An international research team has recorded the deepest cold seeps ever documented in the Arctic, finding active gas hydrate mounds at 3,640 meters below the surface on the Molloy Ridge in the Greenland Sea. The discovery, made during ROV dives on 18 May 2024 as part of the Ocean Census Arctic Deep expedition, revealed chemosynthetic life thriving in conditions previously thought too extreme for such ecosystems. The finding raises sharp questions about how warming Arctic waters could alter methane stored in deep-sea sediments and what that means for both marine biology and climate science.
Why the deepest Arctic cold seeps change the picture for methane research
Cold seeps are locations on the ocean floor where methane and other hydrocarbons leak from subsurface reservoirs, often forming solid gas hydrates when they meet cold, high-pressure water. Until now, confirmed gas hydrate cold seeps in the Arctic had been found at shallower depths, leaving a gap in scientific understanding of how deep these systems can exist and how much methane they hold. The Freya Hydrate Mounds, as the site has been named, sit on the Molloy Ridge, a slow-spreading mid-ocean ridge that reaches some of the greatest depths in the Arctic basin. Their existence at 3,640 meters establishes a new depth record for this type of seep system in the region and anchors future work on deep hydrate stability.
The tension behind this discovery is straightforward. Arctic Ocean temperatures are climbing faster than in many other ocean basins, and gas hydrates are sensitive to thermal changes. If deep hydrate deposits like those at Freya are destabilizing, the released methane could feed into ocean chemistry and, potentially, the atmosphere. The research team’s hypothesis, tested through direct sampling and sonar mapping, is that these mounds formed after the last glacial maximum and may now be responding to shifting thermal conditions on the seafloor. According to the peer‑reviewed analysis, confirming whether destabilization is actively occurring at this depth would require repeat ROV sonar surveys over a period of roughly five years, a timeline the researchers have outlined but that has not yet been carried out. No repeat survey data exist yet, so the rate of change at Freya remains an open question rather than a confirmed trend.
ROV Aurora dives and the EXTREME24 expedition record
The discovery came during the Ocean Census Arctic Deep expedition, designated EXTREME24, which deployed the remotely operated vehicle Aurora to survey the Molloy Ridge. An institutional summary of the cruise describes how the team combined seafloor mapping with targeted ROV dives to search for both hydrothermal vents and methane seeps along this section of the mid-ocean ridge. On 18 May 2024, ROV Aurora conducted dives and direct sampling at the Freya Hydrate Mounds. Days earlier, on 12 and 13 May 2024, the same expedition sampled the nearby Jotul hydrothermal vent field, building a paired dataset of two very different deep-sea chemosynthetic environments on the same ridge system.
The Nature Communications paper details the methods and findings. Researchers used ROV-mounted cameras, sampling arms, and sonar to map the mounds and collect biological and geological specimens. High-definition video documented the seafloor morphology, while manipulator arms retrieved hydrate pieces, sediment cores, and representative fauna. Multibeam bathymetry, the technique of using sonar to map the shape of the seafloor, helped the team pinpoint the mounds before the ROV descended, revealing subtle topographic highs associated with gas escape. NOAA’s National Centers for Environmental Information maintains an archive of such bathymetric data, though specific cruise identifiers linking the EXTREME24 expedition to that database have not been publicly matched, leaving some details of data archiving for future clarification.
What the team found on the seafloor was striking. The Freya mounds host chemosynthetic communities, organisms that derive energy not from sunlight but from chemical reactions involving methane and hydrogen sulfide. Tubeworms and microbial mats were among the life forms recorded at the site, forming dense patches around active seepage points. These organisms are well known at shallower cold seeps, but their presence at 3,640 meters in the Arctic expands the known range of such ecosystems significantly. The biological findings suggest that methane seepage at this depth is not a fleeting event but sustained enough to support permanent communities, complete with trophic interactions and specialized symbioses between microbes and invertebrates.
In addition to visual observations, geochemical measurements confirmed that methane-rich fluids are actively rising from the subsurface. Porewater profiles from sediment cores showed steep gradients in dissolved methane and sulfate, consistent with ongoing anaerobic oxidation of methane by microbial consortia. Pieces of exposed gas hydrate, recovered by the ROV, further indicated that solid methane-bearing phases are stable under present pressure and temperature conditions at the site. Together, these lines of evidence underpin the interpretation of Freya as an active, long-lived cold seep system rather than a waning or relic feature.
What the Freya depth record does not yet answer
Several important questions remain unresolved. The published paper and institutional release confirm the depth record and the presence of active seepage, but they do not include long-term methane flux measurements that would quantify how much gas is escaping from the mounds over time. Without those numbers, scientists cannot yet calculate the contribution of sites like Freya to the Arctic methane budget or assess whether they are minor local sources or regionally significant emitters. The supplementary information referenced in the study includes species counts and geochemical profiles, but detailed extraction of those datasets has not appeared in publicly available summaries, limiting independent assessments of variability and uncertainty.
The hypothesis that the Freya mounds formed after the last glacial maximum rests on geological reasoning about when the Molloy Ridge became ice-free and when conditions allowed hydrate formation. Sediment accumulation rates, regional deglaciation history, and the timing of fluid migration pathways all factor into this reconstruction. Direct dating of the mounds, through methods like radiometric analysis of carbonate crusts or trapped organic material, would strengthen that timeline and could reveal whether seepage has been continuous or episodic over thousands of years.
Whether the mounds are currently destabilizing is a separate and harder question. Detecting changes in mound geometry or seepage intensity would require returning to the site with the same sonar instruments and comparing the new data against the May 2024 baseline. Time-lapse imaging, repeat multibeam surveys, and in situ chemical sensors could capture shifts in bubble flux or hydrate exposure. At present, however, the only available observations come from a single expedition window, which provides a snapshot rather than a time series. As a result, any statements about accelerating methane release from Freya remain speculative and are not supported by the existing dataset.
Another uncertainty concerns how deep Arctic hydrate systems like Freya will respond to ongoing ocean warming. At 3,640 meters, bottom-water temperatures change more slowly than in shallower shelf seas, and the pressure there makes hydrates relatively stable. Model projections cited by the research team suggest that even modest long-term warming could eventually push parts of the hydrate stability zone toward its upper threshold, but the timescales involved may span centuries. Distinguishing between natural variability and anthropogenically driven trends will require both repeated measurements and refined models that incorporate local geology, fluid flow, and oceanographic conditions.
For now, the Freya Hydrate Mounds primarily reshape the basic map of where Arctic cold seeps can exist and what kinds of life they support. They demonstrate that methane-fueled ecosystems extend into some of the deepest reaches of the basin and that gas hydrates can remain stable under extreme pressure while still feeding active seepage. As follow-up expeditions are planned, researchers will be watching for any signs that these deep deposits are beginning to shift, turning a remarkable discovery into a critical test case for how the changing Arctic may unlock buried methane stores.
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*This article was researched with the help of AI, with human editors creating the final content.
