Biogeochemistry of seep-impacted sediments at a cold-water coral site off the Vesterålen coast, Northern Norway

Methane seepage in the Hola area off the coast of Vesterålen (N. Norway) has long been known for its peculiar association with cold-water coral mounds, but only recently it was possible to explore the distribution of seafloor ecosystems using a Remotely Operated Vehicle (ROV) and to conduct microhabitat-specific samplings for biogeochemical investigations. Here, we describe the results from sediment (carbon-nitrogen systematics) and pore fluid geochemistry (sulfate, dissolved inorganic carbon, methane) and interpret them in relation to the seafloor ecosystems. Microbial mats are the dominant seep-related community and form small white patches of a few tens of cm in diameter located at various distances from the coral mounds. Seep carbonates are widespread at this location and form extensive pavements. The seafloor distribution of methane bubbling and chemosynthetic communities seem controlled by fractures in the carbonates. Microbial mats are associated with intense sulfate-driven anaerobic oxidation of methane producing shallow sulfate-methane transitions coupled with highly 13C-depleted dissolved inorganic carbon in the pore water. Acknowledgments: this research was supported by Eman7 project (Research Council of Norway grant No. 320100) and AKMA project (Research Council of Norway grant No. 287869).