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  • 1
    Publication Date: 2019-09-23
    Description: Large reservoirs of methane present in Arctic marine sediments are susceptible to rapid warming, promoting increasing methane emissions. Gas bubbles in the water column can be detected, and flow rates can be quantified using hydroacoustic survey methods, making it possible to monitor spatiotemporal variability. We present methane (CH4) bubble flow rates derived from hydroacoustic data sets acquired during 11 research expeditions to the western Svalbard continental margin (2008-2014). Three seepage areas emit in total 725-1,125 t CH4/year, and bubble fluxes are up to 2 kg.m(-2).year (-1). Bubble fluxes vary between different surveys, but no clear trend can be identified. Flux variability analyses suggest that two areas are geologically interconnected, displaying alternating flow changes. Spatial migration of bubble seepage was observed to follow seasonal changes in the theoretical landward limit of the hydrate stability zone, suggesting that formation/dissociation of shallow hydrates, modulated by bottom water temperatures, influences seafloor bubble release. Plain Language Summary It has been speculated that the release of methane (a potent greenhouse gas) from the seafloor in some Arctic Ocean regions is triggered by warming seawater. Emissions of gas bubbles from the seafloor can be detected by ship-mounted sonars. In 2008, a methane seepage area west of Svalbard was hydroacoustically detected for the first time. This seepage was hypothesized to be caused by dissociation of hydrates (ice-like crystals consisting of methane and water) due to ocean warming. We present an analysis of sonar data from 11 surveys conducted between 2008 and 2014. This study is the first comparison of methane seepage-related hydroacoustic data over such a long period. The hydroacoustic mapping and quantification method allowed us to assess the locations and intensity of gas bubble release, and how these parameters change over time, providing necessary data for numerical flux and climate models. No trend of increasing gas flow was identified. However, we observed seasonal variations potentially controlled by seasonal formation and dissociation of shallow hydrates. The hydrate formation/dissociation process is likely controlled by changes of bottom water temperatures. Alternating gas emissions between two neighboring areas indicate the existence of fluid pathway networks within the sediments.
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  • 2
    Publication Date: 2014-08-28
    Description: In the Northern Adriatic Sea, the occurrence of gas seepage and of unique rock outcrops has been widely documented. The genesis of these deposits has recently been ascribed to gas venting, leading to their classification as methane-derived carbonates. However, the origin of seeping gas was not clearly constrained. Geophysical data collected in 2009 reveal that the gas-enriched fluid vents are deeply rooted. In fact, the entire Plio-Quaternary succession is characterized by widespread seismic anomalies represented by wipe-out zones, and interpreted as gas chimneys. They commonly root at the base of the Pliocene sequence but also within the Palaeogene succession, where they appear to be associated to deep-seated faults. We suggest that there is a structural control on chimney distribution. Chimneys originate and terminate at different stratigraphic levels; commonly they reach the seafloor, where authigenic carbonate deposits form locally. Gas analyses of some gas bubble streams just above the rock outcrops reveal that gas is composed mainly of methane. Geochemical analyses performed at four selected outcrop sites show that these deposits formed as a consequence of active gas venting. In particular, geochemical analyses indicate carbonate precipitation from microbial oxidation of methane-rich fluids, although a straightforward correlation with the source depth of gas feeding the authigenic carbonates cannot yet be clearly defined. © 2014 The Authors.
    Print ISSN: 0950-091X
    Electronic ISSN: 1365-2117
    Topics: Geosciences
    Published by Wiley
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  • 3
    Publication Date: 2009-11-10
    Description: Three-dimensional (3D) seismic data analysis has identified 415 seismic chimneys in the Nyegga area, on the mid-Norwegian continental margin. The majority of the seismic chimneys connect with pockmarks at the modern seafloor, suggesting that the seismic chimneys represent the acoustic imprint of highly focused, cross-stratal fluid flow. A semi-quantitative spatial analysis of the seismic chimney complex, measuring parameters describing the dimension, geometry and root zone, provides constraints on the geological controls on chimney formation. Most of the seismic chimneys in the Nyegga region descriptively fall into the categories of 'blowout chimneys', because they diagnostically link to topography-controlled leakage locations within strata showing acoustic indications of free gas. We find that approximately 60% of the fluid-escape chimneys emanate from free gas layers below the gas hydrate stability zone. The timing of highly focused fluid expulsion through chimneys and the formation of the Nyegga pockmark field postdate the last glacial maximum at 25 000 years before present. © 2010 The Authors. Journal Compilation © Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.
    Print ISSN: 0950-091X
    Electronic ISSN: 1365-2117
    Topics: Geosciences
    Published by Wiley
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