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High-resolution mapping of shallow gas accumulations and gas seeps in San Simón Bay (Ría de Vigo, NW Spain). Some quantitative data

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Abstract

San Simón Bay in the innermost part of the Ría de Vigo is characterized by an abundance of very shallow gas accumulations and methane seeps. During the expeditions of April–June–September 2004 within the Spanish-funded Gs2G project, detailed very high-resolution seismic and field investigations were carried out to study the shallow gas and the seeps. Direct gas fluxes also were measured from bubble streams. For the first time, the surface area and gas front depth of a shallow gas field has been mapped and quantified in the inner bay of Ría de Vigo. This field overlaps spatially with the distribution of Holocene mud within the bay. Seismic data show 3.6 km2 affected by acoustic turbidity but this surface can be extended up to 9.5 km2 of San Simón’s muddy subtidal area. Mounded turbidity superimposed on the main gas field has been mapped and characterized as anthropogenically (mussel rafts) mediated gas accumulations. Different acoustic anomalies have been identified and interpreted as being due to gas escapes from the present seabed sediment. The very high resolution of the seismic data makes it possible to identify a new type of seep, here named ‘acoustic smoke.’ A direct relationship can be observed between the gas front of accumulations and escape features, both acoustic seeps and pockmarks. The methane flux has been estimated from the subtidal environment in San Simón based on detected acoustic targets and direct measurements of current bubble flow. The total estimated methane flux from the seabed into the water column ranges from 10.1 to 48.8 t/year, and into the atmosphere from 7.0 to 34.2 t/year. The intertidal San Simón environment is also actively venting methane, as indicated by the presence of bubbling during high tide and white patches of Beggiatoa sp.

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Acknowledgements

This paper is a contribution to the projects REN2003-03233, CTM2004-21110-E MAR, VEM2003-20093-C03-03, REN2003-02822 of the Spanish MCYT and PGIDIT06TAM31202PR of the Galician Government (XUGA). The authors also wish to thank Drs. Alan Judd, Jorn Bo Jensen and David Casas for their constructive comments that have improved the quality of the manuscript. The authors are grateful to the crew of B/O Mytilus for unfailing engagement during the cruise and especially to Francisco González for his technical support.

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  1. Departamento de Xeociencias Mariñas e Ordenación do Territorio, Universidade de Vigo, 36200, Vigo, Spain

    Jorge Iglesias & Soledad García-Gil

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  1. Jorge Iglesias
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  2. Soledad García-Gil
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Correspondence to Soledad García-Gil.

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Iglesias, J., García-Gil, S. High-resolution mapping of shallow gas accumulations and gas seeps in San Simón Bay (Ría de Vigo, NW Spain). Some quantitative data. Geo-Mar Lett 27, 103–114 (2007). https://doi.org/10.1007/s00367-007-0065-3

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