A pinnacle-like structure dominated by the chemosymbiotic bivalve Thyasira from the Arctic Sea

The West‐Svalbard continental slope represents one of the northernmost gas hydrate provinces in the world. It is located on the western Svalbard margin in the eastern Fram Strait at ~79°N, north of the Knipovich Ridge and Molloy transform fault, situated on a hot and relatively young oceanic crust. The project Advancing Knowledge of Methane in the Arctic (AKMA), funded by the Norwegian Research Council, explores Arctic methane sources, processes, ecosystems and geological history in part of this province. During a dedicated research expedition (CAGE21-1-AKMA) a spectacular pinnacle-like structure has been identified at 914 m of water depth in the area of the north Kniponich Ridge, during the exploration by remotely operated vehicle. 

The structure is located at the base of a small-scale escarpment, typified by the presence of carbonate slabs. It is more than 1 m in height, has a diameter of at least 50 cm, and appears isolated on a flat seafloor with muddy-dominated heterogeneous sediment. From the video, the structure was apparently composed of dead bivalves cemented in a substrate of finer particles. Many bivalves were still articulated and with their valves closed, although no sign of living bivalves could be detected. Living regular echinoids, one large crinoid, several sponges and a few soft corals colonized the surface at the time of observation. Small samples have been collected from the surface of this fossil bivalve pinnacle. These samples supported the video-based interpretation of a fossil structure composed of cemented and carbonate-encrusted dead valves, among which the most abundant and the largest specimens belong to Thyasira cf capitanea, both juvenile and adult. The valves, either articulated or disarticulated, are often cemented by a white thick (3 mm) authigenic carbonate crust that binds together the mollusk shells, but also encrusts some of them internally. This means that these crusts continued to grow also after the death of the mollusks, when some of the valves were open. Additional interesting benthic fauna recognized in the cement includes small gastropods and foraminifers. The genus Thyasira has already been described as a typically chemosymbiotic group of species and as such, it has been reported from active cold seeps in the Arctic, as well as in other geographic areas and in the geologic record. We interpret the pinnacle as fossil evidence of a site of past methane emission, possibly exhumed by recent erosional or gravity-driven resedimentation processes.