Natural methane gas release from the seafloor is a widespread phenomenon that occurs at cold seeps along most continental margins. Since their discovery in the early 1980s, seeps have been the focus of intensive research, partly aimed at refining the global carbon budget. However, deep-sea research is challenging and expensive and, to date, few programs have successfully monitored the variability of methane gas release over several weeks or more. Long-term monitoring is necessary to study the mechanisms that control seabed gas release. Located at 800 m depth on the Cascadia accretionary prism offshore Oregon, Southern Hydrate Ridge is one of the most studied seep sites where persistent, but variable gas release has been observed for more than 20 years. Using a series of instruments connected to the Ocean Observatories Initiative's (OOI) Regional Cabled Array observatory, we monitored the venting activity at Southern Hydrate Ridge over several months. We will present results from the systematic monitoring, which include in particular acoustic sensing of bubble plumes and time-lapse photography of selected vents at the seafloor. The data reveal a very dynamic system characterized by frequent and significant changes in seabed morphology and highly variable gas emissions. Acoustic data show how bubble plume variability is linked to the local tidal cycles. Photo and video imagery reveal how intense gas ebullition contributes to rapidly shaping the seabed morphology. This work is funded by the German Ministry of Education and Research (Bundesministerium für Bildung und Forschung). The OOI is funded by the National Science Foundation.

We studied seafloor characteristics, water column anomalies, and sediment methane geochemistry in the German sector of the central North Sea during a research cruise with the German research vessel Heincke in summer 2019. An extensive hydroacoustic mapping campaign revealed the presence and distribution of flares in the water column, indicative for gas bubble releases as well as for geophysical subsurface indications of elevated gas concentrations. We analyzed the spatial distances of detected flares to subsurface salt diapir locations, seismically identified gas accumulations, and abandoned well sites. Continuous and discrete measurements of dissolved methane concentrations in the water column support the identification of seepage from the seafloor. Our data demonstrate that dissolved methane concentrations in the upper water column were not enriched above the studied well sites. At one area, characterized by the presence of shallow gas pockets, we observed methane concentrations ten times enriched compared to background values close to the seafloor. Our results indicate an active natural seep system in the northwestern part of the German North Sea, which is related to updoming salt structures rather than leaking wells, and further underlines that natural seeps may challenge the identification of potentially leaking wells. Due to the shallow water depths of 30 to 50 m in the study area, at least part of the released methane is probably contributing to the atmospheric inventory. This conclusion is based upon our observations of flares reaching close to the sea surface and a slight oversaturation of surface waters in the flare-rich area.

The Mariana forearc provides a unique natural laboratory to study slab dehydration in an active subduction zone by its deep-rooted mud volcanism. To test if mantle wedge serpentinites would record the source fluid composition and thus the dehydration reactions in the slab, we investigated silicon (Si) isotopic compositions (δ³⁰Si) in serpentine veins by in-situ femtosecond laser ablation ICP mass spectrometry. Our samples were recovered during IODP Expedition 366 and originate from three mud volcanoes that root in different depths, so that the pressure/temperature conditions in their source regions vary.

The δ³⁰Si values differ strongly between the mud volcanoes but also between different serpentine generations within individual samples. Serpentine that formed under low water/rock ratios has δ³⁰Si similar to pristine olivine. In contrast, serpentine veins that formed under higher water/rock ratios show large ranges in δ³⁰Si that vary significantly but systematically between the mud volcanoes and thus with the metamorphic grade at depth. Average δ³⁰Si of such serpentine veins are ‑0.10 ‰, ‑1.94 ‰, and ‑0.80 ‰ to ‑0.93 ‰ with increasing depth-to-slab. We interpret these across-forearc changes to record the Si isotopic compositions of the fluid sources, that are at shallow depth (inferred slab temperatures of ~80°C) the dehydration of (biogenic) opal and release of pore fluids, at intermediate depth (~150°C) clay mineral breakdown, and at the deepest point (>250°C) decomposition of clay minerals and altered oceanic crust. These data imply that Si isotope signatures of wedge serpentinites can be used as a reliable proxy for slab dehydration processes.

