean acidification and climate change continue to threaten marine ecosystems, understanding such natural analogs becomes increasingly critical for anticipating and mitigating future shifts in marine carbon reservoirs and ecosystem health.
Subject of Research: CO2 seepage, hydrate formation, marine geochemistry, volcanic activity impacts, coral reef acidification
Article Title: Large CO2 seeps and hydrate field on the seafloor offshore Mayotte Island
Article References:
Cathalot, C., Rinnert, E., Scalabrin, C. et al. Large CO2 seeps and hydrate field on the seafloor offshore Mayotte Island. Nat. Geosci. (2026). https://doi.org/10.1038/s41561-026-02004-2
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41561-026-02004-2
Keywords: CO2 hydrate, seafloor venting, volcanic activity, marine acidification, benthic ecosystems, coral degradation, gas hydrates, underwater carbon cycle
