International audienceWe present results from multibeam bathymetric data acquired during 2005 and 2006, in the region of maximum slip of the 26 Dec. 2004 earthquake (Mw 9.2). These data provide high-resolution images of seafloor morphology of the entire NW Sumatra forearc from the Sunda trench to the submarine volcanic arc just north of Sumatra. A slope gradient analysis of the combined dataset accurately highlights those portions of the seafloor shaped by active tectonic, depositional and/or erosional processes. The greatest slope gradients are located in the frontal 30 km of the forearc, at the toe of the accretionary wedge. This suggests that long-term deformation rates are highest here and that probably only minor amounts of slip are accommodated by other thrust faults further landward. Obvious N–S oriented lineaments observed on the incoming oceanic plate are aligned sub-parallel to the fracture zones associated with the Wharton fossil spreading center. Active strike-slip motion is suggested by recent deformation with up to 20–30 m of vertical offset. The intersection of these N–S elongated bathymetric scarps with the accretionary wedge partly controls the geometry of thrust anticlines and the location of erosional features (e.g. slide scars, canyons) at the wedge toe. Our interpretation suggests that these N–S lineaments have a significant impact on the oceanic plate, the toe of the wedge and further landward in the wedge. Finally, the bathymetric data indicate that folding at the front of the accretionary wedge occurs primarily along landward-vergent (seaward-dipping) thrusts, an unusual style in accretionary wedges worldwide. The N–S elongated lineaments locally act as boundaries between zones with predominant seaward versus landward vergence
Over the last 20 years, the PAGEs-endorsed IMAGEs program (International Marine Past Global Change study, http://www. images-pages.org/) supported 18 seagoing expeditions onboard the 120-meter-long Research Vessel Marion Dufresne. this vessel, operated by the French Paul-Emile Victor Polar Institute, was equipped with an in-house-developed, unique sediment coring facility, called CALYPsO, which allowed for the retrieval of high-quality, long marine cores at sites of high sedimentation rates. the vessel currently holds the world record for the longest marine core ever retrieved-64.5 m. these cores enabled the comparison of marine and ice-core records at the same resolution for the first time, and tremendously improved the understanding of past oceans dynamics.
Understanding the dynamics and fate of methane (CH4) release from oceanic seepages on margins and shelves into the water column, and quantifying the budget of its total discharge at different spatial and temporal scales, currently represents a major scientific undertaking. Previous works on the fate of methane escaping from the seafloor underlined the challenge in both, estimating its concentration distribution and identifying gradients. In April 2019, the Envri Methane Cruise has been conducted onboard the R/V Mare Nigrum in the Western Black Sea to investigate two shallow methane seep sites at ∼120 m and ∼55 m water depth. Dissolved CH4 measurements were conducted with two continuous in-situ sensors: a membrane inlet laser spectrometer (MILS) and a commercial methane sensor (METS) from Franatech GmbH. Additionally, discrete water samples were collected from CTD-Rosette deployment and standard laboratory methane analysis was performed by gas chromatography coupled with either purge-and-trap or headspace techniques. The resulting vertical profiles (from both in situ and discrete water sample measurements) of dissolved methane concentration follow an expected exponential dissolution function at both sites. At the deeper site, high dissolved methane concentrations are detected up to ∼45 m from the seabed, while at the sea surface dissolved methane was in equilibrium with the atmospheric concentration. At the shallower site, sea surface CH4 concentrations were four times higher than the expected equilibrium value. Our results seem to support that methane may be transferred from the sea to the atmosphere, depending on local water depths. In accordance with previous studies, the shallower the water, the more likely is a sea-to-atmosphere transport of methane. High spatial resolution surface data also support this hypothesis. Well localized methane enriched waters were found near the surface at both sites, but their locations appear to be decoupled with the ones of the seafloor seepages. This highlights the need of better understanding the processes responsible for the transport and transformation of the dissolved methane in the water column, especially in stratified water masses like in the Black Sea.
<p>Methane is an important greenhouse gas and an energy resource. Methane in sea water can originate from microbially-mediated&#160;organic matter (OM) degradation processes at shallow depth&#160; within the sediments, or from thermal cracking of refractory OM at deeper depth. On continental margins, this methane is stored in specific sedimentological bodies or as gas hydrates, or is released at the seafloor as submarine geological seeps followed by its oxidation in the water mass. However, methane released at the seafloor may not entirely be oxidized in the water column and a fraction of it may ultimately reach the atmosphere. The factors that govern the magnitude of methane transfer through the water column to the atmosphere remain poorly known. It has been identified that the amount of methane transferred to the atmosphere is strongly dependent on sites, and the thickness of the water column plays a critical role.</p><p>The Black Sea shelf and margin are known to host a large number of strong methane seepages. It has therefore been identified as a perfect candidate to investigate the fate of methane released from the seafloor to the atmosphere. This area can also act as a proxy for investigating the fate of methane in potential scenarios of hydrate destabilization in a changing climate, which can become a societal problem in the future. In the frame of ENVRIplus H2020 project (www.envriplus.eu) we developed a joint pilot experiment to measure methane transfer from the seafloor to the atmosphere, in a pilot study involving European research infrastructures ICOS, Eurofleets, EMSO and ACTRIS. We investigated the influence of depth by mapping CH4 concentration and bubble distribution at two different sites, at 60m and 100m water depth, respectively. The pilot experiment developed joint monitoring strategy for methane detection at various levels starting from the seafloor and moving across the water column, the water/air interface and the atmosphere. An EK80 echosounder was used to identify emission areas through massive bubble plumes. The methodology applied integrates (1) sampling from the geosphere, hydrosphere and atmosphere for laboratory measurements of methane concentration by well-proven standard methods together with &#948;13CH4 analysis, (2) in situ measurements of methane concentration into the water column and the atmosphere, and (3) the deployment of a seafloor observatory for a short monitoring period (4-5 days) to evaluate the temporal variability of gas fluxes.</p><p>During the cruise we found several occurrences of bubble plumes extending near the surface. Our measurements indicate that dissolved methane concentration drastically decreases from the seafloor to the water surface, highlighting its degradation and dispersion along the pathway to the atmosphere. The atmospheric data suggests a consistent input of marine methane to the atmosphere at the shallower site,. Our study highlights the observational challenges both for the measurement of methane from in situ and laboratory methods, and for the estimation of sea surface fluxes.</p>
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