Methane Seepage, Carbonate Accretion, and Ecosystem Development at the Sentinel Seep in the Norwegian Trench

Understanding natural methane seepage systems is key to constraining subsurface fluid flow, benthic carbon cycling, and the development of seep-related ecosystems. On the Norwegian continental shelf, methane seepage reflects the coupling between hydrocarbon migration in the subsurface and seafloor expressions such as gas flares, authigenic carbonates, and chemosynthetic communities.

The Sentinel Seep was initially discovered in 2024 during the WELLFATE expedition, when hydroacoustic water-column data revealed active gas emissions in the Norwegian Trench of the northern North Sea. Follow-up investigations during the 2025 WELLFATE expedition aboard RV Kronprins Håkon enabled detailed characterization of the site through ROV-based observations, documenting its geomorphology, geological framework, and associated biological communities.

The seep is located within a pronounced ploughmark in an area where the Holocene sediment cover is anomalously thin. Seismic data indicate that the seep is situated at the seafloor near the southwestern edge of a large (~1,800 km²) amplitude anomaly along the Upper Regional Unconformity (URU), interpreted as shallow gas accumulation.

The seepage field covers an area of approximately 800 m (N–S) by 200 m (E–W) and contains extensive carbonate crusts and mound structures. Active gas release occurs at multiple locations and is associated with widespread microbial mat coverage. The site hosts abundant Lophelia corals, including isolated colonies exceeding 2 m in diameter as well as dense coral clusters.

Numerous fragments of abandoned and entangled fishing gear are present across the area, with some elements extending into the water column and posing operational risks. This debris has likely reduced subsequent trawling activity, unintentionally contributing to the preservation of the habitat. In this way, diffuse methane seepage has promoted carbonate buildup that physically shields the area from fishing disturbance - a protective function that inspired the name Sentinel Seep.

The Sentinel Seep represents a valuable natural laboratory for investigating the links between shallow gas accumulations, fluid migration pathways, authigenic carbonate formation, and the establishment and persistence of seep-associated ecosystems on the Norwegian margin.