Subduction is a key process for the plate tectonic cycle and is responsible for the bimodal composition of the Earth’ crust. Whereas active subduction zones can be directly observed at many places, their initiation and the early evolution of the associated volcanic arc can only be studied from the geological record. One key location to study the geological processes related to subduction initiation and subsequent arc emergence and maturation is the Izu-Bonin-Mariana (IBM) subduction zone system in the Western Pacific. Here, a unique record of rocks that formed during the earliest stages of the newly formed subduction zone are preserved in the forearc. However, much less was known about the spatial extent of these lithologies and thus the temporal evolution and the dynamics of subduction zone initiation in the IBM system. In 2014, IODP Expedition 351 added an important perspective by drilling in a rear-arc location, thus complementing the geological record across the proto-IBM arc. In this talk, I will provide a synthesis of the scientific achievements gained through this expedition. The technically challenging drilling recovered a 1.45 km-long section of hemipelagic and volcaniclastic sediments, and 150 m of oceanic igneous crust. New oceanic crust formed analogous to the so-called forearc basalts during subduction initiation, and age and composition of the basaltic crust allow us to constrain the dynamics of subduction zone initiation. The volcaniclastic sediments above provide us with important insights into the compositional and temporal evolution of the volcanic arc over its full lifespan.

The active volcanic arcs of the Scotia- and Caribbean Plate are two prominent features along the otherwise passive margins of the Atlantic Ocean, where subduction of oceanic crust is verifiable. Both arcs have been important oceanic gateways during their formation. Trapped between the large continental plates of North- and South America, as well as Antarctica, the significantly smaller oceanic plates show striking similarities in size, shape, plate margins and morphology, although formed at different times and locations during Earth’s history.

Structural analyses of the seafloor are based on bathymetric datasets by multibeam-echosounders, including data of GMRT, AWI, BAS, MARUM/Uni-Bremen, Geomar/Uni-Kiel and Uni-Hamburg. Bathymetric data were processed to create maps of ocean floor morphology with resolution of 150-250 meters in accuracy. The Benthic Terrain Modeler 3.0, amongst other GIS based tools, was utilized to analyse the geomorphometry of both plates. Furthermore, we used bathymetric datasets for three-dimensional modelling of the seafloor to examine large-scale-structures in more detail. The modelling of ship-based bathymetric datasets, in combination with the GEBCO 2014 global 30 arc-second grid, included in the GMRT bathymetric database, delivered detailed bathymetric maps of both areas.

With the help of the fine- and broad-scale bathymetric position index, we present the first detailed interpretation of combined bathymetric datasets of the entire Scotia Sea, the Caribbean and adjacent areas, such as the South Sandwich Plate. We identified typical morphological features of the abyss, based on determination of steep and broad slopes, ridges, boulders, flat plains, flat ridge tops and depressions in various scales.

The solitary intraplate volcano Vesteris Seamount is located in the Central Greenland Basin and rises around 3000 m above the seafloor with a total eruptive volume of ~500 km³. Newly acquired high-resolution bathymetry allows through backscatter data and raster terrain analysis to distinguish several volcanic structures. The Vesteris Seamount is a northeast to southwest elongated stellar-shaped seamount with an elongated, narrow summit radially surrounded by irregular volcanic ridges, separated by volcanic debris fans. Whole rock geochemical data of 78 lava samples form tight liquid lines of descent with MgO concentrations ranging from 12.6 to 0.1 wt. %, implying that all lavas evolved from a similar parental magma composition. Video footage from ROV dives show abundant pyroclastic deposits on the summit and on the flanks whereas lavas are restricted to flank cones and dike intrusions. The seamount likely forms above a crustal weak zone and the local volcanic stress field increasingly affects the constructive and destructive features on the surface. The evolution of Vesteris Seamount reflects the transition from deep, regional crustal stresses in the older features to local, volcanic stresses in the younger structural features. The Vesteris Seamount enables to understand the structural and magmatic evolution of intraplate volcanoes distant from plate boundaries by combining detailed geological sampling, high-resolution bathymetry and underwater video coverage.