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An estimated 500–2500 gigatons of methane carbon is sequestered in gas hydrate at continental margins and some of these deposits are associated with overlying methane seeps. To constrain the impact that seeps have on methane concentrations in overlying ocean waters and to characterize the bubble plumes that transport methane vertically into the ocean, water samples and time‐series acoustic images were collected above Southern Hydrate Ridge (SHR), a well‐studied hydrate‐bearing seep site ∼90 km west of Newport, Oregon. These data were coregistered with robotic vehicle observations to determine the origin of the seeps, the plume rise heights above the seafloor, and the temporal variability in bubble emissions. Results show that the locations of seep activity and bubble release remained unchanged over the 3 year time‐series investigation, however, the magnitude of gas release was highly variable on hourly time scales. Bubble plumes were detected to depths of 320–620 m below sea level (mbsl), in several cases exceeding the upper limit of hydrate stability by ∼190 m. For the first time, sustained gas release was imaged at the Pinnacle site and in‐between the Pinnacle and the Summit area of venting, indicating that the subseafloor transport of fluid and gas is not restricted to the Summit at SHR, requiring a revision of fluid‐flow models. Dissolved methane concentrations above background levels from 100 to 300 mbsl are consistent with long‐term seep gas transport into the upper water column, which may lead to the build‐up of seep‐derived carbon in regional subsurface waters and to increases in associated biological activity.

The Lost City Hydrothermal Field (LCHF) is a novel serpentinite-hosted vent field located on the Atlantis Massif southern wall. Results of 2 m resolution bathymetry, side scan, and video and still imagery, integrated with direct submersible observations provide the first high-resolution geologic map of the LCHF. These data form the foundation for an evolutionary model for the vent system over the past >120,000 years. The field is located on a down-dropped bench 70 m below the summit of the massif. The bench is capped by breccia and pelagic carbonate deposits underlain by variably deformed and altered serpentinite and gabbroic rocks. Hydrothermal activity is focused at the 60 m tall, 100 m across, massive carbonate edifice ''Poseidon,'' which is venting 918C fluid. Hydrothermal activity declines south and west of the Poseidon complex and dies off completely at distances greater than 200 m. East of Poseidon, the most recent stage of hydrothermal flow is characterized by egress of diffuse fluids from narrow fissures within a low-angle, anastomosing mylonite zone. South of the area of current hydrothermal activity, there is evidence of two discrete previously unrecognized relict fields. Active venting sites defined by carbonate-filled fissures that cut the carbonate cap rock at the summit of the massif mark the present-day northernmost extent of venting. These spatial relationships reflect multiple stages of field development, the northward migration of venting over time, and the likely development of a nascent field at the massif summit.

25A 2003 high-resolution magnetic survey conducted by the Autonomous Underwater Vehicle ABE 26 over the low-temperature, ultramafic-hosted hydrothermal field Lost City reveals a weak positive 27 magnetic anomaly. This observation is in direct contrast to recent observations of strong positive 28 magnetic anomalies documented over the high-temperature ultramafic-hosted hydrothermal vents 29 fields Rainbow and Ashadze, which indicates that temperature may control the production of 30 magnetization at these sites. The Lost City survey provides a unique opportunity to study a field 31 that is, to date, one of a kind, and is an end member of ultramafic-hosted hydrothermal systems. 32Our results highlight the key contribution of temperature on magnetite production resulting from 33 serpentinization reactions. Whereas high temperature promotes significant production and 34 partitioning of iron into magnetite, low temperature favors iron partitioning into various alteration 35 phases, resulting in a magnetite-poor rock. Moreover, the distribution of magnetic anomalies 36 confirms results of a previous geological survey indicating the progressive migration of 37 hydrothermal activity upslope. These discoveries contribute to the results of 25 years of magnetic 38 exploration of a wide range of hydrothermal sites, from low-to high-temperature and from 39 basalt-to ultramafic-hosted, and thereby validate using high-resolution magnetics as a crucial 40 parameter for locating and characterizing hydrothermal sites hosting unique chemosynthetic-41 based ecosystems and potentially mineral-rich deposits. 42 43 1) Introduction 44The discovery of hydrothermal activity along the Galapagos Rift (Corliss et al., 1979) paved the 45 way for large-scale, deep-sea exploration of oceanic ridges, revealing a myriad of hydrothermal 46 vent fields primarily hosted on basaltic crust, with lesser gabbroic and ultramafic material (Kelley 47 and Shank, 2010). In contrast to intermediate-and fast-spreading systems where basaltic rocks 48 dominate, along slow-to ultraslow-spreading centers, the tectonically-dominated geology 49 (Karson and Elthon, 1987; Tucholke et al., 1998; Escartin et al., 2008) gives rise to a higher 50 abundance of hydrothermal systems hosted by variable amounts of ultramafic and gabbroic 51 material, such as the well-known Rainbow and Ashadze hydrothermal fields (Charlou et al., 52 2002; Charlou et al., 2010; Fouquet et al., 2008). These high-temperature venting systems 53 (>350°C) are characterized by sulfide chimneys emitting low pH fluids rich in carbon dioxide, 54 methane and hydrogen; chemical signatures that are hallmarks of fluid interaction with mafic and 55 ultramafic material in the subsurface (Charlou et al., 2002; Fouquet et al., 2010; Kelley and 56 Shank, 2010;Ohara et al., 2012). 57Although the magnetic signature of basalt-hosted hydrothermal sites is well constrained (Tivey et 58 al., 1993; Tivey and Johnson, 2002; Tivey and Dyment, 2010;Szitkar et al., 2014a; Szitkar et al., 59 2015),...

We document the discovery of an active, shallow, seafloor hydrothermal system (known as the Seven Sisters Vent Field) hosted in mafic volcaniclasts at a mid-ocean ridge setting. The vent field is located at the southern part of the Arctic mid-ocean ridge where it lies on top of a flat-topped volcano at ~130 m depth. Up to 200 °C phase-separating fluids vent from summit depressions in the volcano, and from pinnacle-like edifices on top of large hydrothermal mounds. The hydrothermal mineralization at Seven Sisters manifests as a replacement of mafic volcaniclasts, as direct intraclast precipitation from the hydrothermal fluid, and as elemental sulfur deposition within orifices. Barite is ubiquitous, and is sequentially replaced by pyrite, which is the first sulfide to form, followed by Zn-Cu-Pb-Ag bearing sulfides, sulfosalts, and silica. The mineralized rocks at Seven Sisters contain highly anomalous concentrations of ‘epithermal suite’ elements such as Tl, As, Sb and Hg, with secondary alteration assemblages including silica and dickite. Vent fluids have a pH of ~5 and are Ba and metal depleted. Relatively high dissolved Si (~7.6 mmol/L Si) combined with low (0.2–0.4) Fe/Mn suggest high-temperature reactions at ~150 bar. A δ13C value of −5.4‰ in CO2 dominated fluids denotes magmatic degassing from a relatively undegassed reservoir. Furthermore, low CH4 and H2 (<0.026 mmol/kg and <0.009 mmol/kg, respectively) and 3He/4He of ~8.3 R/Racorr support a MORB-like, sediment-free fluid signature from an upper mantle source. Sulfide and secondary alteration mineralogy, fluid and gas chemistry, as well as δ34S and 87Sr/86Sr values in barite and pyrite indicate that mineralization at Seven Sisters is sustained by the input of magmatic fluids with minimal seawater contribution. 226Ra/Ba radiometric dating of the barite suggests that this hydrothermal system has been active for at least 4670 ± 60 yr.

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