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Microbial sulfate reduction is generally limited in the deep sea compared to shallower marine environments, but cold seeps and hydrothermal systems are considered an exception. Here, we report sulfate reduction rates and geochemical data from marine sediments and hydrothermal vent fields along the Arctic Mid Ocean Ridges (AMOR), to assess the significance of basalt-hosted hydrothermal activity on sulfate reduction in a distal deep marine setting. We find that cored marine sediments do not display evidence for sulfate reduction, apart from low rates in sediments from the Knipovich Ridge. This likely reflects the overall limited availability of reactive organic matter and low sedimentation rates along the AMOR, except for areas in the vicinity of Svalbard and Bear Island. In contrast, hydrothermal samples from the Seven Sisters, Jan Mayen and Loki's Castle vent fields all demonstrate active microbial sulfate reduction. Rates increase from a few 10s to 100s of pmol SO42- cm-3 d-1 in active hightemperature hydrothermal chimneys, to 10s of nmol SO42- cm-3 d-1 in lowtemperature barite chimneys and up to 110 nmol cm-3 d-1 in diffuse venting hydrothermal sediments in the Barite field at Loki's Castle. Pore fluid and sediment geochemical data suggest that these high rates are sustained by organic compounds from microbial mats and vent fauna as well as methane supplied by high-temperature hydrothermal fluids. However, significant variation was observed between replicate hydrothermal samples and observation of high rates in seemingly inactive barite chimneys suggests that other electron donors may be important as well. Sediment sulfur isotope signatures concur with measured rates in the Barite field and indicate that microbial sulfate reduction has occurred in the hydrothermal sediments since the recent geological past Our findings indicate that basalt-hosted vent fields provide sufficient electron donors to support microbial sulfate reduction in high- and low-temperature hydrothermal areas in settings that otherwise show very low sulfate reduction rates. [ABSTRACT FROM AUTHOR]

The article provides an overview of the PMEL Earth-Ocean Interactions (EOI) program and its research on seafloor and subseafloor processes in relation to ocean health, biogeochemical cycles, ecosystem diversity, and climate change. The program has made significant contributions to the study of methane seeps, hydrothermal vents, and submarine volcanoes. It emphasizes the need for better observational data on global ocean processes and highlights the importance of understanding hydrothermal iron sources and transport mechanisms. The article also mentions the discovery of hydrothermal activity on the Australian-Antarctic Ridge and the planned expedition to investigate hydrothermal venting and primary productivity in the Southern Ocean. Overall, the article underscores the significance of deep-sea exploration in understanding seafloor processes and their impact on ocean biogeochemistry, sustainable resources, and greenhouse gas accumulation. [Extracted from the article]

Methane gas plumes have been discovered to issue from the seafloor in the Puget Sound estuary. These gas emission sites are co‐located over traces of three major fault zones that fracture the entire forearc crust of the Cascadia Subduction Zone. Multibeam and single‐beam sonar data from cruises conducted in years 2011, 2018, 2019, 2020, and 2021 identified the acoustic signature of 349 individual bubble plumes. Dissolved CH4 gas from the plumes combines to elevate seawater methane concentrations of the entire Puget Sound estuary. Fluid samples from adjacent terrestrial hot springs and deep‐water wells surrounding the estuary contain a helium‐3 isotope signature, suggesting a deep fluid source located near the underlying Cascadia Subduction Zone plate interface. However, limited data from this pilot study suggest that Puget Sound seawater emission sites lack both similar chemical isotope signatures and elevated thermal anomalies that would be expected from association with a deep plate‐interface reservoir. A shallow reservoir within the Holocene sediments that cover the older Puget Sound basement with horizontal transfer to the thinly covered Alki Point and Kingston Arch anticlines is also a possibility, as has been suggested for other methane seep areas. The existence of vigorous marine methane plumes arising from areas of thin sediment cover associated with deeply penetrating forearc fault zones but presenting no thermal or chemical anomalies found in other similar forearc environments, remains an unresolved paradox. Plain Language Summary: Puget Sound is a large inland sea located in western Washington State where seawater circulation is dominated by vigorous tidal forcing from the North Pacific Ocean. The deep Puget Sound is the largest estuary in North America measured by contained water volume and the second largest estuary after Chesapeake Bay in terms of area. Shipboard sonar images have revealed approximately 349 bubble plumes of methane gas and fluid rising from the seafloor of the estuary. Large clusters of bubble plume sites are concentrated over the major regional fault zones that penetrate the western North American plate beneath Puget Sound, including the South Whidbey Island Fault, the Seattle Fault, and the Tacoma Fault Zones. Although the forearc Puget Basin is surrounded by terrestrial hot springs and water wells that show a clear chemical isotope signature of fluid arising from the underlying Cascadia Subduction Zone plate interface, based on our limited sampling there is currently no evidence for similar chemical or temperature anomalies in the Puget Sound plumes and the source of the methane bubble plumes is still unidentified. Key Points: Extensive methane bubble plumes have been discovered on the Puget Sound seafloorThe emission sites of these plumes are associated with major fault zones that penetrate the Cascadia forearcDissolved methane arising from the plumes is mixed throughout the estuary by tides and local mixing [ABSTRACT FROM AUTHOR]